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Volcanic stratigraphy and lithogeochemistry of the Seneca Zn-Cu-Pb Prospect, Southwestern British Columbia McKinley, Sean D. 1996

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V O L C A N I C S T R A T I G R A P H Y A N D L I T H O G E O C H E M I S T R Y O F T H E S E N E C A Z N - C U - P B P R O S P E C T , S O U T H W E S T E R N B R I T I S H C O L U M B I A by SEAN D. M C K I N L E Y B.Sc.(Hons) , Queen's University, 1992 A THESIS S U B M I T T E D IN P A R T I A L F U L F I L L M E N T OF T H E R E Q U I R E M E N T S FOR T H E D E G R E E OF M A S T E R OF S C I E N C E in T H E F A C U L T Y OF G R A D U A T E S T U D I E S (Department of Geological Sciences) We accept this thesis as conforming to_the-^equired standard: THE UNIVERSITY OF BRITISH COLUMBIA March 1996 © Sean D. McKinley, 1996 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of G&OL-OGICAL- S c i e n c e s The University of British Columbia Vancouver, Canada Date A P R I L . ' g" } A B S T R A C T The Seneca Prospect is a volcanic-hosted Zi i -Cu-Pb deposit 120 km east of Vancouver in southwestern British Columbia. Volcanic strata at Seneca form part of the Weaver Lake Member of the Lower to Middle Jurassic Harrison Lake Formation of the Harrison Terrane. The rocks at Seneca comprise four principal facies: 1) vent to vent-proximal facies consisting of basaltic to rhyolitic lavas and associated breccias; 2) vent-proximal to distal facies consisting of volcaniclastic debris flows and siltstones; 3) coeval intrusions consisting of basaltic andesitic to rhyolitic sills and dikes, and 4) distal marine facies consisting of a pumice-bearing argillaceous unit. The volcanic strata can be subdivided into three intervals from bottom to top as follows: 1) the Footwall Interval below the mineralized horizon which comprises subaqueously deposited basaltic lavas and felsic debris flows; 2) the Seneca Horizon which hosts the mineralized zones, and 3) the Hangingwall Interval which is consists largely of felsic flows, intrusions and volcaniclastic rocks. Volcaniclastic rocks in the Footwall Interval are dominated by coarse, poorly-sorted debris flows whereas the volcaniclastic rocks of the Hangingwall Interval are mostly massive to well-bedded ashes and volcanically-derived turbidites. The Seneca volcanic sequence is a bimodal suite of rocks of calc-alkaline to transitional calc-alkaline-tholeiitic affinity (Zr /Y ratios >3.5, L a N / Y b N ratios > 2.0). Pearce element ratio analysis of the mafic rocks shows that the chemical variation in a least-altered subset can be explained by the fractionation of plagioclase, olivine and clinopyroxene although variation within basaltic and basaltic andesitic subgroups can be explained by plagioclase fractionation alone. Major element variations in the least altered felsic rocks can be explained by fractionation involving the crystallization of feldspar, quartz ± pyroxene and/or hornblende. Trace element trends can be accounted for by 30 to 40 % fractional crystallization of the assemblage feldspar-hornblende-magnetite+apatite. Mass change calculations revealed both a vertical zonation and spatial differences in the hydrothermally altered stockwork zones. In general, the stockworks can be subdivided into upper and lower alteration zones. The upper quartz-sericite zone has experienced net mass gain with mass gains of Si02 and K 2 0 and mass losses of N a 2 0 and C a O resulting from silicification and the destruction of feldspar. The lower sericite-chlorite zone has had small net mass gains or losses as a result of mass gains of K 2 0 and M g O and losses of N a 2 0 , C a O and in places S i 0 2 . The M g O gains throughout the Vent Zone are much smaller or absent in the Fleetwood Zone, perhaps indicating that the larger Vent Zone hydrothermal system was more capable of incorporating seawater magnesium than the Fleetwood Zone stockwork systems, which may have been sealed by overlying flows or volcaniclastic sediments. Mineralization in the Pit Area consists of zones of disseminated to conformable massive sphalerite, pyrite, chalcopyrite, galena and barire hosted by the strongly altered ore zone conglomerate (OZC) . Stratigraphic relationships indicate that these zones may have formed contemporaneously with the stockwork sphalerite-pyrite-chalcopyrite mineralization in the Fleetwood and Vent Zones. The stockwork zones possibly were vertical conduits for hydrothermal fluids which then migrated laterally through the permeable O Z C where they interacted with seawater and formed the Pit Area sulphide mineralization. The volcanic rocks which host the Seneca deposit have geological and geochemical similarities to younger rocks of the Lau Basin and Tofua Arc in the southwest Pacific and the Hokuroku Basin in Japan. These similarities suggest that the Seneca volcanic sequence and sulphide mineralization may be in rifted sub-basins within a calc-alkaline volcanic arc formed at a destructive plate margin involving two oceanic plates with little or no continental crustal influence. iv TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS iv LIST OF TABLES vii LIST OF FIGURES vii LIST OF PLATES x ACKNOWLEDGMENTS xii DEDICATION xui CHAPTER 1. INTRODUCTION I 1.1 SCOPE OF STUDY 1 1.2 TERMINOLOGY 4 1.3 PROPERTY HISTORY 5 1.4 REGIONAL GEOLOGY 5 1.4.1 HARRISON T E R R A N E 5 1.4.2 HARRISON L A K E FORMATION 6 1.4.3 G E O C H R O N O L O G Y 7 1.4.4 STRUCTURE A N D M E T A M O R P H I S M 8 CHAPTER 2. GEOLOGY OF THE SENECA DEPOSIT 9 2.1 INTRODUCTION 9 2.2 DISTRIBUTION OF VOLCANIC FACIES 17 2.2.1 F O O T W A L L I N T E R V A L 17 2.2.2 SENECA HORIZON 19 2.2.3 H A N G I N G W A L L INTERVAL 19 2.3 DESCRIPTION OF VOLCANIC FACIES 20 2.3.1 FACIES 1: L A V A S 20 2.3.1.1 MAFIC LAVAS 20 2.3.1.2 FELSIC LA VA FLO WS A ND DOMES 2 2 2.3.2 FACIES 2: V O L C A N I C L A S T I C ROCKS 24 2.3.2.1 FA CIES 2.I: DEBRIS FLO WS. GRA IN FLO WS AND TURBID/TIC VOLCANICLASriCS 25 2.3.2.2 FA CIES 2.2: VOLCANICLASTIC S1LTSTONE 29 2.3.2.3 FA CIES 2.3: DA CIVIC AND RHYOLITIC PUMICE BEDS 29 2.3.2.4 FA CIES 2.4: ORE ZONE CONGLOMERA TE 31 2.3.3 FACIES 3: S Y N V O L C A N I C INTRUSIONS 31 2.3.3.1 FELDSPAR-I'H YH/C INTRUSIONS (FT) 34 2.3.3.2 MAFIC INTRUSIONS 34 2.3.3.3 QUARTZ-FELDSPAR-PHYRIC INTRUSIONS (OTP) 35 2.3.4 FACIES 4: A R G I L L A C E O U S BEDS 35 2.4 DISCUSSION 36 CHAPTER 3. PETROLOGY OF VOLCANIC SEQUENCE AT SENECA 40 3.1 INTRODUCTION 40 3.2 PETROGRAPHY OF THE VOLCANIC ROCKS AT SENECA 40 3.2.1 FACIES 1: L A V A FLOWS A N D BRECCIAS 40 3.2.1.1: MAFIC LAVAS 40 3.2.1.2: FELS1C LA VAS 42 3.2.2 FACIES 2: V O L C A N I C L A S T I C ROCKS 44 3.2.2.1 FACIES 2.1: VOLCANICLASTIC DEBRIS FLOWS 44 3.2.2.2 FACIES 2.2: VOLCANICLASTIC SILTSTONES 44 3.2.3 FACIES 3: S Y N V O L C A N I C INTRUSIONS 49 3.2.3.1 MAFIC INTRUSIONS 49 3.2.3.2 FELSICINTR USIONS 4 9 3.3 IGNEOUS LITHOGEOCHEMISTRY 56 3.3.1 M E T H O D O L O G Y 56 3.3.2 TECTONIC AFFINITY 60 3.3.3 G E O C H E M I C A L CLASSIFICATION OF UNITS 66 3.3.3.1 MAFIC ROCKS 67 3.3.3.2 FELSIC ROCKS 6 7 3.3.4 IGNEOUS ROCK-FORMING PROCESSES 73 3.3.4.1 MAFIC ROCKS 75 3.3.4.2 FELSIC ROCKS 77 3.4 DISCUSSION 85 CHAPTER 4. ALTERATION 88 4.1 INTRODUCTION 88 4.2 DISTRIBUTION OF ALTERATION 88 4.3 CHARACTERIZATION OF HYDROTHERMAL ALTERATION 89 4.4 QUANTIFICATION OF ALTERATION PROCESSES 93 4.4.1 IZAWA A L T E R A T I O N DISCRIMINATION D I A G R A M 93 4.4.2 PER A N A L Y S I S 96 4.4.3 MASS C H A N G E C A L C U L A T I O N S 99 4.4.3.1 METHODOLOGY 99 4.4.3.2 MASS CHANGE CALCULA 'HONS FOR FELSIC ROCKS 102 4.4 SUMMARY 115 CHAPTER 5. MINERALIZATION 117 5.1 INTRODUCTION 117 5.2 MINERALIZATION OF THE PIT AREA 117 5.3 MINERALIZATION OF THE VENT ZONE 122 5.4 MINERALIZATION OF THE FLEETWOOD ZONE 124 5.5 DISCUSSION 129 VI CHAPTER 6. DISCUSSION AND CONCLUSIONS 131 6.1 INTRODUCTION 131 6.2 STRATIGRAPHY AND GEOCHEMISTRY 131 6.2.1 STRATIGRAPHIC SUBDIVISIONS 131 6.2.2 FACIES INTERPRETATIONS 132 6.2.3 GEOCHEMISTRY OF T H E V O L C A N I C SEQUENCE A T SENECA 133 6.2.4 COMPARISONS OF THE SENECA V O L C A N I C SEQUENCE WITH M O D E R N SETTINGS 134 Lcrn Basin, southwest Pacific 134 Puertocitios Volcanic Province, northeastern Baja California 139 Medicine Lake Volcanic Center, northern California 140 Hokuroku Basin, northern Honshu. Japan 143 6.2.5 S U M M A R Y 145 6.3 ALTERATION AND MINERALIZATION 146 6.4 CONCLUSIONS 151 Volcanic Stratigraphy and Facies Distribution 151 Geochemistry 153 Alteration 153 Mineralization 154 6.5 CLOSING REMARKS 155 REFERENCES 157 APPENDIX A - LITHOGEOCHEMICAL DATA 163 APPENDIX B - MASS CHANGE CALCULATIONS 179 APPENDIX C - ESTIMATES OF PRECISION AND ACCURACY 185 Vll LIST OF TABLES Table 2.1. Summary of the major volcanic facies at the Seneca property 38 Table 3.1. Geochemical composition of least altered samples 58 Table 3.2. Mineral/melt partition coefficients for Zr, Y and Ti for andesitic to rhyolitic melts 80 Table 3.3. Summary of results of fractional crystallization modelling 84 Table 4.1. Summary of sericitization index calculations 97 Table 4.2. Mass change calculations for strongly altered felsic rocks 103 Table 5.1. Summary of typical assay values for selected mineralized intervals in the Pit Area 120 Table 5.2. Summary of selected assay data from the main Fleetwood Zone stockwork 124 Appendix A. 1. Chemical composition of felsic volcanic rocks at Seneca 165 Appendix A.2. Chemical composition of mafic volcanic rocks at Seneca 172 Appendix A.3. Chemical composition of volcaniclastic rocks at Seneca 174 Appendix B. Mass change calculations for major elements 18 1 Appendix C. 1. Analyses of in-house standards 188 Appendix C.2. Analyses of duplicate samples 193 VIII LIST OF FIGURES Figure 1.1. Location map of the Seneca deposit 2 Figure 1.2. Map of the Seneca property 3 Figure 2.1. Geological cross-sections through the Seneca property 10 Figure 2.2. Simplified stratigraphic cross-section through the Fleetwood Zone 1 1 Figure 2.3. Simplified stratigraphic cross-section through the Vent Zone 12 Figure 2.4. Simplified stratigraphic cross-section through the Pit Area 13 Figure 3.1. A F M plot of least altered volcanic rocks at Seneca 61 Figure 3.2. Zr-Ti-Y and Zr-Ti discrimination plots for basalts and basaltic andesites 62 Figure 3.3. TiO 2-(Mn*10)-(P 2O 5* 10) geochemical discrimination plot 63 Figure 3.4. Chondrite normalized rare earth element plots for felsic and mafic rocks 64 Figure 3.5. Zr/Ti02 vs. S i0 2 geochemical discrimination plot 68 Figure 3.6. Harker-type variation diagrams for major elements vs. S i0 2 69 Figure 3.7. Harker-type variation diagrams for selected trace elements vs. S i 0 2 70 Figure 3.8. Binary geochemical plots of ALOj vs. T i 0 2 and S i0 2 vs. T i 0 2 71 Figure 3.9. T i 0 2 vs. Zr and Y vs. Zr plots for least altered felsic data 72 Figure 3.10. Pearce element ratio plots for mafic samples 76 Figure 3.11. Pearce element ratio plots for least altered felsic samples 78 Figure 4.1. Izawa alteration discrimination diagram for felsic rocks 94 Figure 4.2. (2Ca+Na+K)/Zr PER vs. Al/Zr PER plot of felsic data 95 Figure 4.3. A1 2 0 3 vs. T i 0 2 immobile element binary of for felsic data 100 Figure 4.4. Mobile-immobile element plots of least altered felsic data 106 Figure 4.5. Mass change diagrams for all felsic data 107 Figure 4.6. Summary of mass changes for most strongly altered stockwork samples 109 Figure 4.7. Downhole mass changes for drillholes 91-18 and 91-18 110 Figure 4.8. Downhole mass changes for drillholes 91-10 and 86-13 1 1 1 Figure 5.1. Mineral paragenesis diagram for the massive sulphides in the Pit Area 122 Figure 5.2. Graphic geological log of drillhole 91-16 highlighting the stockwork zone 125 Figure 6.1. Variations of silica content time for volcanic glasses, Tonga Platform, SW Pacific 136 Figure 6.2. REE patterns of rocks from Seneca and Miocene volcanic suites (Lau Basin & Baja) 138 Figure 6.3. REE patterns of volcanic rocks from Seneca and Medicine Lake, California 14 1 Figure 6.4. REE patterns of volcanic rocks from Seneca and Hokuroku Basin, Japan 144 Figure 6.5a. Schematic illustration of the early stages in the formation of the Seneca sequence 147 Figure 6.5b. Schematic illustration of the later stages in the formation of the Seneca sequence 148 Figure 6.6. Schematic model for the formation of hydrothermally altered stockwork zones 149 Figure A. 1. Location map of sampled drillholes 165 Figure C. 1. Measured vs. accepted element concentrations for MDRUstandard ALB-1 190 Figure C.2. Measured vs. accepted element concentrations for MDRUstandard P-l 191 Figure C.3. Measured vs. accepted element concentrations for MDRUstandard QGRM-100 192 ix Figure C.4. Plots of major element concentrations for duplicate samples 194 Figure C.5. Plots of selected trace element concentrations for duplicate samples 195 X LIST OF PLATES Plate 2.1a: Footwall Interval - basaltic lava erupted subaqueously by lava fountaining 14 Plate 2. Ib: Seneca Horizon - coarse grained, debris flows 14 Plate 2.2a: Seneca Horizon - Ore zone conglomerate (OZC) 15 Plate 2.2b: Seneca Horizon - basaltic andesite intrusion 15 Plate 2.3a: Hangingwall Interval - Rhyodacite flows 16 Plate 2.3b: Hangingwall Interval - Volcaniclastic siltstone and sandstone 16 Plate 2.4a: Basaltic lava - 'fire fountain debris' 21 Plate 2.4b: Volcaniclastic turbidites 21 Plate 2.5a: Block lapilli tuff unit ( ' B L T ) 26 Plate 2.5b: Coarse felsic fragmental units 26 Plate 2.6a: Pumiceous beds 30 Plate 2.6b: Peperites - sill-sediment contact interactions 30 Plate 2.7: Drillcore samples of synvolcanic intrusions 32 Plate 2.8a: Feldspar-porphyritic (FP) intrusions 33 Plate 2.8b: Quartz-feldspar-porphyritic (QFP) intrusions 33 Plate 3.1a: Photomicrograph of basaltic lava 41 Plate 3.1b: Photomicrograph of vesicular glassy rhyolitic lava 41 Plate 3.2a: Photomicrograph of classic perlite in rhyodacitic glass 43 Plate 3.2b: Photomicrograph of banded perlite in rhyodacitic flow 43 Plate 3.3a: Photomicrograph of crystal-rich volcaniclastic sandstone 45 Plate 3.3b: Photomicrograph of fiamme in coarse grained debris flow 45 Plate 3.4a: Photomicrograph of heterolithic breccia 46 Plate 3.4b: Photomicrograph of bedding contact between reworked basaltic hyaloclastite 46 Plate 3.5: Photomicrographs illustrating the typical glassy fragmental textures of ashes 47 Plate 3.6a: Photomicrograph of volcaniclastic siltstone/ash 48 Plate 3.6b: Photomicrograph of pyritic lamination in volcaniclastic ash 48 Plate 3.7a: Photomicrograph of basaltic andesite 50 Plate 3.7b: Photomicrograph of peperitic basaltic'andesite sill 50 Plate 3.8a: Photomicrograph o f typical FP dacite intrusion 5 1 Plate 3.8b: Photomicrograph of rhyodacite FP intrusion 5 I Plate 3.9a: Photomicrograph of spherulitic groundmass of a Q F P intrusion 53 Plate 3.9b: Photomicrograph of Q F P rhyolite intrusion 53 Plate 3.10: Photomicrograph of group A felsic unit 54 Plate 3.11a: Photomicrograph of group C felsic unit 55 Plate 3.11b: Photomicrograph of group D felsic unit 55 Plate 4. l a : Drillcore sample o f intensely silicified and sericitized rhyodacite breccia 90 Plate 4.1b: Altered and mineralized basaltic fire fountain debris 90 Plate 4.2a: Photomicrograph of a moderately altered FP rhyodacite 91 xi Plate 4.2b: Photomicrograph o f an intensely hydrothennally altered FP rhyodacite 91 Plate 4.3a: Photomicrograph of a strongly quartz-sericite altered felsic rock 1 12 Plate 4.3b: Photomicrograph of a strongly quartz-sericite altered felsic rock 1 12 Plate 4.4a: Photomicrograph of a strongly sericite-chlorite-quartz altered felsic rock I 13 Plate 4.4b: Photomicrograph of a strongly sericite chlorite altered felsic rock 1 13 Plate 5.1a: Representative samples of the mineralized ore zone conglomerate (OZC) 1 18 Plate 5.1b: Photomicrograph of massive sulphides and barite in the upper part of the O Z C 118 Plate 5.2a: Photomicrograph of massive sulphides and barite from O Z C 1 19 Plate 5.2b: Photomicrograph of Sample 85-03-104 massive sulphides and barite 119 Plate 5.3a: Grab sample from the Vent Zone stockwork outcrop 123 Plate 5.3b: Dril lcore samples from D D H 86-28 in the Vent Zone 123 Plate 5.4: Representative drillcore samples from the Fleetwood Zone stockwork 126 Plate 5.5a: Mineralized flow breccia from a Fleetwood Zone stockwork ( D D H 91-18) 127 Plate 5.5b: 33-Zone massive sulphides 127 ACKNOWLEDGMENTS I would like to thank a number of people who have contributed their time and personal expertise to the completion of this study. Dr. John Thompson provided excellent supervision and guidance over the course of the project and was extremely helpful in keeping the project focussed without causing me any undue stress. (John's assistance on the soccer field was also greatly appreciated.) I cannot thank T im Barrett enough for his help during this study. His expert insights, friendly attitude and willingness to discuss at whatever length required virtually any subject from volcanology and lithogeochemistry to baseball, hockey (Go Habs Go !) and Phylum Cephalopoda, interspersed with the occasional attempt to play snooker made this a much more pleasurable project to complete. I also wish to thank Dr. Kelly Russell and Dr. Alastair Sinclair for their guidance. Dr. Russell 's edits greatly improved the validity and flow of the text. Rod Allen provided excellent input on volcanic textures and facies interpretations. Julia Matsubara provided assistance in the field. I would also like to thank other people at M D R U and U B C - Ross Sherlock, Rob Macdonald, Chris Sebert, Sonya Tietjen, Arne Toma, Fiona Childe and Brian Mahoney - for helpful discussions, input, and general assistance during the last couple of years. Inmet Min ing Corp. (formerly, Metall Mining), and in particular Col in Burge and Gary Wells, supplied maps, sections and access to drillcore at the Seneca property which provided the groundwork for the project. Funding for this study was provided by the Mineral Deposit Research Unit ( M D R U ) as part of the Volcanogenic Massive Sulphide Deposits of the Cordillera project. Financial support was given to M D R U by eleven member companies, the Science Council of British Columbia and by a Natural Science and Engineering Research Council ( N S E R C ) grant. I would also like to thank a number of people who provided moral support and kept me relatively sane during the course of this project. I thank my family members - Mom, Dad, Kelly and Conor - for providing constant support and encouragement. Thanks also to all my friends - Rob, Theresa, Leslie, Gwyn, Shannon, Hamish and Bil l to name a few - for their support, sympathetic ears and general entertainment. xi i i DEDICATION This thesis is dedicated to the memory of Johanna Marie Goldthorpe, my best friend and confidante and fellow geologist, who tragically passed away in September, 1992. Johanna's support and confidence in me provided me with the incentive to pursue my Master's degree and without which 1 may not have undertaken this project.' Memories of Johanna's love of life and cheerful outlook have been a constant source of inspiration. I wil l remember you always. 1 C H A P T E R 1 INTRODUCTION The Seneca property in southwestern British Columbia is located approximately 120 kilometres east of Vancouver (Figure 1.1). The property is accessible from the Lougheed Highway at Harrison Mi l l s by the Morris Valley Road and the Chehalis-Fleetwood logging road. The property has been described as a zinc-copper-lead-barite volcanogenic massive sulphide environment similar to the Kuroko-type deposits (Urabe et al. , 1983). Current geological reserves are estimated at 1.5 million tonnes grading 3.57% zinc, 0.63% copper and 0.15% lead (Hoy, 1991). Mineralization occurs as massive and matrix-filling sulphides associated with volcaniclastic sediments and as stockwork-style stringer and massive sulphides hosted in a sequence of felsic to intermediate volcanic rocks of the Harrison Lake Formation (McKinley et al. , 1995). 1.1 S C O P E O F S T U D Y This study constrains the spatial, temporal and geochemical relationships of the lithological units in the study area, the mineralization and the accompanying alteration within the volcanic sequence that hosts the Seneca deposit. Surface exposure on the property is poor and diamond drillcores were used almost entirely to infer relationships and the distributions of the various volcanic facies. The distribution of drillholes on the property is sufficient to allow adequate observation of vertical and lateral facies changes. However, the use of drillcores allows only limited interpretation of structural elements, namely the strikes of faults, dikes and bedded volcaniclastic units. The varying depths of the drillholes in different parts of the property also restricted the study to a 200 to 450 m thick portion of the volcanic sequence. Forty drillholes were logged in detail, and on the basis of igneous textures and contact relationships, extrusive lavas and synvolcanic intrusions were differentiated; clast size and 2 Figure 1.1. Location map of the Seneca deposit showing a simplified geology of the area west of Harrison Lake (modified after Thompson, 1972; geology modified after Journeay and Csontos, 1989). 3 Trace of cross-section of Fleetwood-Vent Zones (Fig. 2.1a) \ D r i l l h d l e 91-16 y s \ F L E E T W O O D Z O N E "0briHh6leN92-3e % 'o^  \ Drillhole 91-06 Om 400m 800m 1200m C V E N T Z O N E : / - v Trace of the cross-section of the Pit Area (Fig. 2.1b) sCORE SHACK . ; ^ Dri l lhole 85-03 Open Pit X N o Drillhole 74-36 PIT A R E A Trough Zone -Spy Figure 1.2. Map of the Seneca property showing the locations of the different zones described in the study and the surface traces of the geological cross-sections shown in Figure 2.1. 4 compositional variations among the volcaniclastic units were used to track volcano-sedimentary facies changes. Major, trace and rare earth element geochemical data are used in this study to infer the tectonic affinity, possible genetic relationships and the effects of hydrothermal alteration among the volcanic rocks in the study area. 1.2 TERMINOLOGY The volcanic stratigraphy at the Seneca property, like other volcanic belts, is quite complex and varied. Since there is such a wide variety o f different lithologies present, naming the units by simple conventional classification schemes, such as that of Fisher (I966)( does not do justice to the many compositional and textural variations observed. This is especially true for the volcaniclastic rocks which show extreme variation As such the volcanic rocks in this study are described using non-genetic, yet descriptive, terminology and are interpreted using the volcanic : facies architecture' approach as outlined by Allen (1993) and by McPhie and Allen (1992). The term 'vent' is used in this study to refer to a volcanic centre or feeder. However, the same term can also be used to describe a centre for hydrothermal activity; this is the case for origin of the name of the Vent Zone at Seneca. When describing facies relationships, however, terms such as vent-proximal are in reference to a volcanic vent. The Seneca property is subdivided into four different areas. From northwest to southeast, these different zones have been named the Fleetwood Zone, the Vent Zone, the Pit Area and the Trough Zone (Figure 1.2) each of which comprise different combinations of lithologies and volcanic facies and styles of mineralization and alteration. 1.3 PROPERTY HISTORY The Seneca Prospect, formerly known as the Lucky Jim property, was discovered in 195 I as an indirect result of logging operations and was optioned by Noranda Exploration Company at that time (Thompson, 1972). The sulphide mineralization was believed to be part of a steeply dipping vein or shear system. In 1961 stripping, trenching, and some underground work were carried out, but the results were not encouraging. The property was held by Noland Mines, Ltd. from 1964 to 1965 and was bought by Zenith Mining Corporation, Ltd. in 1969. Cominco Ltd. optioned the property in 1971 and carried out further exploration based on the concept that the zone represented Kuroko-style conformable mineralization. The property was acquired by Chevron Standard Ltd. in 1977 and further diamond drilling was completed over the next ten years in joint ventures with International Curator Resources Ltd. and B P Canada Inc. Further logging in the area indirectly led to the discovery in 1986 of the Vent zone stockwork mineralization 1.75 kilometres to the west of the original discover)'. In 1991 drilling by Minnova, Inc. (subsequently Metall Mining Corp., and now l.nmet Mining Corp.), I kilometre to the west of the Vent zone, led to the discovery of the Fleetwood zone. In 1992, further drilling in that area intersected the 33-zone massive sulphides. The property is currently held by International Curator Resources Ltd. of Vancouver. British Columbia. 1.4 REGIONAL GEOLOGY 1.4.1 H A R R I S O N T E R R A N E The Harrison Terrane on the west side of Harrison Lake is a sequence of Triassic to Cretaceous volcanic and sedimentary rocks adjacent to Upper Jurassic quartz diorite batholiths situated to the west of the property. The terrane is bounded to the east by the Harrison Fault, a major structural feature juxtaposing the highly deformed and metamorphosed rocks east of the fault with the relatively 6 undeformed rocks west of Harrison Lake (Arthur, 1986). Arthur et al. (1993) suggest that the Middle Jurassic volcanic rocks of this belt are correlative with the Wells Creek volcanics of Washington and perhaps the Bowen Island Group to the west near Vancouver. 1.4.2 H A R R I S O N L A K E F O R M A T I O N The Harrison Formation is a Lower to Middle Jurassic succession that strikes north-northwest, with gentle to moderate easterly dips and which may be up to 2500 metres thick (Monger, 1970; Mahoney, 1994). From oldest to youngest, the Harrison Lake Formation is composed of the Cel ia Cove Member, the Francis Lake Member, the Weaver Lake Member and the Echo Island Member (Arthur, 1986; Mahoney. 1994). This sequence is overlain by Upper Jurassic to Lower Cretaceous conglomerates of the Fire Lake section (Figure I.I; Journeay and Csontos, 1989). Mahoney et al. (1995) describe the Harrison Lake Formation as a volcanic arc having a medium to high-K, calc-alkaline affinity and having Nd and Sr isotope values which suggest a relatively juvenile magmatic system derived from a slightly enriched mantle wedge. The Cel ia Cove Member comprises mostly deep water sedimentary rocks unconformably overlying the Middle Triassic age argillites, sandstones, tuffs and volcanic flows of the Camp Cove Formation. Arthur at al. (1993) propose that these rocks formed part of the western margin of a Triassic ocean basin represented by the Bridge River-Hozameen assemblage to the east. Early and Late Toarcian ammonites have been collected from a calcareous argillite above and below a volcanic flow within the Francis Lake Member (Arthur, 1986). This flow may represent the onset of the volcanism that characterizes the Harrison Lake Formation. Regionally the Weaver Lake Member is dominated by intermediate to felsic volcanic rocks and related intaisions (Mahoney, 1994). This voluminous unit covers an area of approximately 10 km from east to west (from Harrison Lake in the east to the Chehalis River valley in the west) and 15 km north to south (from Mount McRae in the north to the Harrison River in the south). The composition of the flows and related breccias and tuffs van ' from basalt and basaltic andesite to rhyodacite. Volcaniclastic rocks are plentiful and van ' from fine ashes to volcanic breccias. Mahoney (1994) observed an overall change upwards from mafic-dominated volcanic rocks lower in the sequence to felsic-dominated volcanic rocks in the upper parts of the sequence. Although not fully constrained, the predominance of felsic rocks at Seneca property suggests that it is situated within the upper part of the Weaver Lake Member. The Echo Island Member, which outcrops to the north and south of the Weaver Lake Member and on Echo Island in Harrison Lake, comprises mostly finely-banded volcaniclastic sediments and argillites with minor plagioclase-porphyritic flows (Arthur, 1986; Mahoney, 1994). Callovian ammonites in the overlying Mysterious Creek Formation suggest that the Echo Island Member may be Bajocian or Bathonian (Arthur, 1986) 1.4.3 G E O C H R O N O L O G Y A sample of a rhyolite dome complex from the upper part of the Weaver Lake Member on Echo Island was analysed using U-Pb zircon dating and has an estimated age of 166 ± 0.4 M a (Mahoney et al., 1995). A sample of a quartz feldspar porphyry stock was dated by U-Pb at 165 9 +6.4/-0.3 M a (Monger, 1987, as cited by Mahoney et al., 1995). The Harrison Lake Formation is intruded by the Mt . Jasper pluton, a tonalite/granodiorite intrusion which yields a 167 ± 4 M a U-Pb zircon age (Friedman and Armstrong, 1994). Mahoney et al. (1995) report that the rhyolite dome complex from Echo Island and the quartz feldspar stock have similar Nd and Sr isotopic values which, while also considering their similar ages, suggests that they may be comagmatic and that the granitoid rocks are subvolcanic roots to the Middle Jurassic volcanic rocks of the Harrison Lake Formation. 8 1.4.4 S T R U C T U R E A N D M E T A M O R P H I S M Structural interpretations are limited by poor outcrop in the study area. The Harrison Lake Formation is deformed, forming a broad, west-northwest trending, shallowly plunging anticline (Pearson, 1973; Arthur, 1987; Mahoney et al., 1995). The structure in the area around the south slope of Mount Keenan, which includes the study area, is described as a southwestward-dipping homocline (Thompson, 1977). The contrasting structural styles between the Harrison Lake Formation and the overlying Mysterious Creek Formation suggests a compressional event in the region in Early Bathonian to Early Callovian time (ca. 165 to 160 M a ; Mahoney et al., 1995). Penetrative fabrics are absent in the study area and throughout the entire Harrison Lake Formation except for a narrow zone adjacent to the Harrison Fault (Thompson, 1972; Journeay and Csontos, 1989). There is little evidence of metamorphism of the rocks in the study area. Zeolites are present in some of the flows at Seneca, but it is unclear if they are simply amygdules or the result of low grade metamorphic recrystallization. Journeay and Csontos (1989) describe the metamorphism of the entire Harrison Lake Formation as sub-greenschist facies. 9 C H A P T E R 2 G E O L O G Y OF T H E SENECA DEPOSIT 2.1 INTRODUCTION The major lithologic units at Seneca are subdivided on the basis of their proximity to their primary source, which is in this case a volcanic centre or vent(s). The four principal volcanic facies are described as follows: • Facies 1 - Vent to vent-proximal facies: Lavas consisting of basaltic to rhyolitic composition flows, domes and associated in situ hyaloclastites and autoclastic breccias; • Facies 2 - Vent-proximal to distal facies: Volcaniclastic rocks consist of juvenile to reworked coarse volcanic breccias and tuffs to fine grained siltstones and ashes; and • Facies 3 - Coeval Intrusions: Intrusions consist of basaltic to rhyolitic composition sills and dykes that have intmded lavas and wet volcaniclastic sediments. • Facies 4 - Distal Marine. Rocks of volcano-sedimentary origin consisting of an argillite that often contains flattened feldspar (± quartz)-phyric pumice clasts (fiamme). Facies 1 to 3 are generally observed in all drillholes, but their relative abundances vary greatly from hole to hole, often over small distances, thus making correlation on the basis of facies difficult. The fourth facies is often in close proximity to mineralization and is spatially restricted to the Pit Area and the Trough zone and does not correlate to other parts of the property. Observations in drillcore suggest that strata on the property strike approximately to the northwest and are essentially flat lying or moderately dipping in an easterly direction. 12 CO _ cu ^ g " GO 3 J o > cu ^ o :o^ f :o!>-fmf ro; o o Lfi O i - C\J ST o in co o o C\ l I Q a 1^  E o T o o o •IS o in co 8 § ° O n rt „ o C3 — o o X£ -a -a i> —. as ' -t—> a> o 00 .5 T3 * 3 • O «i VI CO o N fi ' cn fl co > « cu Cl ^1 "ob'o hi P< .2 15 •« fi 2^ O CU ?5 CO •S's rt "-3 j_ eg CO cu •° £ cu O « ° i" ^  s .3 to, t<-> cu S3 1 0 3 fl fi - ° § g ' C co oj) co eu ' £ T3 £ 13 CO S" o _u o -g l l l l l i i i i i i B o Q< Q . 00 OJ s . •a fi £ oj cd •C bbJj 2 « S o 2 « ttio C •a « a o •a 3 S 'g </> .2 o 3 o 03 ,—i <? E «) ^ O M • a l a 1 •"-> -D oo oo 00 O -rt a O U M rt *3 fli U 3 J -S 5 ^ &> fi | « * s <U PL, H 1*1 CX oo . S 6 " 00 y cu «> § i veNg s -a . 2 14 Plate 2.1a: Footwall Interval - basaltic lava erupted subaqueously by lava fountaining (Sample 94-FF-01). This rock consists of amoeboid lava clasts with lighter green (chloritized) chilled margins in a matrix of chloritized hyaloclastite which has spalled off the clasts by quench fragmentation. Sample was collected at Morris Creek to the southeast of the Seneca property. Plate 2.1b: Seneca Horizon - coarse grained, debris flows. These rocks are associated with mineralized zones and are dominated by poorly sorted dacitic and rhyolitic lava clasts (often variably altered), vitric clasts (dark green) and sandy material. Mafic clasts are sometimes present. 15 Plate 2.2a: Seneca Horizon - Ore zone conglomerate (OZC). This unit consists of subangular to subrounded felsic clasts of sand to pebble size that have been strongly sericitized and silicified. Tins unit hosts disseminated matrix-filling sulphides to massive mineralization (pyrite-sphalerite-chalcopyrite-galena). Plate 2.2b: Seneca Horizon - basaltic andesite intrusion. These rocks are often associated with the O Z C in the Pit Area. They are intruded as narrow sills that often display peperitic textures as shown in tiiese photos, and incorporate clasts of the units they intrude. 16 Plate 2.3a: Hangingwall Interval - Rhyodacite flows. This unit is common in the Fleetwood and Vent Zones. The rocks consist of massive to autobrecciated feldspar±quartz-phyric lavas often forming thick flow-flow breccia and flow-dome sequences up to 100 metres thick. Flow banding is accentuated by variable silicification and chloritization. Samples are from D D H 87-11 east of the Vent Zone Plate 2.3b: Hangingwall Interval - Volcaniclastic siltstone and sandstone (DDH 85-03). The sample to the left consists of massive volcanic ash; the centre sample is well-bedded crystal-rich volcanic sandstone; the sample to the right is a massive bed of vesicular dacite lava clasts and hyaloclastite. These units were deposited by gravity settling and as turbidites derived from the flanks of a volcanic edifice or from pyroclastic eruptions. 17 2.2 D I S T R I B U T I O N O F V O L C A N I C F A C I E S Figure 2.1 illustrates the distribution of the three principal facies along two northwest to southeast trending longitudinal sections across the property. The sections are continuous except for a 500 m separation where there is no drillhole coverage. Distinct marker units are not evident, but at least three packages of distinct lithologies and facies are identified at specific stratigraphic and are correlatable across the Seneca property (Figures 2.2 to 2.4). Each of these horizons comprises varying proportions of all three volcanic facies, but each horizon has a particular unit, or sub-facies, or a facies association making it unique. The three horizons are named on the basis of their positions relative to the mineralized zones and are, from stratigraphically lowest to highest: the Footwall Interval, the Seneca Horizon and the Hangingwall Interval. 2.2.1 F O O T W A L L I N T E R V A L The Footwall Interval is characterized by the presence of mafic lavas and breccias (Plate 2.1a), or by the presence of coarse volcaniclastic units having a mafic component that has been derived from these mafic lavas. Synvolcanic sills and dikes (Facies 3) of dacitic to rhyolitic composition are common. Felsic flows and volcaniclastic units are also present, but are much less common than the other units. Due to its position stratigraphically below the main mineralized zones, the Footwall Interval hosts some minor mineralization in the form of disseminated and stringer sulphides. Localized areas of strong hydrothermal alteration are present, but overall the horizon is only weakly to moderately altered. The Footwall Interval is best exposed in drillcores from the Fleetwood and Vent Zones.which generally penetrate deeper into the sequence than drillcores from the Pit Area. True extrusive mafic lavas, or 'fire fountain debris', distinguished by their hydroclastic fragmental textures, were observed in the lowermost portions of drillcores in the Fleetwood and Vent Zones, but were not observed in the 18 Pit Area. An outcrop of the texturally distinct fire fountain debris exists several kilometres to the southeast of the Seneca property at a stratigraphically similar position suggesting that the mafic lavas may be a regionally extensive unit. The last, or uppermost, occurrence of this mafic unit marks the upper boundary of the Footwall Interval. 2.2.2 SENECA HORIZON The Seneca Horizon hosts all major sulphide mineralized zones on the property. It is thinner and more discontinuous than the two other horizons and is comprised of felsic flows and flow breccias, poorly-bedded, coarse grained, felsic-dominated volcaniclastic units as well as felsic, and lesser mafic, syiivolcanic intrusions. The base of the horizon is commonly marked by a moderately to poorly sorted volcanic breccia or debris flow unit dominated by angular to subround feldspar-phyric clasts and lesser mafic clasts (Plate 2 .1b). Mafic lavas are uncommon in this horizon and coarse-grained volcaniclastic debris flows are more prevalent than well-bedded volcaniclastic siltstones and turbidites. The Seneca Horizon includes the sequence of rocks overlying the uppermost occurrence of mafic lavas and which underlies the lowermost occurrence of dominantly fine-grained volcaniclastic units. Alternatively, the Seneca Horizon is marked by the presence of stockwork and semi-massive sulphides (pyrite-sphalerite-chalcopyrite±galena) and associated zones of strong silica and sericite alteration. The former criteria are used when mineralization or strong alteration are absent. In the Pit Area the Seneca Horizon is characterized by a coarse volcaniclastic unit, termed the ore zone conglomerate (OZC). This unit is texturally distinct from other units. It contains rounded and subrounded felsic clasts, is generally moderately to strongly hydrothermally altered and hosts disseminated to semi-massive sulphides (Plate 2.2a). The OZC is interpreted to be a mass flow unit having a discontinuous sheet-like, or possibly channelized moiphology (R. Allen, personal communication). This unit is not present in the Fleetwood and Vent Zones to the northwest, but is 19 considered part of the Seneca Horizon because of its similar stratigraphic position to the other mineralized zones and its similar relationship with bounding lithologies; the O Z C is underlain by felsic volcanic breccias and is overlain by the first, or lowermost, occurrence of well-bedded, finer-grained volcaniclastic siltstones and turbidites. Basaltic andesite commonly intrudes the Seneca Hor izon/OZC in the Pit Area. These mafic units are interpreted to be synvolcanic sills based on their massive nature, their common peperitic margins and their lateral continuity (McPhie et al . , 1993) (Plate 2.2b). Also commonly associated with the O Z C is a thin dark brown to black argillite unit which often contains suspended clasts of rhyolitic pumice. 2.2.3 H A N G I N G W A L L I N T E R V A L The Hangingwall Interval comprises essentially all units stratigraphically overlying the mineralized zones of the Seneca Horizon. It is composed almost entirely of dacitic to rhyolitic rocks. Vent to vent-proximal flows and breccias are common in the Fleetwood and Vent Zones (Plate 2.3a); portions of the stockwork zones in these areas are hosted by distinctly banded and brecciated felsic flows. Synvolcanic sills are more prevalent than flows in the Pit Area. The distinctive lithologies common to all areas, however, are well-bedded volcaniclastic turbidites and massive to bedded and laminated gravity-settled volcaniclastic sandstones to siltstones (Plate 2.3b). These units are intercalated with the felsic flows in the Fleetwood-Vent Zones and are intruded by the synvolcanic sills in the Pit Area. Coarser-grained fragmental units are present in the Hangingwall Interval, but they tend to be subordinate to the fine-grained material. This horizon is essentially unaltered and unmineralized, in sharp contrast to the underlying Seneca Horizon. Trough Zone Dril lhole 91-03 (Figure 2.3) located 1.5 km south of the Pit Area intersects a 150 m thick sequence of uninterrupted Facies 2 volcaniclastic beds - the most continuous section of distal facies 20 rocks observed at Seneca (Plate 2.4b). The lowermost 75 to 80 m of the drillcore consists of massive, imbedded, but internally normally-graded volcaniclastic sandstone. Bedding was not observed within this interval. Overlying this is a sequence of well-bedded and laminated volcaniclastic turbidites. Some of the fine grained ash/siltstone beds contain suspended 'rafts' of quartz-feldspar-phyric pumice up to 10 cm in diameter, and are intercalated with quartz-feldspar crystal-rich beds. This sequence also contains several thin argillaceous beds. Unlike all other drillcores examined, D D H 91-03 is unique in that no flows or synvolcanic intrusions were intersected. Despite its distance from drillholes in the Pit Area, D D H 91-03 has some notable similarities to parts of the Pit Area stratigraphy. The volcaniclastic units in both areas display an upward transition from poorly-bedded to well-bedded material and show an overall fining-upward trend. Pumiceous clasts and argillaceous beds are also more common in the upper sections of each area. Although no mineralization or hydrothermal alteration was observed in the Trough Zone, it appears to be roughly correlatable with Seneca-Hangingwall Interval observed in the Pit Area drillcores. 2.3 D E S C R I P T I O N O F V O L C A N I C F A C I E S 2.3.1 L A V A S Lava flows are defined by their contact relationships and the occurrence of flow textures and autobrecciation at their margins. 2.3.1.1 MAFIC LAVAS Unbrecciated basaltic andesites at Seneca tend to be featureless and without pillow forms; the occurrence of extrusive mafic flows has not been confirmed. However, massive coherent mafic flows are inferred by the presence of stretched amygdules and their association with accompanying autoclastic breccia. A unit that consists of subrounded to amoeboid fragments of vesicular basalt 21 Plate 2.4a: Basaltic lava - "fire fountain debris'. This unit formed as lava 'fountained' into the water column. This is inferred by the amoeboid-shaped fragments, chi l led margins on the clasts and the abundance of quench-derived hyaloclastite in the matrix (dark green). (Large divisions on scale are 1 cm). Plate 2.4b: Volcaniclast ic turbidites. These samples from left to right represent a top to bottom sequence from D D H 91-03 in the Trough Zone (distal facies). The sample on the right is part of a thick massive, imbedded interval of volcanic sand and ash likely representing a single continuous eruptive event. The three samples to the left are from normal graded beds and are feldspar and quartz crystal-rich. These upper units likely represent the reworking of volcanic debris as turbidites during the waning and post-eruption stages of volcanic activity at that time. The left sample contains silty rip-ups from the underlying bed. 22 surrounded by angular lava clasts and hyaloclastite (cf. Dimroth et a l . , 1978) occurs in the western part of the property (Plate 2.4a). These fragments are typically 1 to 10 cm in size, are light green or purplish grey in colour and consist of a core of massive basaltic andesite lava with chilled or brecciated rims. The textures of the fragments together with their amoeboid shape and tail-like ends, suggests that they were ejected in a subaqueous environment (Carlisle, 1963; R. Al len, personal communication). However, the basaltic lava clasts often have irregular shapes and lack the tapered or teardrop-shaped morphologies of molten subaerial ejecta. This unit must be proximal to a vent, as the surrounding angular hyaloclastite has not been greatly reworked. This facies is termed 'fire fountain debris' and is only seen in lower parts of the drillholes in the Fleetwood and Vent Zones. It also outcrops several kilometres southeast of the property beside Morris Lake (sample 94-FF-0I) and as large boulders in Sakwi Creek along the Hemlock Valley Road. The drillcores examined in this study generally do not penetrate below this mafic unit and as such the true thickness of the unit has not been determined; observed intersections of the unit indicate a thickness of at least 50 m. Infrequently the upper parts of the unit contain scattered felsic lava clasts indicating that at least a small amount of mixing has occurred between the mafic breccias and adjacent or overlying felsic fragmental units. 2.3.1.2 FELSIC LA VA FLO WS A ND DOMES Dacitic to rhyolitic composition flows and flow domes are more widespread than mafic flows at the Seneca property. • The domes and flows are intruded into and extruded through other lavas, fine-grained volcaniclastic sediments and coarse lava clast breccias (Figure 2.1). Individual felsic flows range from one to several tens of metres thick, and have flow brecciated upper contacts and chilled and/or slightly brecciated lower contacts. The flows and breccias have generally been weakly to moderately silicified. 23 probably by syndepositional seawater interaction. The silicification often enhances the f low banding by creating alternating lighter and darker bands (Plate 2.3a), The cores of the thicker flows are typically massive or weakly to strongly flow-banded, and are light greyish green in colour. They are often accompanied by a lateral transition from autobreccia to reworked hyaloclastite and volcaniclastic sediments at the margins. Such transitions and associations of coherent lava and hyaloclastite have been documented in Canadian Archean rhyolites (de Rosen-Spence et al . , 1980) and in Miocene lavas of the Ushikiri Formation, Japan (Kano et al., 1991). The flows have often incorporated clasts of autobreccia where they have apparently flowed over older flow breccias. It appears that fragments of the autobrecciated flow carapace may have foundered into the less dense molten portion of the flow interior. In such cases the breccias appear to have a 'matrix' of lava having apparently the same composition as the clasts. The autobrecciated lava clasts themselves are often flow banded and are rotated possibly during the advancement of the flow . The uppermost portions of the flow breccias often contain angular felsic hyaloclastic fragments interstitial to the large flow-banded lava clasts. The angular nature of these clasts is indicative of quench-induced fragmentation due to the interaction of lava with cold seawater (Hanson, 1991). Where the flow tops were observed, there is some reworking of the autobreccia and hyaloclastite which created a weak normal grading aiid diffuse lamination of the finer-grained sand-size lava debris. In some instances several coherent flow-flow breccia sequences occur in a vertical succession up to 100 m thick, but less than 200 m in lateral extent, forming a dome-like morphology (cf. Al len, 1992). There can be considerable variability in the thicknesses of flow-flow breccia sequences between adjacent drillholes again suggesting a non-tabular morphology. However, discrete flows exist that are enclosed within volcaniclastic rocks. Such flows tend to be less than 10m thick, have sharp lower contacts and brecciated upper contacts and appear to be more tabular in nature. The volcaniclastic sediments immediately below the flows are often silicified, probably due to interaction between the wet 24 sediments and the hot lava (Kokelaar, 1982). Intersections in which autobrecciated layers are thicker than coherent, massive or banded layers likely represent the margins of flow dome or flow-fronts, whereas intersections in which coherent lavas predominate over autobreccias likely represent the cores of the flows and domes (McPhie et al., 1993). Such relationships between lateral and vertical dimensions and lava textures suggest that the felsic flows have both tabular and dome-like morphologies similar to those described by McPhie and Allen (1992) for the Mount Read Volcanics in western Tasmania. 2.3.2 VOLCANICLASTIC ROCKS There is a wide variety of volcanically-derived sediments and breccias on the Seneca property with clasts ranging from silt size to block size (<15 cm). These rocks show tremendous variation in textural maturity and composition. Such characteristics are important indicators of the processes of transportation and deposition which operated in the area, and of the proximity of a unit to its source. These fragmental units represent the deposition and reworking of volcanic debris derived from lava flows, domes and pyroclastic eruptions, with the probable addition of fine sediments of a more distal origin. They are subdivided into four facies: • Facies 2.1. Monolithic to heterolithic, massive to well bedded/laminated and normal density graded, moderately to poorly sorted lava clast breccias, pebble conglomerates, vitric-crystal tuffs and volcanic sandstones, interpreted to have been deposited as debris flow deposits, subaqueous pyroclastic flows and turbidites (Plates 2.4b and 2 5). • Facies 2.2. Massive to well laminated volcanic siltstone/fine ash deposited subaqueously by gravity settling. 25 • Facies 2.3. Dacitic to rhyolitic pumiceous beds often associated with argillaceous beds (Plate 2.6a). • Facies 2.4. Reworked dacitic debris flow/conglomerate interpreted to have been deposited in a fluvial or deltaic environment. • Facies 2.5. Argillaceous beds consisting of thin layers of dark brown to black fine sand to silty tuffaceous sediments which possibly correspond to more normal, non-volcanic dominated period of sedimentation. 2.3.2.1 FA C1ES 2.1: DEBRIS FLO WS. GRA IN FLO WS A ND TURBIDITIC VOLCANICLA STICS Fragmental rocks composed of sand to block-sized volcanic clasts form a large portion of the volcaniclastic facies at Seneca. The coarsest and most poorly sorted unit is termed the 'block-lapil l i tu f f (BLT ) (Plate 2.5a). This unit consists of large 2-15 cm clasts of felsic lava (FP and lesser QFP) with a matrix of sand-sized clasts of mostly felsic composition with lesser vesicular mafic lava clasts, fine grained volcaniclastic rip-ups and crystal fragments. The large clasts often have angular or cuspate edges similar to the hydroclastic breccias (Hanson, 1991), but can also be rounded; they are generally massive and unvesiculated, but can be flow-banded. Colour variations imposed by different degrees of silicification or chloritization of the clasts give the rock a more polymictic appearance although the unit usually has less than 10 % mafic clasts. These coarse grained beds are up to 10 m thick and are generally in sharp contact with overlying fine grained volcaniclastics or felsic flows and autobreccias. The coarse nature and delicate hydroclastic shapes of the felsic clasts suggests that the B L T unit is essentially locally-derived and has not undergone much transportation, however, the incorporation of clasts of varying compositions implies a certain degree of reworking. The lack of grading or well developed stratification of the unit suggests transportation of the volcanic detritus by a debris flow or subaqueous pyroclastic flow (Lowe, 1982;Fisher and Schminke, 1984; Busby-Spera, 26 Plate 2.5a: B lock lapi l l i tuff unit ( k B L T ' ) . These poorly sorted fragmental rocks were deposited as debris flows and often overlie the basaltic lavas in the lower parts of the stratigraphy. The upper sample contains flow-derived clasts and hyaloclastite. The lower samples are mostly variably altered rhyodacitic clasts, but some mafic material is present. These two rocks may have formed from slumps associated with formation of basin-bounding scarp faults. (Scale divisions = 1 cm). Plate 2.5b: Coarse felsic fragmental units. The upper sample consists of rhyodacitic lava clasts and vitric clasts perhaps derived from a subaqueous pyroclastic eruption. This unit is common in the lower portions of the Pit Area stratigraphy. The lower sample consists of in-situ hyaloclastite and is interpreted as a proximal facies (i.e. has not been reworked) to a felsic flow. 27 1988; McPhie et al . , 1993). Such debris flows were likely localized phenomena, and may have been channelized or topographically-controlled, since there is little lateral continuity to the unit. The essentially monolithologic nature of the B L T and textural similarities between the large clasts in the B L T and those of the hydroclastic and flow breccias suggest that the felsic detritus was derived from the brecciated margins of flows or emerging cryptodomes which may have slumped due to oversteepening or perhaps seismic events (Dimroth et al. , 1978; Cas et al . , 1990; McPhie et al. , 1993). The large felsic clasts may be blocks and bombs derived from a more distal, subaerial emption, but the proximity of the B L T unit to observed felsic flows favours a more local source. Better sorted, coarse volcaniclastic units are much more common. Although such units themselves are relatively poorly sorted, they do not show the extreme range of clast sizes observed in the B L T . Dense, mostly non-vesicular, feldspar-phyric clasts are the dominant fragment type in these felsic lava clast breccias (Plate 2.5b). Altered dark green vitric clasts are also common. These clasts contain similar amounts and types of phenocrysts as the lighter coloured felsic lava clasts, and thus are inferred to be quenched, glassy equivalents of the lavas. Vitr ic clasts are altered to chlorite (± sericite), and they often have an internal foliation parallel to the length of the clast which may result from the flattening of an originally vesicular and/or semi-molten pumiceous bomb. Clast sizes range from less than 1 mm to 5 cm, in general, and the beds tend to lack internal stratification or grading. Bed thicknesses vary from one to ten metres. Contacts of the beds can be either sharp or show a rapid change in grain-size to the overlying finer grained beds. The monomict nature and lack of internal stratification or grading of these coarse beds suggests that transportation of the volcanic debris was by turbulent debris flows (Walker, 1984), as opposed to gravity settling of volcanic clasts from a subaerial eruption which likely would have resulted in at least some density grading in the beds. A dacite lava breccia (Facies 2.1; Plate 2. lb) occurs stratigraphically below the O Z C in the Pit Area, and generally above the major andesitic units in the Fleetwood and Vent zones. Typical ly the 28 unit is clast supported (up to 90% clasts up to 10 cm in diameter) and consists dominantly of subangular dense fragments of feldspar-phyric dacite lava and lesser amounts of dark green vitric or pumiceous clasts, andesitic fragments and occasional silty rip-up clasts. The dacite clasts vary in colour from light grey to reddish tan, possibly representing subaerial deposition and later reworking. The unit is moderately to poorly sorted, suggesting deposition by debris flows. The true thickness of the unit is difficult to determine due to synvolcanic intaisions, but individual intersections are in the some 5 to 10 metres thick. Andesite lava breccias are less common at higher stratigraphic levels, and where they do occur, contain smaller clasts that are more rounded. Fine-grained volcaniclastic sediments (Facies 2.1 and 2.2) are common throughout the property, particularly in the upper parts of the examined stratigraphy and are volumetrically more significant than the B L T and coarse-grained, non-stratified lava clast breccias at Seneca. These units are composed mostly of three clast types: dense lava clasts, crystal fragments and pumiceous or glassy debris. Siltstone intraclasts derived from underlying beds have also been observed in the sandy beds, but are relatively rare. The relative amounts of these clast types can vary greatly between individual beds. Where discernible, non-vesicular feldspar (± quartz)-phyric lava clasts and feldspar crystals are the dominant fragment types. Crystal fragments are apparently entirely plagioclase and quartz The volcaniclastic sediments form light to dark grey beds of silt to sand-sized material. Individual beds of sand-sized detritus range from 2 cm to 5 m thick, and vary from massive to well laminated and graded. The basal contacts of normal graded beds are often sharp and are characterized by coarse sand to gravel-sized material, often with a component of dacite pumice fragments. These beds grade upward through massive or weakly laminated sands to well-laminated and occasionally cross-bedded fine sand, silt and mud. Graded beds become more common higher in the stratigraphy. Loading structures have been observed where coarser grained beds have sunk into finer silty beds. 29 Although they were observed in the upper portion of the Pit Area stratigraphy, cross-beds on the whole are rare. The well-bedded and laminated, often graded, and rhythmic nature of these volcaniclastic sand-dominated beds suggests they were deposited as discrete turbidites or grain flows (Walker, 1984; Fisher, 1984; Busby-Spera, 1988) that were generated by subaqueous eruptions and slumping due to slope oversteepening. 2.3.2.2 FACIES 2.2: VOLCANICLASTIC SILTSTONE Massive to well-laminated volcaniclastic siltstone beds are quite common at Seneca. They are distinguished by their lack of discernible macroscopic grains, and are light to dark grey in colour. These units contain glassy shards and crystal fragments that are the only features distinguishable from the massive, fine-grained matrix. Individual beds can be distinguished where there are subtle grain size or colour differences. Beds of volcanic siltstone vary from less than one to several metres in thickness, and vertically continuous series of fine grained beds reach up to 10 m thick. The massive nature of these beds suggests that they may have been deposited by gravity settling from suspension (Ledbetter and Sparks, 1979; Branney, 1991). This material likely rained down through the water column during the waning stages of eruptions. It is not clear whether the fine ash was erupted subaerially or subaqueously, but its close spatial association with the coarser volcaniclastics suggests that it may be a fine grained equivalent of the coarse detritus that was density sorted within the eaiption column. 2.3.2.3 FACIES 2.3: DACITIC AND RHYOLITIC PUMICE BEDS Pumice beds are common in the volcaniclastic sequence. The pumice clasts are feldspar (± quartz)-phyric and have an altered glassy groundmass. They occur either as bombs in fine-grained detritus or as discrete beds usually less than 1 m in thickness (Plate 2.6a). The pumice is often flattened giving the beds a flow-like texture. Although the individual beds are quite thin, they 30 Plate 2.6a: Pumiceous beds. The upper sample consists of rhyolite pumice shards and ash. Such beds are common in die volcaniclastic sequence and likely represent single, explosive eruptive events some distance from the depositional site The lower sample consists of flattened pumiceous material within a dark brown to black argillaceous bed. This unit is often associated with the OZC in the Pit Area. P . F M T 1 M F T R F . 9 Plate 2.6b: Peperites - sill-sediment contact interactions. From left to right are rhyolitic, rhyodacitic and andesitic peperite zones which form when synvolcanic sills intrude wet, unconsolidated volcaniclastic beds. The sill margins become chilled and quench brecciated while the fine sediments are fluidized and silicified. 31 punctuate virtually all parts of the upper volcaniclastic sequence in all areas. As such the pumiceous beds are useful as a basis for stratigraphic correlation in that each bed likely represents an individual eruptive event, either subaqueous or subaerial, that would distribute volcanic debris over a wide area. 2.3.2.4 FA CIES 2.4: ORE ZONE CONGLOMERA TE The 'ore zone conglomerate' (OZC), located entirely in the Pit Area, hosts disseminated to massive sulphide mineralization, and varies from I to 15 metres in thickness. The OZC consists of moderately silicified, mostly subrounded dacite lava clasts ranging from sand size up to 3 cm in diameter in a sandy or silty matrix (Plate 2.2a). The unit can be matrix or clast supported, and also contains clasts and matrix that have been replaced and/or infilled by sulphides. The term ore zone conglomerate refers to the entire unit which hosts sulphide mineralization and that much of the unit contains only sparsely disseminated sulphides, but is texturally distinct from other volcaniclastic units. 2.3.3 FACIES 3: S Y N V O L C A N I C INTRUSIONS Synvolcanic intrusions are distinguished from flows by their contact relationships and textural features such as peperite (Plate 2.6b). Commonly, the contacts are bedding parallel and the units lack flow banding and autobreccia. Chilled margins and contacts at high angles to stratigraphy provide simple criteria for the recognition of dikes. The felsic intrusions comprise a volumetrically large portion of the volcanic sequence at Seneca and complicate stratigraphic interpretations by splitting up and displacing units and by 'inflating' the stratigraphy. Many drillcore intersections of massive felsic intrusions may in fact be relatively thin dikes that have steep dips which increases the apparent thicknesses in drillcore. 32 Plate 2.7. Synvolcanic intrusions. Photo illustrates the range in compositions of the synvolcanic sills and dikes that occur at Seneca. These range from QFP rhyolite (left) to FP dacite/rhyodacite (middle) to basaltic andesite (right). 33 Plate 2.8a: Feldspar-porphyritic (FP) intrusions. These units are dacitic to rhyodacitic in composition and occur as both synvolcanic sills and dikes. They contain plagioclase and hornblende/pyroxene phenocrysts with occasional quartz and are generally massive with sharply defined chilled margins. (Large scale divisions = 1 cm). Plate 2.8b: Quartz-feldspar-porphyritic (QFP) intrusions. These units are rhyolitic in composition and form synvolcanic sills, intrusive domes and dikes. They contain plagioclase phenocrysts and up to 10 % quartz phenocrysts. They are generally more coarsely porpyritic than the FP intrusions shown above. (Large scale divisions = 1 cm). 34 2.3.3.1 FELDSI'AR-I'HYRIC INI R USIONS (FR) The most common intrusions are feldspar-phyric dacite to rhyodacite porphyry sills (FP) (Plate 2.7a). The sills range from one to several tens of metres thick, and are often columnar jointed when exposed in some outcrops. Mineralogically, and often texturally, these rocks are identical to the dacite flows described earlier and are only distinguishable by their contact relationships. Dacite sills, where they cut other intrusions or flows, commonly have chilled contacts over widths of 10 cm to more than 1 m, and are only slightly brecciated. Where the sills intrude the volcaniclastic sediments and breccias, the contacts commonly have peperitic textures. In peperitic zones the contacts tend to be quenched and brecciated with angular to cuspate hyaloclastic fragments less than 1 cm to 20 cm in size with a matrix of the finer sediment (Plate 2.6b). Kokelaar (1982) documents similar textures and suggests that the unconsolidated sediments are fluidized due to the heat from the sills and can thus flow more easily into the matrix of the brecciated margins of the intrusion. The hyaloclastite fragments and surrounding sediments are often silicified. These peperites reach thicknesses of several metres and usually occur at the top of the sills. Quench-induced fractures extend from the brecciated zone into the massive sil l. These fractures have a silicified envelope which destroy the porphyritic texture of these rocks and replace it with a 'pseudobrecciated' texture. The textures indicate that the sills have intruded into wet, unconsolidated sediments (McPhie and Al len, 1992: McPhie et al. , 1993). 2.3.3.2 MAFIC INTRUSIONS Maf ic intrusions are less common than felsic and tend to occur in the lower part o f the •stratigraphy. Similar to the dacite porphyries, they have both crosscutting and bedding-parallel contacts with chilled margins. The andesites are generally massive and dark green with chlorite-filled amygdules 0.5 to 1 mm in diameter. Where the andesites intrude sediments, they exhibit quenching, brecciation (Plate 2.2b), and mixing similar to the felsic intrusions, except that brecciated zones tend to be more extensive (in the range of several metres). These hydroclastic breccias consist of angular or cuspate-shaped fragments with a 'shirr}'" matrix of fine grained volcaniclastic sediments. In many of the drillholes in the Pit Area, these mafic sills intrude the OZC and units immediately above and below it. 2.3.3.3 Q UA RTZ-FELDSPA R-PHYRIC INTR USfONS (QFP) The third type of intrusion is quartz-feldspar-phyric rhyolite porphyry (QFP) (Plate 2.7b). These are less common than the other two types and their mode of emplacement is uncertain. They occur at higher levels in the stratigraphy and as a result their upper contacts are not always seen. Their size and massive nature suggest that they may be synvolcanic sills, but they may also represent emergent domes. Thick intersections of coarsely porphyritic QFP occur in the Fleetwood zone. These units do not correlate with adjacent drillholes and are thus interpreted as moderately or steeply dipping dikes. The rhyolite porphyries are easily distinguishable from the dacite porphyries by their greyish brown groundmass and by the presence of up to 10% subrounded quartz phenocrysts 2 to 7 mm in diameter. The rhyolite porphyry bodies range from a few to more than 30 metres thick. They have not been observed to exhibit the same sediment interaction textures as the dacites and andesites. 2.3.4 FACIES 4: A R G I L L A C E O U S BEDS Thin dark brown to black beds of fine grained sediments less than 1 metre thick were observed in the lower parts of the Hangingwall Interval in the Pit Area and in the Trough Zone (Plate 2.6a, bottom). These beds are also associated with the OZC in the Seneca Horizon. This unit was not observed in the Fleetwood Zone and, therefore, may represent a more distal basinal facies formed by normal sedimentary processes without a large volcanic input, perhaps during a period of volcanic quiescence. Flattened felsic clasts are often observed within the argillaceous beds. 36 2.4 DISCUSSION The distribution of the different volcanic facies across the Seneca property and at different stratigraphic intervals is summarized in Table 2.1. The prevalence of both mafic and felsic lavas, flow breccias and coarse hydroclastic breccias in the Fleetwood and Vent Zones suggests a vent to vent-proximal facies. Maf ic lavas in the Footwall Interval appear to.be continuous between these two zones. However, the felsic lavas that dominate the Hangingwall Interval form two, or possibly more, separate flow-dome complexes centred roughly around drillholes 91-18 and 86-22. The brecciated margins of these flows were eroded and redeposited to form the coarse hydroclastic beds at the base of the Seneca Horizon as well as some of the coarse distal debris flows. The felsic flows appear to have been largely constructive in nature, forming domes consisting of a series of superposed alternating flows and breccias. Some of the more massive, near-surface (near paleo-seafloor) sills that lack internal structure or brecciation may in places have'emerged through the volcaniclastic cover to form true flows similar to the mechanism described for cryptodomes (McPhie and Al len, 1992). Other intrusions common at all stratigraphic intervals were likely feeders for these and subsequent flows, or were high level sills that did not reach the paleo-seafloor. Volcaniclastic units have two possible sources: 1) redeposited hyaloclastite from margins of flow-domes, and 2) gravity-settled fallout from pyroclastic eruptions. The nature of the angular, poorly sorted breccias in the lower portions of the stratigraphy favour transportation as a debris flow. The commonly normal graded and well-bedded finer grained beds of the upper volcaniclastic interval favours transportation of volcanic debris as turbiclites or deposition of pyroclastic debris by gravity settling. The imbedded volcaniclastic siltstone beds were likely deposited by gravity settling of volcanic ash possibly erupted from a distal pyroclastic source. These beds were likely below wave base since they are often bounded by well laminated beds. The volcaniclastic sequence examined in the Trough Zone is similar to the unwelded subaqueous pyroclastic flows described by Fisher and 37 Scliminke (1984) and portions of the volcaniclastic apron described by Busby-Spera, 1988. These deposits are described as having a single massive coarse-grained lower division up to 300 m thick that lacks internal structure and which is overlain by an upper division composed of thin and often normal-graded fine to coarse-grained ash beds. As such, the entire sequence observed in the Trough Zone may represent a single eruptive event characterized by an initial high energy surge which deposited the coarser massive debris flow material, followed by the deposition of finer volcaniclastic turbidites during the waning stages of the eruption. Such an eruption may have marked the onset of the felsic-dominated extrusive volcanism that is prevalent in the upper parts of the sequence elsewhere on the property. This same eruptive event may have deposited the lava clast breccias that are common in the interval stratigraphically above the mafic lavas. Following, or partly contemporaneous, with this event was a period of more effusive volcanism that formed the felsic flows and breccias common in the Fleetwood and Vent Zones. The intercalation of pumice-bearing fine volcaniclastic beds with the flows suggests that ash and coarser pyroclastic debris continued to rain down during this effusive period, perhaps 'draping" volcaniclastic material over the flows. However, this materia) may have originated from many kilometres away and may not necessarily reflect the volcanic processes acting in a particular area at that time. The ultimate origin of the fine-grained volcaniclastic beds is purely speculative at this point. The similarities between the finer-grained volcaniclastic units in the Hangingwall Interval of both the Fleetwood Zone and the Pit Area suggest that these stratigraphic intervals are correlatable; the debris flow and gravity settling processes that deposited these units are favourable for a more areally extensive deposition of volcanic detritus which would account for the occurrence of these units in all areas of the Seneca property. These areas differ in that felsic flows are more common in the Hangingwall Interval of the Fleetwood Zone and the Vent Zone than in the Pit Area; the Pit Area appears to have been more distal and as such does not contain an abundance of extrusive vent to vent 38 c o CJ CO Q 3 x> TJ 3 cd o ca Xl TJ CJ TJ CJ CJ .2 "3 2 o CJ +1 CO x> •° s s « ca CJ a .£ 'co -- cj u 3 C +1 co ca j$ cj TJs .£ -a co o 2 J= cs o s TJ CJ TJ •a CJ X) o 3 o TJ o o o a. TJ o CJ i i "S ° ca u- ,c — —. -3 O s -a a; re s o cj u IS 73 CJ o S £ V TJ" .£ CJ co TJ co 3^ 2 cj £ x> ca 2 TJ CJ u" "p £' Uh o o 'u cs TJ >t 3 o o rted; C3 11 so I, 11 so CS 1 o CJ >. CB u* CJ CJ CS £ CJ TJ 00 O < s £ CO co" ix CJ CO c s JS o N o u. CJ cs tJ u-o la\ OO s CJ o o. 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S CJ CO ca cj o Cu E o o Cu CJ T3 O Cu Xl CJ TJ co3 O Crt 3 • * XI 3 co 3 CS O — CJ -3 3 CJ cr X) P CJ CO CJ CJ -3 •3 s cj TJ 5 3 CJ CJ ' £ cs O CS CJ CJ • '£ -3 3 o -3 CJ -3 '-3 •3 3 T3 S p 3 •3 cj 00 3 C o 3 3 lu u 3 o Cu 3 o Uh Cu B T3 O 3 •3 •3 o o •3 O O u« Cu .2 •3 CJ £ o "cs E 'i< o -3 CJ 3 o o '•3 IS 3 3 CJ pu, it CJ c c as • _CJ O > CJ 3 O co T3 3 3 TJ 3 cs CJ c o CJ ' £ ,co cj , 3 > TJ CJ X> a 3 Cu CJ CJ 3 o co TJ 3 3 co CJ 'H co is CJ E _o oo 3 O CJ O cj '3 o co 5 3 Q_ 3 U, .2 O CJ J= " 2 3 CJ ? '3 3 — c«' Q ^ 3 O co s 3 >3 3 CJ X: 3 3 CJ i5« < IT) U to CJ — 3 fa. U fN proximal facies rocks. The Trough Zone appears to be more distal than all of the other areas since it lacks the synvolcanic intrusions or feeders that are abundant in the Pit Area. These observations indicate that there is an overall facies change northwest to southeast across the Seneca property from vent to vent-proximal facies rocks in the Fleetwood Zone to distal facies-dominated rocks in the Pit Area and Trough Zone. 40 C H A P T E R 3 P E T R O L O G Y OF T H E VOLCANIC SEQUENCE AT SENECA 3.1 INTRODUCTION Thin section analysis of the volcanic rocks at Seneca proved useful in confirming macroscopic observations, and in outlining subtle igneous textures, mineralogical characteristics and degrees of alteration that allowed for the subdivision of units that were macroscopically indistinguishable. This chapter will outline the petrographic characteristics of the major volcanic units at Seneca and wil l use geochemical data to infer the tectonic affinity of the rocks and the possible igneous processes that may have led to the variability in the data set. 3.2 PETROGRAPHY OF THE VOLCANIC ROCKS AT SENECA 3.2.1 FACIES 1: L A V A FLOWS AND BRECCIAS 3.2.1.1 MAFIC LAVAS Basaltic and andesitic lavas are invariably amygdaloidal (Plate 3.1a). The fire fountain debris contains 10 to 20 % calcite and chlorite-filled amygdules that are up to 2 mm in diameter, but generally less than 0.5 mm. The groundmass is essentially aphyric and contains fine plagioclase laths that often display a trachytic texture. The groundmass is strongly chloritic. Large phenocrysts are rare; small euhedral plagioclase laths are present up to 5 %, and ferromagnesian phenocrysts were not observed. Plagioclase phenocrysts from mafic rocks that are in the proximity of the Fleetwood and Vent Zone stockworks are variably altered to sericite ± epidote and calcite. Where the lavas are brecciated, the matrix comprises predominantly angular 41 Plate 3.1a: Basaltic lava. This photomicrograph of fire fountain lava (Sample 91-16-231; see also Plate 2.4a) shows the strongly chlorite-calcite amygdaloidal and essentially aphyric nature of these subaqueously erupted basalts. Only fine plagioclase microlites and an occasional larger phenocryst are discernible. (Plane polarized light; Field of view = 5mm). Plate 3.1b: Vesicular glassy rhyolitic lava. This photomicrograph of a chilled and weakly in-situ brecciated rhyolite flow (Sample 92-33-70) shows the vesicular nature of this unit and the typical variable quartz and chlorite alteration of the glass. (Plane polarized light; Field of view = 1.25 mm). 42 and subangular chloritized glass shards believed to be derived from the lava clasts by spalling of their quenched, glassy rims. The rims of the lava clasts are noticeably finer grained than the massive cores suggesting they were quenched upon contact with the water. 3.2.1.2 FELSIC LAVAS The felsic lavas contain 5 to 15% subhedral to euhedral plagioclase phenocrysts that are typically 1 to 2 mm long. Quartz phenocrysts are less common, but may comprise up to 7% of the rock. They are generally subrounded and less than 5 mm in diameter. Chloritized laths and euhedral phenocrysts are also usually present (up to 5%, but more commonly <2%) and average 1 to 2 mm in size. These mafic phenocrysts are inferred to be altered hornblende based on some of their crystal forms, but some may also be altered pyroxene. The felsic lavas are moderately to strongly silicified (Plate 3.1b). The groundmass of the lavas consists of mosaic-textured quartz with lesser sericite and chlorite. Flow banding is visible in thin section and is enhanced by variations in the silica and chlorite alteration. Relict perlitic cracking is common in these rocks suggesting that the groundmass of the lavas were originally glassy. This texture forms in response the volume increase associated with the hydration of glass (McPhie and Allen, 1993). The perlitic fractures are highlighted by chlorite alteration. Both classical and banded perlite are present. Classical perlite consists of arcuate cracks with chloritic cores and is present more commonly in the massive lavas (Plate 3.2a). Banded perlite consists of a network of rectilinear cracks subparallel and oblique to the flow banding (McPhie and Allen, 1993) (Plate 3.2b). 43 Plate 3.2a: Classic perlite in rhyodacitic glass. Photomicrograph of Sample 91-16-61 shows the arcuate fine cracks that form due to volume expansion associted with the hydration of volcanic glass. The cracks and some of the cores of perlite are accentuated by weak chlorite alteration. (Plane polarized light, Field of view = 1.25 mm). Plate 3.2b: Banded perlite in rhyodacitic flow. Photomicrograph of Sample 91-18-64 shows the rectilinear cracks that form when perlite develops from a flow-banded felsic glass. (Plane polarized light; Field of view = 1.25 mm). 44 3.2.2 F A C I E S 2: V O L C A N I C L A S T I C R O C K S 3.2.2.1 FACIES 2.1: VOLCANICLASTIC DEBRIS FLOWS AND TURBIDITES Coarse grained fragmental rocks are composed of lava clasts, crystal fragments and glassy, hyaloclastic debris (Plate 3.3). Lava clasts are comprised primarily of feldspar and quartz-feldspar-phyric debris. Trachytic mafic clasts are also present, both as discrete beds and in heterolithic beds mixed with felsic clasts (Plate 3.4a). The lava clasts are variably silicified and sericitized and are texturally and mineralogically the same as the lava flows and synvolcanic intrusions. Lava clasts and feldspar phenocrysts in rocks from the Trough Zone tend to be much less altered than those in rocks from the rest of the property. The pumiceous clasts, or fiamme, generally have and elongate, flattened texture and ragged margins (Plate 3.3b). They are also feldspar ± quartz-phyric and have a chloritic groundmass. Although they are spatially associated with the mineralized zones, these coarse units do not contain any sulphide clasts. 3.2.2.2 FA CIES 2.2: VOLGA NIC LA STIC SIL TSTONES The very fine grained volcaniclastic ash beds are generally massive in thin section and textures are not easily discernible. Where present, laminations are defined by subtle grain-size variations. The beds are composed almost entirely of subround to angular glassy shards and finer felsic ash, but they are occasionally interbedded with reworked mafic hyaloclastite. Three types of glass shards are present: platy shards, cuspate or vesicular shards and tube pumice fragments (Plates 3.5 and 3.6a). The cuspate shape of many of the shards is due to quench fragmentation of glass or to the shattering of vesicular glass which leaves the arcuate remnants of the vesicle walls. The tube pumice fragments are composed of glass shards with elongate tube vesicles, and are relatively rare. The shards are surrounded by a matrix of much finer chloritized ash. Plate 3.3a: Crystal-rich volcaniclastic sandstone. Photomicrograph of sample 91-03-14 from the Trough Zone shows illustrates the predominance of quartz and plagioclase crystals in these normal graded, turbiditic beds. The lack of lithic lava clasts in this bed may imply a certain degree of density sorting during transportation. (X-Nicols; Field of view = 5 mm). Plate 3.3b: Fiamme in coarse grained debris flow. Photomicrograph of felsic lava clast-dominated unit from the footwall of the Pit Area shows foliated texture of a flattened pumice fragment. The abundance of glassy material in such units may imply eruption as a subaqueous pyroclastic flow. (X-Nicols, Field of view = 5 mm) 46 rhyolite (lower left) surrounded by chlorine basaltic andesite debris. (Plane polarized light; Field of view = 5 mm). Plate 3.4b: Bedding contact between reworked basaltic hyaloclastite (upper half) and fine grained felsic ash/volcaniclastic siltstone. The mafic material may be a reworked and transported equivalent of the fire fountain debris lower in the sequence. (Plane polarized light; Field of view = 5 mm). 47 Plate 3.5: Two photomicrographs illustrating the typical glassy fragmental textures of the volcaniclastic siltstones and ashes. The upper photo contains platy shards (labelled p) and vesicular fragments (v). The lower photo contains platy shards (p), cuspate shards (c) and a fragment of tube pumice (t) that has elongate tube-like vesicles. (Plane polarized light; Field of view = 625 p.). 48 Plate 3.6b: Pyritic lamination in volcaniclastic ash. Photomicrograph illustrates fine pyrite lamination which is a common feature of the volcaniclastic sequence immediately above the stockworks in the Fleetwood and Vent Zones. Left side of photo is up direction. (Plane polarized light; Field of view = 1.25 mm). 49 Thin pyrite laminations are present in some of the ash beds (Plate 3.6b). They are generally observed in strata close to the mineralized Seneca Horizon. Although in thin section the pyrite appears to possibly have a replacement origin, the laminae themselves do not exhibit any compositional or textural contrast to the adjacent laminae that would favour preferential replacement. The origin of these laminae is not clear. 3.2.3 F A C I E S 3: S Y N V O L C A N I C I N T R U S I O N S 3.2.3. 1 MAFIC INTRUSIONS Maf ic intrusions, in contrast to mafic flows, are poorly to non-vesicular. They tend to contain a greater amount of larger euhedral plagioclase phenocrysts and microlites up to 0.5 mm in size (Plate 3.7a). These are often aligned, giving the rock a weakly trachytic texture. Primary ferromagnesian phases were not observed although they may have been present and have since been altered to chlorite. Some of these intrusions are magnetic, suggesting the presence of magnetite although it was not confirmed in thin section. Peperitic textures are common at the contacts between these intrusions and unconsolidated sediments (Plate 3.7b). 3.2.3.2 FELSIC INTRUSIONS FP Intrusions Dacitic to rhyodacitic composition feldspar-phyric synvolcanic intrusions (FP) are very common at Seneca. They contain 5 to 15 % sub- to euhedral plagioclase phenocrysts, but more commonly less than 10 % (Plate 3.8a). Small quartz phenocrysts are also occasionally present, but generally make up less than 3 % of the rock. The groundmass of the FP intrusions has a felty, and occasionally weakly trachytic, texture defined by varying abundances of plagioclase microlites. 50 Plate 3.7a: Basaltic andesite. Photomicrograph of Sample 83-17-34 shows the abundance of plagioclase phenocrysts and the poorly vesicular nature of these units which were generally emplaced as sills (see also Plate 2.2b). (X-Nico l s ; F ie ld of view = 5 mm). Plate 3.7b: Peperitic basaltic andesite s i l l . Photomicrograph of Sample 83-10-56 shows the brecciation of the mafic s i l l that occurs during interaction with wet sandy volcaniclastic material which becomes fluidized and incorporated into the interstices as shown by the brown areas. (Plane polarized light; F ie ld o f view = 5 mm). Plate 3.8b: Rhyodacite FP intrusion. Photomicrograph showing the typical porphyritic texture of these rocks with euhedral plagioclase phenocrysts (weakly sericitized) and chloritized mafic phenocrysts (pyroxene/hornblende). (Plane polarized light; Field of view = 5 mm). 52 The plagioclase phenocrysts are 0.5 to 2 mm in size and commonly display albite twinning. They are variably sericite altered; even the least altered samples display a weak 'dusting' of sericite, whereas the the strongly altered samples from the stockwork zones contain relict phenocrysts that have been completely replaced by sericite. The presence of potassium feldspar phenocrysts was not confirmed petrographically. Inferred ferromagnesian phases are present, but have been completely altered to chlorite (Plate 3.8b). These relict phenocrysts make up 2 to 5 % of the rocks and are generally less than 1 mm in size. The original mineralogy of these mafic minerals, inferred from the remnant crystal shapes, appears to have been hornblende and clinopyroxene. The F P intrusions often contain inclusions which display a variably glomeroporphyritic texture. These clusters are up to 2 mm in size and consist of intergrown plagioclase laths and chloritized mafic minerals that can comprise up to 5 % of the rock (Plate 3.10). The origin of these clusters is not clear, but they may represent small 'xenoliths' of a cumulate layer that were carried up close to surface during the emplacement of the sills and dikes. The possible lithogeochemical significance of these clusters will be discussed in the next chapter. QFP Intrusions Quartz-feldspar-phyric intrusions contain 7 to 15 % plagioclase phenocrysts and 5 to 10 % quartz phenocrysts. In contrast to the FP units, the groundmass of the Q F P rocks is typically spherulitic, indicating it was originally glassy (Plate 3.9a). The spherulites consist o f quartz and variably altered feldspar that occasionally shows a radiating texture. Plagioclase phenocrysts are 1 to 3 mm in size and are sub- to euhedral. They are generally weakly to moderately sericite altered and display albite twinning. • Plate 3.9a: Sphemlitic groundniass oi a OFP intrusion. Photomicrograph of Sample 91-18-302 shows a typical texture of the rhyolitic synvolcanic sills and dikes which tend to have a more glassy groundmass compared with FP intrusions. (Plane polarized light; Field of view = 1.25 mm). Plate 3.9b: QFP rhyolite intrusion. Photomicrograph of Sample 92-26-227 shows large embayed quartz phenocryst, a common feature of the QFP intrusions, and a weakly sericitized plagioclase phenocryst and some calcite to the left. (X-Nicols; Field of view = 5 mm). >4 Plate 3.10: Group A felsic unit. Photomicrographs of Sample 92-27-85, an FP dacite that was classified as group A based on its major element chemistry. This more intermediate composition group often contains cumulophyric clusters of minerals (upper right) which contain plagioclase and mafic minerals (now chloritized). The groundmass generally has a felted texture and is less glassy than the more silicic rocks. (Upper: plane polarized light, Lower: X-Nicols; Field of view = 5mm). 55 Plate 3.11a: Group C felsic unit. Photomicrograph of Sample83-02-320, an FP rhyodacite, showing plagioclase phenocrysts in an aphanitic groundmass. This compositional group of rocks generally lacks the cumulophyric inclusions seen in groups A and B and the abundant quartz of group D rocks. (X-Nicols; Field of view = 5 mm). Plate 3.11b: Group D felsic unit. Photomicrograph of sample 91-02-85. a QFP rhyolite dome, showing weakly sericitized plagioclase phenocrysts and euhedral quartz crystals with silica overgrowths. This sample has the glassy, spherulitic groundmass typical of this most silicic compositional group. (X-Nicols; Field of view = 5 mm). 56 Quartz phenocrysts are up to 5 mm in size, but more commonly 1 to 2 mm (Plates 3.9b and 3.1 lb). They are commonly rounded or hexagonal in cross-section. The embayments that are common among the larger quartz phenocrysts are indicative of rapid crystal growth (McPhie and Allen, 1993). Inferred ferromagnesian phenocrysts are present and make up 1 to 2 % of the rocks. They are also completely altered to chlorite, but are inferred to represent relict hornblende and clinopyroxene crystals. However, in contrast to the FP units, the QFP units lack cumulophyric crystal clusters. 3.3 IGNEOUS L.1THOGEOCH.EMJSTRY Lithogeochemical data is used in this study to distinguish different lithologic units, to determine the tectonic affinity of these units, and to determine possible igneous processes that may have produced the primary, non-hydrothermal chemical variations within the volcanic rocks. Petrography has shown that most samples at the Seneca property have undergone at least a weak sericitic alteration. In this chapter major and trace element data for a subset of least altered rocks are used to best determine the primary igneous lithogeochemical trends by minimizing the effects of this ubiquitous alteration. Major and trace element data for these samples are included in Table 3.1. The locations of the sampled drillholes and the geochemical data for the entire sample set is included in Appendix A. Calculations of analytical uncertainty are included in Appendix C. 3.4 METHODOLOGY Ninety-eight samples of drillcore were analysed in four separate batches between September, 1993 and November, 1994 using X-ray fluorescence at Geochemical Laboratories, Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec. Major 57 ^ d ri f Ox o o m | H T oo K 1 „ — oo » O — — p _ C> M CS O 00 V) CS CS cs u <u c CJ C/2 g 00 — •» m «-> , 9 o ~ UJ in h n rt< CJ o v£> 00 -3" vA ° ^ S m oo CJ CJ ce I1 o cj "es u U Trt <")' Xi cs H © cs vo r-- P-rs © oo oo rs ON 00 — i S 5 m o ci oo « to h « vo "t oo -r ON r-i m ON O m m o r-i — O m O vD C\ -i-CN VO r*S O r*S oooooor-i — t O 'A ro©t^t^Or*i©vqc-i d 6 - ^ - o — i ©' r— — O O © T r-» t co '/-i co oo ON r-i r-i >/•!t n o o r - - O O ^ c o c o ON O O © 'fi © O rl r*1 ON ON O MD m 'A ON T CO O *rr -rr rn m vO \0 — '"' O O O r-i ON r— © ON r- r-© ^ ^ ^ © O N O N N O O O © O •*: rn O — O *T — o m i*"t *o r-0O >/1 r- ON t o\ t co r>i -r —: — -rr rt- o — ON r i O ON ON ON ro r-l tr ""l ON O r i co r-ON O -rt rn O r i © ri 3 -T3 T) CC CO O lO 'Ci xi C T ON ro O (1 « £ £ c c m 'T; -o x> r-- ON ON f i NO NO "O CO I— \D CO "3" C-l \0 NO C C C \o r- — co oo r- r- co r-i X! c c c D \ N T3 T J « CO O M i X C C r i ~ r-j O r*~ *f "D "O MT> *f <*1 "O T3 r- m rn r-i NO P- \0 '/-i rt — ON " r-l — — JD JD C C ilS f i 11-2 S £ -i i i ! lit g i o ' t * « CO Tl Tl 2 b 5 O O -o E = x> Z 1« UJ t-58 VI 00 o T r» o r- rj **? •—'• *~ 1 n - J 3 oo — r i-N£) ON "O ci NO O r m so t T (N O 0 0 t j oo <N r> ^ NO o ON 9 od a « rs — o o o o <o ON M o >n 2 d d r i CN a o -o o. c NO O m t— 0\ f l ID 0O O — "t ON 1 r i oo r- -tf -rt — in — r- — x> . o ci <n O O *tf O O r-I ON -tf © o NO r i >n o O -tf O •n — O ""i M — \ o *t ON OO r-i m r l — -tf — d 06 o\ d NO oi . rr, o O ON CO m — — I © r-~ © © NO r-i o t o- m ON — r i m o i in' o O -tf © •O O r-J vo *tf i n NO O "O TJ o r - O N O N r — - t f r - - — T J T J oo r-i m NO m >n vO r-i — O — O i oo o rn t— 1^ o *o h • ^ c c c c c c c c c c O "t ^ — o — r- t p TJ vO NO TJ TJ C C C OO r*"l oo o r-i o — O ON — r-i oo ON Ol M J3 X C c NO — TJ TJ r- rn O -tf "tf NO rn ON NO ON ON 1 O ON O ON ON in . O t TJ O TJ 00 -tf o- TT -tf ND TJ TJ « M — ON f l X> X> C r-i r— —; o C -tf NO c — NO -tf m O ON i/". TJ TJ O — X! JD O NO — CN rni ON• R < > •r, o T co NO "1 NO 0 0 O ^ . ^ r-— o — © -tf NO TJ TJ TO <0 ca TO •— r-i _o xi c c c c c c c c O -tf -tf O r- NO *tf r-i r-i d in' NO d 1 55 H < u. — T O O •n O O r-i -tf TJ -rt co -tf IS 2 i | o S il u J> • » as 2. , A U £ U Z U </5 O 2. N 59 " 1 Z o\ r-J h/) — w-i C J OO i n » O N m rs ^ I« r-- </-> o " ™ m m o CN ro o xo — xo rs g Ov - « E S u 5 V O C J 2 3 O O O "*> r-i • m o - T " T r n N O T J v O v O ON rn oo o> ON — — "3" r-l m O rt -i r-i o t co J -J co r-ON rn CO vO O ON o o o o o "t H XI vO T) CO H T r- rt rt o i CO NO Tf r- NO "O T ) r - o o c o o o - — •— 3 : — NO ON T J ON CX) CO iO XJ X) ON CO \ 0 ON NO T Ti O t— O O m o o \o o 3 2 o xi t r~ O ON CO rn O • ON r-l rn -r]- TJ-K O O O '/-i OO TT ON O 1- o r-i r-l -1- T J <ri — rn rn rt ON J D OO \JD r- rt m r- J D J D C C c c c •/•i \o r-o o r - o o c o m m o o ' CO O O ON O rf ON NO '*"! CO -T m — -r CJ O rn o NO ON <r, cO " S O O O O n S o o -n "oc DA — •g i ai TJ CL D£ C "a 3 T3 o> O _>. is c 73 Sc & t_ > c U o « c c IL c 3 u E 3 "EL •O z; c o c > a a © .-u •-> e w s tv a> —j l< £ f e 60. elements plus the trace elements C u , Zn , N i , Cr , V and Sc were measured using glass beads. The trace element concentrations of Zr, Y , Nb , Rb, Sr, G a and Pb were measured using pressed pellets. The rare earth elements were analysed by instrumental neutron activation analysis at Activation Laboratories Ltd., Ancaster, Ontario. The sample set was chosen to approximately reflect both the entire compositional range and the relative abundance of the various synvolcanic intrusive, extrusive and volcaniclastic units observed in the examined stratigraphic interval at Seneca. Fewer volcaniclastic samples were analysed because their geochemistry is more difficult to interpret due to the influence of non-igneous processes such as mixing of various clast compositions during transportation. As such, the volcaniclastic samples in the data set are restricted almost entirely to massive, fine-grained Facies 2.2 volcanic siltstones. These samples are not included in the least altered data subset. A l l samples were taken from drillcore, except for 94-FF-0I which was collected from an outcrop of mafic lava near Morr is Creek, 4 km to the southeast of the property. Petrography and plots of the lithogeochemical data were used to determine which samples have undergone the least alteration. Samples with the least amount of sericitization of feldspars and/or silicification of the groundmass were chosen for the least altered subset. Data for samples on all lithogeochemical plots have been recalculated on an anhydrous basis. 3.5 TECTONIC AFFINITY Previous studies (Irvine and Baragar, 1971; Winchester and Floyd, 1977; Wood et al. , 1979) have illustrated the use of certain major and trace elements to discriminate between rock suites from different tectonic settings. The hydrothermal alteration present around Seneca limits the use of diagrams based on mobile elements. These mobile elements wil l not be ignored completely, however, and will be discussed in the subsequent chapter dealing with alteration and 61 FeO* Na20 + K 20 MgO (from Irvine & Baragar , 1971 , f ig. 2) LEGEND • Basalts, andesites O FP dacites & rhyodacites O Q F P rhyolites • Aphyric rhyolite dikes Figure 3.1. AFM plot of least altered volcanic rocks from the Seneca property. The plot illustrates the calc-alkaline nature of the volcanic sequence. Ti / 100 Within plate basa l t s : D O c e a n f loor basa l t s : B L o w - K tholei i tes: A , B Ca lc -a l ka l i ne basa l t s : B, C f rom P e a r c e & C a n n , 1973 Zr Y * 3 18000 15000 h-12000 9 0 0 0 6 0 0 0 3000 f rom P e a r c e & C a n n , 1973 (fig. 2) -i r T O c e a n f loor basa l ts : D, B L o w - K tholei i tes: A , B Ca lc -a l ka l i ne basa l t s : A , C _L 25 50 75 100 125 150 Zr (ppm) 175 200 225 2 5 0 Figure 3.2. Zr-Ti - Y and Zr-Ti discrimination plots for least altered basalts and basaltic andesites at Seneca. These plots illustrate the compositional contrasts among the mafic rocks from low-K tholeiites (basalts) to calc-alkaline basalts (basaltic andesites). Fil led squares are basalts and open squares are basaltic andesite. 63 Figure 3.3. TiO2-(MriO*10)-(P2O5*10) discrimination plot for all mafic samples at the Seneca property. Compositional fields are taken from Mullen (1983). This plot demonstrates the calc-alkaline to weakly tholeiitic nature of the basalts and basaltic andesites at Seneca. 64 100, "T 1 T~ - i 1 1 1 r -Sample 94-FF-01 1 0 h O Basalt • Basalt ic andesite -j 1 i i i _ _ i i i i i _ La Ce Pr Nd Sm E u (Gd) Tb Dy Ho Er Tm Yb Lu Figure 3.4. Rare earth element patterns of a) basalts and basaltic andesites and b) dacites to rhyolites from the Seneca property. R E E abundances are normalized to the chondritic values of Evensen et al. (1978). Gd values have been estimated using the formula Gd = 1/3 Sm+2/3Tb. 65 mass changes. A lso, Harker plots of the major elements versus S i 0 2 wil l be used in this chapter to illustrate the range in the geochemical data. The low overall abundances of some trace elements (eg. Ta , Th , U) in the rocks at Seneca also hinders the use of some of these methods of tectonic discrimination. The A F M plot of Irvine and Baragar (1971) (Figure 3.1) shows that the volcanic rock samples from Seneca have a calc-alkaline affinity, although some of the mafic samples may have a more tholeiitic nature having less N a 2 0 and K 2 0 and more FeO and M g O . Figure 3.2 illustrates that the mafic rocks at Seneca range in composition from low-K tholeiites to calc-alkaline basalts. The mafic samples plot in the fields of calc-alkaline basalts to island arc tholeiites in the T i 0 2 -(MnO 2 *10) - (P 2 O 5 *10) discrimination plot (Figure 3.3; Mul len, 1983). These trends are consistent with the rocks having been formed in a volcanic island arc setting at or near a destaictive plate boundary (Irvine and Baragar, 1971; Pearce and Cann, 1973). Figure 3.4 depicts the rare earth element abundances relative to chondrite for both mafic and felsic rocks at Seneca. Chondrite values are taken from Evensen et al. (1978). The mafic samples (Figure 3.4a) show flat, chondritic to slightly fractionated, light REE-enriched patterns. L a N / Y b N ratios range from 1.2 to 1.9 indicating a tholeiitic to transitional affinity (Jakes and G i l l , 1970). The felsic rocks (Figure 3.4b) all have an enrichment in the light R E E , and LaN/YbN ratios range from 2.0 to 3.7 indicating a transitional to calc-alkaline affinity (Jakes and G i l l , 1970). One sample of fine grained volcaniclastic siltstone/ash (Sample 91-16-94, Appendix A.3) was analysed for R E E s . This sample has approximately the same overall abundances and the same flat trend amongst the H R E E s as the felsic samples (Figure 3.4). However, the volcaniclastic sample does not exhibit the enrichment in L R E E s that characterizes the dacitic to rhyolitic lavas and intaisions. The R E E pattern of this sample coincides well with the pattern for sample 66 Z r and Y are immobile elements and are good indicators of the tectonic affinity of a volcanic suite (Pearce and Cann, 1973; MacLean, 1990). Pearce and Cann (1973) and Pearce (1982) summarized the trace element abundances for rocks of different tectonic settings. Their data showed that abundances of Z r and Y for rocks from volcanic arcs were lower than those for rocks from oceanic ridges and basins. Zr and Y abundances for ocean floor basalts range from 64 to 129 ppm and 22 to 47 ppm respectively; Zr and Y abundances for mafic volcanic arc rocks range from 33 to 107 ppm and 15 to 24 ppm respectively (Pearce and Cann, 1973). Using these trace element abundances as a guide, the volcanic rocks at Seneca are consistent with formation in a volcanic arc. The data summarized by Pearce and Cann (1973) also demonstrated that in a volcanic arc setting, rocks with a tholeiitic affinity (lower K 2 0 , higher Fe /Mg values) had Z r / Y ratios of less than 3.5, whereas rocks of calc-alkaline affinity (higher K 2 0 ) had Z r / Y ratios of greater than 3.5. Using this Z r / Y ratio of 3.5 as a potential boundary between tholeiitic and calc-alkaline rocks in a volcanic arc setting, it is apparent that almost all of the felsic rocks (rhyodacites and rhyolites) at Seneca are calc-alkaline in nature. The mafic rocks (basalts and basaltic andesites) can be subdivided into two groups based on this Z r / Y boundary. The more basaltic composition flows and breccias have Z r / Y ratios of less than 3.5, whereas the more andesitic composition sills have Z r / Y ratios of greater than 3.5. 3.6 GEOCHEMICAL CLASSIFICATION OF UNITS Using the Z r / T i 0 2 - S i 0 2 discrimination diagram of Winchester and Floyd (1977), it is apparent that the volcanic rocks at Seneca are bimodal with a basaltic to basaltic andesitic population and a dacitic to rhyolitic population which are separated by a 'gap' corresponding to compositions from 53 to 63 wt. % S i0 2 (F ig . 3.5). The bimodal nature of these rocks is also apparent on silica variation diagrams (Figures 3.6 and 3.7). Although bimodal geochemical suites 67 are not uncommon among rocks of volcanic terranes, this characteristic is not consistent with the data set of Mahoney (1994) which encompassed the entire Harrison Lake Formation of which these rocks are a part. As such, this bimodal signature may simply be a local phenomenon and may not be representative of the entire regional volcanic sequence. The volcanic rocks at Seneca can be separated into a number of subgroups based on their major and immobile minor and trace element geochemistry. T i 0 2 , coupled with Zr and A l , is a useful monitor of fractionation and with S i 0 2 can be used to subdivide the felsic and mafic composition groups of units. 3.6.1 M A F I C R O C K S The mafic rocks are subdivided into two subgroups: I) 'less evolved 1 basaltic composition rocks with lower T i 0 2 , Z r and S i 0 2 contents and Z r /Y ratios less than 3.5, and 2) 'more evolved' basaltic andesitic composition rocks with higher T i 0 2 , Z r and SiO? contents and Z r /Y ratios between 3.5 and 4.8 (Figures 3.8 and 3.9). T i 0 2 , P 2 0 s Zr and Y contents all increase through the range from 44 to 56 wt. % S i 0 2 suggesting that they are incompatible with minerals crystallizing from mafic composition melts. M g O , F e 2 0 3 and A 1 2 0 3 all decrease from 44 to 56 wt. % S i 0 2 suggesting that they are compatible in this range of melt compositions (Figure 3.6). 3.6.2 F E L S I C R O C K S The least altered felsic samples (dacites to rhyolites) are^subdivided into four subgroups (denoted A to D on Figures 3.8 and 3.9) based on their relative T i 0 2 contents and their petrographic characteristics such as the presence of the cumulophyric clusters described previously. Groups A , B and C comprise mostly dacitic to rhyodacitic FP rocks whereas group D comprises mostly rhyolitic FP and Q F P rocks. These groups have unique, non-overlapping T i 0 2 68 .001 .01 .1 1 10 Z r / T i 0 2 Figure 3.5. Z r / T i 0 2 vs. S i 0 2 discrimination plot of the least altered volcanic rocks at Seneca. The plot illustrates the range in compositions in the volcanic sequence and highlights the compositional gap that exists from 53 to 63 wt. % S i 0 2 . The symbols are the same as in Figure 4.1. 69 40 50 60 70 S i Q 2 (wt%) 80 o ro O 12 10 8 6 4 2 O CM ro 3.5 2.5 O 1 5 o CM CL 0.3 0.2 0.1 40 a a l • P V. > • i 50 60 70 80 S i Q 2 (Wt %) Figure 3.6. Harker-type variation diagrams of the major elements versus S i 0 2 . 70 8 • a 9 ° 9 40 50 60 70 S i 0 2 (Wt. %) 80 270 „ 210 E CL 3 150 90 30 50 _ 40 E CL 3 30 o ° 20 10 E a . CL • — p — 1 — w—•—•—•—r • B • -• - • • • -°a • • 0 -• • o -- o 8 v.. • • i 1 1 1 1 1 -• • ' * 1 — i - -• • • * • • • _ • 0 * - $ • • V . • i i i 1 0 « 1 1 1 1 1 1 1 i i i i i * • • • • • • 1 1—.—.—•—i— 40 50 60 70 S i Q 2 (wt. %) 80 Figure 3.7. Harker-type variation diagrams of selected trace elements versus SiO 71 a) b) 25 20 t 15 5 CO o 10 0 80 h £ 60 CM 9 40 c/> 20 Basalt a;** Basaltic Rhyolite Andesite n =4 Dacite Rhyolite -co Dacite B -D o ^ "\ Basaltic B , 9 ° Andesite Basalt 0 0.2 0.4 0.6 0.8 T i0 2 (wt%) 1.2 Figure 3.8. a) Binary immobile element plot of A l j O , vs. T i 0 2 . b) Binary immobile-mobile element plot of SiO, vs. T\02. Both plots show subdivision of mafic and felsic compositions based on petrographic differences and T iO, contents. Mafic rocks are broken down into basalts and basaltic andesites; felsic rocks are divided into 4 groups A to D (see text for discussion). 72 a) 0 i -1 i i i d i i i I I I I I 1 Basaltic Andesite - - - - r D i • -t—» CM o f -0.8 0.6 0.4 Basalt / ' „ / X i l / Dacite \ ^ ® V © ' a * * \ 3 ® -0.2 i 1 1 ) 1 \m 1 1 1 Rhyolite . I 1 l 1 l 0 I I I" 1 1 I i b) 35 30 • 1a > ® o (ppm) 25 20 <VV - fib 3^ -©-> 15 _  u - - — _ --10 / 5 / / / / / / ^ -** I I I I i i i 0 20 40 60 80 100 120 140 Zr (ppm) Figure 3.9. a) Zr-Ti02 plot shows increase in Ti02 from basalt to basaltic andesite followed by a decrease from basaltic andesite to rhyolite; b) Zr-Y plot illustrating the increase in Zr/Y ratios from basaltic to rhyolitic compositions. Both plots show the felsic subgroups A to D and the fractional crystallization paths 1 to 3 that were modelled and summarized in Table 4.3 (see text for discussion). 73 contents which decrease from A to D. A I 2 O 3 decreases from A to D while SiO"2 increases. There is an overall decrease in all the major element contents, except for possibly K 2 0 , in the range from dacitic rocks to rhyolitic rocks (Figures 3.6 to 3.8). Z r contents increase over this range. In contrast to the mafic rocks, Y contents decrease with increasing silica content in the dacitic to rhyolitic samples (Figure 3.7), and thus, there is an overall gradual increase in the Z r / Y ratios from dacitic to rhyolitic compositions (Figure 3.9b). On a Z r - T i 0 2 plot (Fig. 3.9a), groups A and B form clusters with very little compositional variation, whereas groups C and D form linear to curvilinear trends defined by variations in Zr content and only small variations in T i 0 2 content. These groups also have petrographic differences as outlined in the preceding section. Most of the FP and Q F P samples have Z r / Y ratios of greater than 4.2 and are thus classified as calc-alkaline as discussed above (Figure 3.9b). The only two exceptions are the two intermediate, dacitic composition samples which comprise group A . The Z r / Y ratio of 4.8 that forms the upper limit for mafic composition samples also forms a distinct boundary between group B and group C and D felsic rocks. While group C samples have Z r / Y ratios less than 5.4 and plot close to this boundary, group D samples are much more variable and have Z r / Y ratios ranging from 4.8 to 7.5. It is apparent then that the transition from dacitic to rhyodacitic to rhyolitic compositions corresponds to a gradual increase in the Z r / Y ratio controlled by an overall greater increase in Zr contents relative to Y in groups A to C followed by a greater decrease in Y contents relative to Z r in group D. 3.7 IGNEOUS R O C K - F O R M I N G PROCESSES Trends in the major and trace element geochemistry of volcanic rocks are largely controlled by the fractional crystallization of different mineral phases at different points in the evolution of a body of magma. Elements that are incompatible (i.e. are not partitioned into a 74 precipitating phase) are enriched in the residual melts during progressive crystallization and will have a positive correlation when plotted against S i 0 2 on variation diagrams (Figures 3.6 and 3.7). Z r and H f show such a positive correlation over the entire compositional range (Figure 3.7). Other elements such as T i 0 2 , Y and P 2 O 5 exhibit a positive correlation with S i 0 2 in the range of mafic samples (< 57 wt. % Si0 2 ) , but exhibit negative correlations over the range of felsic compositions (> 65 wt. % Si0 2 ) suggesting that there are changes in the assemblage of precipitating phases during magmatic evolution such that elements that are initially incompatible are partitioned into later-crystallizing phases. This study will use Pearce element ratio analysis and fractional crystallization modelling to explain the trends in the major and trace elements. Since it is not clear that the mafic and felsic rocks are directly related by fractionation, the pedogenesis of each compositional group will be addressed separately. Pearce Element Ratio Analysis Pearce element ratios (PERs) are a useful tool in determining which minerals have contributed to chemical variations in a related suite of volcanic rocks and the extent to which they were involved. P E R analysis associates primary chemical variability in a suite of genetically related rocks to the addition or removal of an assemblage of minerals. PERs utilize molar quantities and have a denominator constituent which is conserved by processes of mass transfer. The axes of P E R plots are chosen based on the stoichiometry of the minerals involved in the hypothesis that is being tested. The procedure for selecting a conserved element and testing petrologic hypotheses using PERs is described in detail by Russell and Nicholls (1988), Stanley and Russell (1989) and Russell et al. (1990). 75 Zr was chosen as the best conserved element for the volcanic rocks at Seneca because no other elements were truly conserved (Nb concentrations are quite low and have a greater degree of analytical uncertainty). Z r contents increase steadily between basaltic and rhyolitic compositions suggesting that it is incompatible over the entire compositional range and is being enriched in the increasingly fractionated melts. T i is only a conserved element in the most basaltic composition rocks and is therefore not used in P E R calculations. Since Zr can be partitioned into accessor}' phases such as hornblende, it is not truly a conserved element. However, the effects of the fractionation of the major minerals such as feldspar and quartz wil l have the greatest effects on the bulk composition of the rocks. It should be noted that the variability of Z r contents among some of the more sil icic rocks (groups C and D, Figure 3.9) may be due to the fractionation of small amounts of zircon. Zircon has only been identified in one sample, but may be present as a minor phase in the aphanitic glassy groundmass of the Q F P rhyolites. 3.7.1 M A F I C R O C K S Pearce element ratio analysis was used to interpret the pedogenesis of the basalts and basaltic andesites. The fractionation of plagioclase, which is the only discernible phenocryst phase in these mafic rocks, is inferred to have had the greatest effect on the bulk geochemistry during magmatic evolution. Thus PERs were used to test petrogenetic models which involve plagioclase fractionation. Figure 3.10a is a P E R plot which uses Zr as the conserved element and models the effects of the fractionation of a combination of plagioclase, olivine and clinopyroxene. On such a plot, rocks that are related by the fractionation of these minerals in any proportion wil l lie along a line having unit slope (i.e. m=l). Displacements in the vertical sense from the linear fractionation trend can be attributed to C a or Na-metasomatism. It is apparent that the basalts from the Fleetwood Zone are less evolved and can be related to the basaltic andesites from the Pit Area by 76 a O E, £. a o + o _+ a. 1.8! 1.6 1.4-1.2 1.0 0.8 0.6 0.4 0.2 0 Fleetwood Zone basaltic samples • Pit Area / • basaltic andesites / • Plagioclase fractionation (m=1) 0.5 1.0 1.5 Si/Zr P E R (molar) (PI+OI+Cp) = (1.5Ca+2.75Na+0.25AI+0.5Fe+0.5Mg) 2.0 o E "ST z CO + ra O CM 1.4 i 1.2 1.0 0.8 0.6 0.4 0.2 0 Fleetwood Zone « basaltic samples • Pit Area ^ basaltic andesites P l a g i o c l a s e f r a c t i o n a t i o n (m=1) J N a , C a m e t a s o m a t i s m 0.5 1.0 1.5 Si/Zr P E R (molar) 2.0 Figure 3.10. P E R plots for the least altered mafic samp les at S e n e c a , a) shows the effects of the fractionation of an a s s e m b l a g e of p lag ioc lase , ol ivine and pyroxene and b) mode ls the effects of p lag ioc lase fractionation a lone. 77 the fractionation of some combination of the assemblage plagioclase, olivine and clinopyroxene. Although olivine and clinopyroxene were not observed in the rocks, it is possible that they have been completely removed by fractionation, but have still influenced the geochemical trends. Figure 3.10b shows the effects of fractionating plagioclase alone; samples related by plagioclase fractionation lie along lines with unit slopes, and, again, vertical displacements from these lines are due to Ca and Na metasomatism. The data points on this plot can be divided into two separate groups each lying along a line of unit slope. These two groups correspond to the basaltic Fleetwood Zone samples and the more andesitic Pit Area samples. 3 . 7 . 2 F E L S I C R O C K S Pearce element ratio analysis is used to explain the variation of the major elements in the least altered felsic rocks (dacitic to rhyolitic compositions) while combinations of binary geochemical plots and Rayleigh fractionation modelling are used to explain the variations in trace element contents. Pearce Element Ratio Analysis Zr was chosen as the best conserved element among the felsic rocks for use in PER analysis. It is used with caution, however, because it may be a compatible element in the most silicic samples. The compatibility of both TiO: and P20} in the felsic rocks rules them out as possible alternate conserved elements. Figure 3.1 la is a phase discrimination PER plot in which chemical variations in genetically-related rocks can be related to the fractionation of a combination of minerals. The effects of fractionation of various mineral phases are shown as a series of vectors with different slopes. The fractionation of some combination of these minerals will result in a trend which has a 78 a) b) ro O E_ cr LU 0 . + 30 20 i . 10 CO + ro O 5! Fractionation trend (plag±pyroxene+hornblende) Group D samples O Group A samples • Group B samples • Group C samples • Group D samples 20 40 60 Si/Zr PER (molar) 80 100 -r 6 ro o E. 5 DC S 4 2- 3 + ro ^ 2 Feldspar fractionation (m=1) 4 6 Al/Zr PER (molar) 10 Figure 3.11. PER plots of the least altered felsic samples at Seneca, a) is a phase discrimination diagram which illustrates the effects of quartz addition on the group D samples, and b) demonstrates the modelling of feldspar fractionation on the felsic samples. 79 slope corresponding to the sum of the vectors of the minerals involved. Plagioclase fractionation alone would yield a trend with a slope of 1 on this plot. In Figure 3.11a, samples in groups A, B and C, which were defined above, lie along a line which has a slope of less than 1. This indicates that an additional phase to plagioclase, such as pyroxene, may be involved in the fractionation process. It is possible, based on this plot alone, to explain the slope of the trend as a result of orthopyroxene fractionation alone, but this is not consistent with results of other PER plots (i.e. modelling feldspar fractionation) and petrographic observations which indicate plagioclase is the most abundant phenocryst phase. Fractionation of hornblende would also cause such a displacement from the plagioclase fractionation line. Since Si is included in the abscissa, addition or removal of quartz to or from the system in the form of small phenocrysts or secondary silicification will result in horizontal displacements on this plot. Group D felsic samples which contain variable amounts of quartz have higher Si/Zr PER values and plot away from the main fractionation trend suggesting quartz has been added to the system. Figure 3.1 lb is a PER plot which tests the effects of feldspar fractionation. Al l but one of the samples plot on or near a trendline of unit slope indicating that plagioclase and K-feldspar fractionation can explain the chemical variation in these rocks. This is in accordance with the trend described above for Figure 3.11a in which the samples also plotted close to the feldspar fractionation line, but were drawn off due to the effects of the cyrstallization of quartz and possibly pyroxene or hornblende, none of which would effect the slope of the trend on Figure 3.1 lb. Fractional crystallization modelling of trace elements The equilibrium distribution of a trace element between mineral and melt is often described by the Nemst distribution (partition) coefficient defined by: K,| = C i , „ / C i l 13.1] 80 Andesitic Melts (<63 wt. % Si02) Fs Hb Ap Mg CPx Zr 0.013 1.40 - 0.20 .162 Y 0.060 2.50 - 0.50 1.50 Ti 0.050 3.00 - 9.00 0.40 Dacitic to Rhyolitic Melts (>63 wt. % Si02) Fs Hb Ap Mg CPx Zr 0.100 4.0 0.1 0.800 0.6 Y 0.100 6.0 40 2.00 4.0 Ti 0,050 7.00 0.1 12.50 0.70 Table 3.2: Mineral/melt partition coefficients for the trace elements Zr , Y and T i between selected minerals and andesitic and dacitic to rhyolitic composition melts (summarized from Rollinson, 1993 after Pearce and Norry, 1979); Fs=feldspar, Hb=hornblende, Ap=apatite, Mg=magnetite, CPx=clinopyroxene. where Kd is the Nernst distribution coefficient, Ci,„ is the concentration of the trace element / in the mineral and Cn is the concentration of of the trace element / in the liquid (Mclntire, 1963). Apatite has a strong affinity for partitioning Y , illustrated by the very high partition coefficient of 40 for Y between apatite and a rhyolitic liquid (Table 3.2). Therefore, the crystallization of even small amounts of apatite wil l greatly deplete Y from the residual melts. Conversely, the presence of small amounts of apatite in the rocks will yield higher concentrations of Y for those samples. Hornblende wil l also partition Y , although the effect of hornblende crystallization on the content of Y is not as great as for apatite because of the lower partition coefficient of 6.0 for Y between hornblende and liquid. Z r contents are also affected by the precipitation of hornblende; Z r has a partition coefficient of 4.0 between liquid and hornblende. The effects of fractional crystallization of several mineral phases on the trace element concentrations of rocks can be modelled using the Rayleigh fractionation equation: C L = C n * F ( n - I ) [3.2] 81 where C L is the weight concentration of trace element in the liquid, C 0 is the weight concentration of the trace element in the parental liquid (before fractionation), F is the weight fraction of melt remaining and D is the bulk distribution coefficient of the fractionating mineral assemblage (Rollinson, 1993). The bulk distribution coefficient is defined by: D, = x,Kd, + x 2 K d 2 +... [3.3] where xi is the percentage proportion of mineral 1 in the rock and Kd i is the Nernst partition coefficient for element / in mineral 1 (Rollinson, 1993). The model for Rayleigh fractionation assumes that crystallized phases are removed from the melt and that no assimilation or magma replenishment from an external source occurs. To approximate the effects of fractional crystallization on the overall trace element abundances in the felsic rocks, the fractionation of varying proportions of the assemblage feldspar, hornblende, magnetite and apatite was modelled using the formula for Rayleigh fractionation (Equation [3.2]) and the partition coefficients summarized in Table 3.2. The variability in the trace element contents within each of the four felsic subgroups (A to D) wil l be tested by modelling the fractional crystallization of the minerals hornblende, magnetite and apatite, whereas the overall change in major and trace element abundances from group A to group D is likely controlled by the fractional crystallization of feldspar and quartz. Apatite and hornblende partition Zr and Y in varying proportions and magnetite strongly partitions T i . The precipitation of even small amounts of these phases wil l produce distinct changes in the trace element trends. For simplicity, 'feldspar' is used to represent a combination of fractionating plagioclase and K-feldspar; since petrographic evidence suggests that plagioclase is the most abundant feldspar in these rocks and since both plagiocalse and K-feldspar have similar low affinities for the trace elements Z r and Y , the partition 82 coefficients for Z r and Y between plagioclase and melt are used here to model the effects of total feldspar fractionation. Fractional crystallization was modelled in three major steps, each with its own initial and final melt compositions. These steps represent stages in the overall magmatic evolution from andesite to dacite and dacite to rhyolite, and are illustrated in Figures 3.8 and 3.9. The results of this modelling are summarized in Table 3.3. Since T i has been established as a reliable monitor of fractionation (i.e. it shows a systematic variation with respect to S i 0 2 and A1 2 0 3 ) , the changes in T i were accounted for first by fractionating magnetite only. Since magnetite fractionation alone does not account for the changes in Zr and Y , various amounts of feldspar, hornblende and apatite were also fractionated. Hornblende must be fractionated to offset the enrichment of Z r brought about by feldspar fractionation (fractionation of feldspar removes N a , C a , A l and S i , thus enriching Zr , Y and T i in the melt). Thus, as increasing amounts of feldspar are removed from the system, increasing amounts of hornblende and decreasing amounts of magnetite must be removed to yield the trace element trends observed in the volcanic rocks at Seneca. Apatite is included to balance the effects of fractionation on Y concentrations. Step 1: Andesite to dacite Step 1 represents the magmatic evolution from andesitic to dacitic compositions. Initial trace element concentrations of 82 ppm Zr, 21.5 ppm Y and 5280 ppm T i and 'target' final dacitic concentrations of 98 ppm Zr , 22 ppm Y and 4260 ppm T i were used. The fractionation of 17.5 % magnetite can account for the bulk of these changes in trace element abundances (Table 3.3), but petrographic evidence suggests that other minerals such as plagioclase and hornblende were likely involved. In addition, fractionation of magnetite only does not explain the steady decrease in A I 2 O 3 from andesitic to rhyolitic compositions. It appears that the trends can be best accounted for by fractionation of 8 to 15 % feldspar, 7.5 to 12 % hornblende, 5.5 to 9.5 % magnetite and small 83 amounts of apatite. The two samples that comprise group A have similar Z r and T i concentrations, but quite different Y concentrations (Fig. 3.9b). To account for the variability of Y , the effects of accumulation rather than removal of apatite were modelled (Table 3.3, steps la and lb). As such it appears that the Y abundances of these two samples can be accounted for by the accumulation of 1.5 and 4.1 % apatite in the rocks. The presence of apatite, which has been confirmed petrographically, supports such a model since these rocks have higher P 2 0 5 contents than rocks that would lie on the inferred fractionation trend from andesite to dacite. In addition, of these two samples, the rock with the higher Y content also contains considerably more apatite. Step 2: Group A to Group B felsic rocks The second step of the fractionation modelling represents the transition between dacitic and rhyodacitic composition rocks (group A to group B). Initial concentrations of 98 ppm Zr , 21.5 ppm Y and 4260 ppm T i and final melt or daughter rock concentrations of 1 17 ppm Zr , 24 ppm Y and 3120 ppm T i were used. As with step 1, the change in the trace element abundances modelled in step 2 can be explained by fractionation of magnetite only; in this case 19.5 % magnetite would be required to be removed to yield the observed trace element trends. However, once again this model does not explain the major element trends. It appears that a fractionating assemblage of 5 to 10 % feldspar, up to 10 % hornblende and 10 to 15 % magnetite best explains the observed trace element trends. The variability of Y within group B samples can again be explained by the accumulation of small amounts (<0.5%) of apatite in the rocks. Step 3: Group B to Groups C and D Step 3 represents the transition between rhyodacitic and rhyolitic composition rocks (group B to groups C and D). Initial concentrations of 117 ppm Zr , 24 ppm Y and 3 120 ppm T i and final melt/daughter rock concentrations of 133 ppm Zr, 26 ppm Y and 1800 ppm T i were modelled. 84 Step Co (ppm % mineral 1 ractionated Ci(ppm) Zr Y Ti Fs Hb Mg Ap Zr Y Ti 1 la lb 82 20 5280 - - 17.5 - 96.8 22.7 4202 8 7.5 9.5 0.25 98.1 21.5 4286 15 12 5.5 - 98.3 21.3 4279 82 20 5280 15 12 6 + 1.5 97.5 26.5 4214 82 20 5280 15 12 6 +4.1 96.0 36.8 4315 2 98 21.5 4260 - - 19.5 - 117.7 24.5 3119 - 2 18 - 1 16.5 24.1 3124 7.5 8.4 1 1 - 117.1 23.4 3147 10 10 9 - 117.0 23.1 3206 12 1 1 8.5 +0.5 1 17.1 24.5 3121 20 14.5 4.9 +0.5 1 17.1 23.9 3113 3 117 24 3120 - 1 1.5 15.7 +0.7 133.0 26.1 1811 10 15.5 9.5 - 132.8 22.7 1800 15 16.5 7.5 +0.8 133.0 25.7 1837 Table 3.3. Summary of the results of the effects of fractional crystallization on the abundances of the trace elements Zr, Y and T i . The trace element variations are examined for three steps representing magmatic evolution from andesite to dacite to rhyolite (steps 1 to 3 are indicated on Figure 3.9). The effects of fractionating different amounts of the minerals minerals feldspar, hornblende, magnetite and apatite on an initial trace element abundance, Co, were modelled using Rayleigh fractionation. C L represents the trace element abundance in the residual liquid or subsequent daughter rock following fractionation. Values marked with + indicate accumulation of a mineral in the daughter rock. (Fs=plagioclase + K.-feldspar, Hb=homblende, Mg=magnetite, Ap=apatite). The modelling of the trace element trends of this step was more difficult. This is attributed to the variability in the Zr and Y contents that is present in group C and D samples that is not present in groups A and B (Fig. 3.9). However, the final results showed that a fractionating assemblage similar to steps 1 and 2 can explain the trends (Table 3.3). In contrast to group A and B rocks, quartz is more common as a phenocryst phase in group D rocks, and to a lesser extent in group C rocks, suggesting it too may be a fractionating phase. However, like feldspar, the partition coefficients of the trace elements between quartz and a felsic melt are very low, and thus the effects of quartz fractionation would be very silmilar to those of feldspar fractionation. 85 3.8 DISCUSSION Major and trace element discrimination plots and ratios demonstrate that the volcanic rocks at Seneca are of dominantly transitional to calc-alkaline affinity and were formed in a volcanic island arc setting. Variation diagrams indicate that the suite is bimodal with a compositional 'gap' from 53 to 63 wt. % Si0 2 . It is not clear whether the volcanics sampled at Seneca are representative of the entire Weaver Lake Member as the data of Mahoney (J994) suggests that intermediate compositions are present elsewhere in the volcanic belt. As such, perhaps the bimodal nature of these rocks is entirely a local phenomenon. Mafic rocks are subdivided into basalts (<60 ppm Zr, Zr/Y < 3.5) and basaltic andesites (>60 ppm Zr, Zr/Y > 3.5). PER analysis shows that the two groups of mafic rocks can be related to each other by fractionation of the assemblage plagioclase-olivine-clinopyroxene, but that trends within the two groups can be explained by plagioclase fractionation alone. Felsic rocks with dacitic to rhyolitic compositions are subdivided into four subgroups that exhibit decreasing contents of T i 0 2 with increasing Zr and S i0 2 contents. PER analysis does not disprove the hypothesis that groups A, B and C are related by the fractionation of the assemblage feldspar-quartz±pyroxene and/or hornblende. The fractionation of feldspar alone can explain most of the major element variation. Trace element trends within the felsic rocks can be accomodated by 30 to 40 % fractional crystallization of the assemblage feldspar-hornbIende-magnetite±apatite (feldspar>hornblende>magnetite). Quartz may also be involved, but will have the same effects on the trends as feldspar fractionation (i.e. residual enrichment of Zr, Ti and Y). The results of the PER analysis and the modelling of trace element fractionation do not produce identical results. The PER analysis suggests that plagioclase fractionation has the greatest effect on the geochemical trends (Figure 3.11b) whereas the fractional crystallization modelling 86 suggests that the trace element trends can be accomodated by fractionation of similar amounts of plagioclase, hornblende and magnetite. In this modelling, the amounts of fractionated magnetite seem to be reasonable since there are few other minerals i f any that can account for the rate of decrease in the T i contents of the felsic rocks. Magnetite is present in some of these rocks, and where it was not observed petrographically, its presence was inferred by the magnetic nature of some samples. The flaw in the model may be in the assumption that hornblende is the only fractionating mineral that partitions Zr (clinopyroxene also partitions Zr , but to a lesser degree than hornblende). The fractionation of even very small amounts of zircon would greatly deplete Zr in the residual melts and would eliminate the need for hornblende as an abundant fractionating phase. Zircon fractionation could also balance the effects on the Zr contents if greater amounts of feldspar were fractionated. However, the fractionation of much larger amounts of feldspar likely would have caused larger Eu anomalies than are observed in these rocks. The more silicic samples of groups C and D have more variable Zr contents than the other groups and form two distinct linear trends. It is possible that zircon fractionation has caused these trends. It seems unlikely that this variability is due to alteration because any alteration that might be present would be expected to also effect the samples in groups A and B. If zircon fractionation is the source of these trends, then a change in the physical conditions or composition of the magma body must have occurred between the formation of groups B and C since groups A and B show-very little Zr variation. Watson (1979) states that any felsic magma wil l likely contain zircon crystals since the saturation level of zircon in these composition melts is low and is strongly dependant upon the molar ( N a 2 0 + K 2 0 / A l 2 0 3 ) of the melts. Watson's experimental studies showed that a change from 1.0 to 2.0 in the molar (Na 2 0+K 2 07A1 2 0 3 ) of melts in equilibrium with zircon resulted in a 500-fold increase in the solubility of Z r in these liquids. Therefore, considering the felsic rocks at Seneca, it is possible that the molar ( N a 2 0 + K 2 0 / A l 2 0 j ) in groups C and D were 87 sufficiently different from groups A and B to cause a decreased solubility of Z r in the melts and thus facilitating zircon crystallization in groups C and D. However, it is difficult to reliably calculate and test these molar ratios in the rocks at Seneca due to the mobility of the alkalies even in the least altered samples. In summary, it is not clear whether or not fractional crystallization alone can explain the relationships between the mafic and felsic rocks and between the four felsic subgroups. Hall (1987) suggests that the derivation of rhyolites from differentiation alone from a basaltic parent is possible, but that only small volumes of rhyolite can be produced. This would not explain the large volumes of felsic rocks present in the study area. Another process such as a continual replenishment of the basaltic parent would have to be invoked. It is also apparent that the P E R analysis and fractional crystallization modelling only adequately describe a 'global fractionation trend' for the dacitic to rhyolitic composition rocks. There appears to have been some separation or 'pooling' of different smaller felsic magma batches from a larger source. Fractionation within the source magma body yielded the global fractionation trends. The smaller magma bodies perhaps rose to higher cmstal levels at different stages in this global evolutionary trend and subsequently continued to evolve and fractionate further. Such a scenario seems to be necessary to explain the subtrends that are present amongst the minor and trace elements. 88 CHAPTER 4 ALTERATION 4.1 I N T R O D U C T I O N Strong hydrothermal alteration on the Seneca property is restricted to discordant, laterally discontinuous zones around the mineralized stockworks in the Fleetwood and Vent zones and to the areally extensive ore zone conglomerate in the Pit Area. Quartz-sericite-pyrite is the dominant alteration assemblage with lesser amounts of chlorite, epidote and calcite. This chapter will describe the petrographic and geochemical characteristics.of this alteration and will attempt to quantify the effects of the hydrothermal processes using the mass balance method of MacLean (1990). 4.2 D I S T R I B U T I O N O F A L T E R A T I O N Pit Area The most areally extensive zone of alteration at Seneca occurs in the Pit Area and is associated with the ore zone conglomerate. Moderate to intense silicification and serialization is the dominant alteration style in this unit. In places the sericite alteration has completely destroyed the matrix material leaving only some of the more resistant lava clasts as evidence of the unit's original fragmental texture. The alteration is confined to a shallowly dipping interval of varying thickness which was intersected in drillholes at depths varying from near surface to a maximum depth of 150 m. The surface projection of this zone an area of approximately 500 m by 500 m around the Pit Area. One drillhole located 1 km to the northeast of the Pit Area intersected a strongly altered and weakly mineralized horizon that is lithologically very similar to the OZC intersected in drillhole 85-03 (Pit Area) suggesting that the Seneca horizon may be much more laterally extensive. The thickness of the altered OZC varies from 2 metres to over 15 metres. Although the alteration in this unit is very strong and is widespread, it is essentially stratabound and there is no apparent stockwork zone immediately 89 underlying the O Z C that could be interpreted as a feeder zone. It appears that the permeability of the coarse, less well sorted O Z C provided a more favourable conduit for the hydrothermal fluids compared with its bounding strata of felsic flows and fine grained volcaniclastic rocks. Fleetwood and Vent Zones In contrast to the Pit Area, hydrothermal alteration in the Fleetwood and Vent Zones is discordant and is not as areally extensive. Alteration is confined to discrete stockwork zones that can reach over 50 metres in vertical extent. These zones of stockwork-related hydrothermal alteration all lie along a northwest-southeast striking trend; drillholes located to the northeast of this did not intersect any zones of strong alteration. The most extensive individual stockwork outcrops at surface and is intersected by numerous drillholes in the Vent Zone. This area of strong silicification and sericitization extends over an area of 100 to 200 metres in diameter and can be traced to depths of over 100 metres. The stockwork alteration in the Fleetwood Zone to the northwest of the Vent Zone is less extensive both laterally and vertically. Although alteration in the Fleetwood Zone occurs at depths greater than 100 metres below surface, lithological relationships suggest that it occurs at the same stratigraphic interval as the alteration in the Vent Zone which occurs at surface and extends down to over 100m below surface. The stratabound alteration in the Pit Area is also along strike of the trend of the stockworks in the Fleetwood and Vent Zones suggesting a possible larger-scale structural control on the hydrothermal activity, as well as a possible genetic relationship between the different alteration zones. However, no large-scale controlling structures were recognized in the limited drillcore. 4.3 CHARACTERIZATION OF HYDROTHERMAL ALTERATION Sericite-quartz ± pyrite is the dominant hydrothermal alteration assemblage at Seneca. This alteration generally causes a bleaching of the rock or the formation of sericitic envelopes around clasts (Plate 4.1). The only real variability is in the intensity of the alteration and in the amounts of chlorite Plate 4.1a: Drillcore sample of intensely silicified and sericitized rhyodacite breccia with matrix-filling quartz and sulphides from the Fleetwood Zone stockwork (DDH 91-16). The late quartz-sulphide phase cuts a previous alteration which formed the altered envelopes around the breccia clasts Plate 4.1b: Altered and mineralized basaltic fire fountain debris. This unit underlies the Vent Zone stockwork and has apparently been affected by the same hydrothermal processes that formed the stockwork. It is the position of this unit that allows correlation of the Vent mineralization with the Fleetwood mineralization. (Note the texlural similarities with Plates 2. la and 2.4a). Plate 4.2a: Photomicrograph of a moderately altered FP rhyodacite. This sample is transitional between the least altered samples shown in the previous chapter and the intensely altered rocks such as shown below. The matrix is altered to sericite-quartz, but the phenocrysts remain relatively unaltered. (X-Nicols; Field of view = 1.25 mm). Plate 4.2b: Photomicrograph of an intensely hydrothermally altered FP rhyodacite. The phenocrysts and groundmass have been converted to quartz, sericite and minor chlorite, but the remnant feldspar phenocrysts are still discernible. (X-Nicols; Field of view = 1.25 mm). 92 and quartz present. Sericitization of feldspar is almost ubiquitous throughout the property and is present to a small degree even in the least altered samples. Least altered samples generally have small amounts of sericite present in the aphanitic groundmass and often the feldspar phenocrysts have a 'dusting' of sericite. As the intensity of alteration increases, the glassy groundmass becomes almost completely converted to a combination of quartz and sericite ± chlorite and the phenocrysts become increasingly sericitized (Plate 4.2). Phenocrysts in the most intensely altered samples from the stockworks are completely converted to sericite ± quartz (Plate 4.2b). However, remnant phenocrysts are still discernible macroscopically and microscopically even in the most strongly altered rocks. The Fleetwood Zone stockworks in particular have undergone a large degree of silicification, especially in their upper portions. The alteration in upper parts of the Fleetwood stockworks is often associated with flow breccia and hyaloclastite, lithologies that likely favoured movement of hydrothermal fluids. In these zones, the interstices to the angular, hydroclastic fragments are filled by quartz and fragments with altered selvedges are cut by quartz veins (Plate 4. la). Quartz in the interstices is also often intergrown with sulphides both of which appear to have formed late in the alteration history. The lower parts of these altered stockworks are hosted by more massive rocks in which silicification is not as strong. The rocks are often softer due to sericitization and quartz-sulphide veins are more common. The host rocks of the Vent Zone stockwork are also quite massive. Quartz-sericite alteation is pervasive, but greater amounts of chlorite are present here than in the Fleetwood Zone. Silicification is also stronger in the upper parts of this zone than in the lower parts. Quartz-sulphide veins are very common in the Vent Zone and often have dark alteration envelopes. (It is not clear petrographically if diese selvedges are simply more chloritic than the adjacent rocks). The underlying mafic lavas, where observed, are also often moderately altered. Most commonly these rocks have been bleached by 93 silicification and are accompanied by disseminated and veinlet sulphides (Plate 4. lb) . Some epidote alteration is also present, but it is not clear i f this is directly related to the hydrothermal activity. 4.4 QUANTIFICATION OF ALTERATION PROCESSES The geochemical variability that characterizes the volcanic rocks at Seneca is attributed to a combination of igneous differentiation processes and varying degrees of hydrothermal alteration. The previous chapter outlined possible igneous processes that could account for the variability amongst a set of least altered samples. This chapter will account for the remaining chemical variations by relating the altered samples to this least altered subset by the processes of mass gain and mass loss caused by hydrothermally-induced mineralogical changes and the variable mobility of the major elements. Alteration indices and mass change calculations will be used to characterize the chemical changes and to illustrate variations in alteration effects in different areas of the Seneca property. This chapter wil l deal entirely with alteration amongst the felsic samples (dacites to rhyolites) since they comprise the greatest portion of the data set and the mafic samples have not undergone a large degree of hydrothermal alteration. 4.4.1 I Z A W A A L T E R A T I O N D I S C R I M I N A T I O N D I A G R A M Changes in the bulk compositions of felsic samples can be related to mineralogical changes represented by the coordinates A I 2 O r M g O - ( C a O + N a 2 0 + K 2 0 ) where whole rock data have been recalculated as molar proportions (Izawa et al. , 1978). On such a diagram least altered samples plot close to the feldspar end-member (Figure 4.1). Increasing degrees of alteration destroys the feldspar and displaces samples away from the feldspar end-member due to the loss of CaO and N a 2 0 . The most intensely altered samples plot along a tie-line between chlorite and sericite compositions representing maximium loss of CaO and N a 2 0 and varying gains of M g O (chloritization) and/or K 2 0 94 • least altered fj moderately altered • strongly altered AIA Figure 4.1. Izawa alteration discrimination diagram. The plot illustrates the mineralogical changes that occur within the felsic rocks at Seneca with increasing degrees of alteration. Geochemical data are recalculated as molar proportions. Least altered samples plot close to the feldspar end-member; intensely altered samples plot along a tie-line between chlorite and sericite compositions. 95 0.400 0.350 « 0.300 o E rr 0.250 LU Q_ N 0.200 * 0.150 + CO O 0.100 0.050 0.000 F e l d s p a r f rac t iona t ion (m=1) • * C a , N a m e t a s o m a t i s m S e r i c i t e a l te ra t ion , - - (m=1/3) ^ S tockwo rk a l te red s a m p l e s Ch lo r i t e a l te ra t ion (m=0) 0.1 0.2 0.3 A I / Z r P E R (molar) 0.4 0.5 Figure 4.2. (2Ca+Na+K)/Zr vs. Al/Zr PER plot. The diagram illustrates the departure of felsic samples from a fractionation trend with unit slope toward a line with a slope of 1/3 representing the total loss of Na and Ca by the complete sericitization of feldspar. Due to the lack of strong chlorite alteration of felsic rocks at Seneca, samples do not plot below this sericite alteration line. 96 (sericitization). Such trends are clearly visible on Figure 4.1. The intensely altered stockwork samples (solid symbols on Figure 4.1) can be subdivided into two groups on this plot; one group of samples from the Fleetwood Zone stockworks plots closer to the sericite end-member composition, whereas the second group taken entirely from the Vent Zone stockwork plots closer to the chlorite end-member suggesting a relatively greater involvement of MgO in the hydrothermal processes that formed this stockwork. Although this diagram is useful in relating some components of the bulk chemistry to mineralogical changes in the rock during alteration, it may be slightly misleading in that it does not consider additions or losses of other elements such as S i 0 2 which can be significant in V M S systems. 4.4.2 PER A N A L Y S I S An alternative method of illustrating the effects of hydrothermal alteration on the alkali content of the felsic rocks is by using Pearce element ratio analysis. A plot of Al/Zr PER versus (Na+K)/Zr PER was used in the previous chapter to establish a primary trend amongst the least altered samples that can be related to fractionation of dominant!}' feldspar. A similar plot that has an abscissa of (2Ca+Na+K)/Zr PER (Figure 4.2) can best display the effects of varying degrees of sericitization or chloritization on the composition of the felsic rocks at Seneca. Destruction of feldspar by Ca and Na metasomatism displaces a sample from its unaltered precursor composition on the fractionation trend having a unit slope toward the abscissa (assuming A120:, is immobile). Samples that have been completely sericitized, such as those from the Fleetwood and Vent Zone stockworks, lie along a line with a slope of 1/3 representing a total loss of Na by the reaction: 3NaAISi r,O s + K + + 2rT <=> KAI 3 Si 3 CMOH) 2 + 3Na + + 6Si0 2 (Stanley and Madeisky, 1994) 97 Sample Location 87-11-75 V E N T 39.29 87-12-301 F L E E T W D 61.07 87-12-69 F L E E T W D 53.63 87-12-147.4 F L E E T W D 90.55 91-10-234 F L E E T W D 79.56 91-10-210 F L E E T W D 30.41 91-10-180 F L E E T W D 42.47 91-10-118 F L E E T W D 2.13 91-10-99 F L E E T W D 16.21 91-16-61 F L E E T W D 62.25 91-16-152 F L E E T W D 37.96 91-16-159 F L E E T W D 90.50 91-16-161 F L E E T W D 79.32 91-16-307 F L E E T W D 81.31 92-27-71 F L E E T W D 45.58 92-27-85 F L E E T W D 50.15 92-27-177 F L E E T W D 55.19 92-31-281 F L E E T W D 23.25 92-31-244 F L E E T W D 63.91 92-31-225 F L E E T W D 60.47 92-33-75 33-ZONE 38.21 92-39-71 F L E E T W D 55.53 92-39-200 F L E E T W D 23.36 93-VT-01 V E N T 39.0(1 93-VT-02 V E N T 100.29 93-VT-03 V E N T 108.11 93-VT-04 V E N T 99.05 93-FW-46 F L E E T W D 60.51 93-FW-49 F L E E T W D 44.98 93-FW-51 F L E E T W D 27.44 93-FW-52 F L E E T W D 97.78 Sample Location 83-02-277 PIT A R E A 37.38 83-02-320 FIT A R E A 32.00 83-02-186 PIT A R E A 30.23 83-02-67 PIT A R E A 33.79 83-06-50 PIT A R E A 27.19 83-07-27 PIT A R E A 21.16 83-07-54 PIT A R E A 45.80 83-10-91 PIT A R E A 68.88 83-11-24 PIT A R E A 44.21 83-11-40 PIT A R E A 36.15 85-03-70 PIT A R E A 31.75 85-03-126 PIT A R E A 19.73 91-02-28 PIT A R E A 29.39 91-02-85 PIT A R E A 18.15 91-02-170 PIT A R E A 26.86 93-SN-44 PIT A R E A 61.27 93-SN-45 PIT A R E A 40.31 93-SN-46 PIT A R E A 42.83 Table 4.1. Summary of the sericitization index (r S E R) calculations highlighting the greater average degree of sericitization of feldspar that has occurred in the Fleetwood and Vent Zones relative to the Pit Area. The index is calculated using the formula rSER = (1 - |(Na + K)/Z) / ((Al/Z) -x0)]) x 3/2 x 100%, where Z is the conserved element and x„ is the abscissa intercept on a (Na + K)/Z vs. (Al/Z) PER plot. Sericitization Index Stanley and Madeisky (1994) present a sericite alteration index in which the degree of sericitization is related to the slope of the line between a data point and the abscissa intercept on a (Na+K)/Zr vs. Al/Zr plot and can be calculated using the following equation: 98 r SER = (1 - [(Na + K)/Z) / ((Al/Z) - x0)J) x 3/2 x 100% where r S E R is the relative amount of sericitization of feldspar, Z is the conserved element and x ( 1 is the abscissa intercept (Stanley and Madeisky, 1994). Although Ca is a component of the ordinate axis in Figure 4.2, C a has not been incorporated into the sericitization index in this study because of the presence in some of these rocks of minerals such as calcite and epidote which do not appear to have been hydrothermally derived and which would partially obscure the effects of the true hydrothermal alteration. This index has been calculated for a representative set of the F P and Q F P flow and intrusion samples from Seneca and the results are included in Table 4.1. The amount of sericitization varies from less than 10 % for the least altered, post-hydrothermal intrusions to greater than 90% for the strongly altered stockwork samples. Although samples such as 91-10-1 18 and 92-39-200 are close to known areas of hydrothermal alteration, they show relatively small amounts of sericitization. This suggests that either they were intruded into the sequence after the main hydrothermal activity and thus did not experience the degree of alteration of other similar synvolcanic intrusions (e.g. sample 91-16-307), or these intrusions had a lower permeability making them more resistant to alteration. Stockwork samples such as 93-VT-02, -03 and -04 and 91-16-159 have experienced the greatest degree of sericitization. The sericitization index also highlights the difference in the overall degree of alteration between the Pit Area and the Fleetwood and Vent Zones. The set of samples in Table 4.1 were chosen to represent all of these areas. In general, the Pit Area has lower sericitization indices, less than 40 % (often < 35%>), compared to the Fleetwood and Vent Zones where indices are generally greater than 45 % and frequently are > 50 %. The hydrothermal activity that created the stockwork zones also created a zone of moderate to strong sericite alteration throughout the entire Fleetwood and Vent Zone but had a much smaller effect on rocks in the more distal Pit Area. 99 4.4.3 M A S S C H A N G E C A L C U L A T I O N S 4.4.3.1 METHODOLOGY The method of MacLean (1990) is used in this section for calculating material changes in altered rock series. The method uses immobile and incompatible elements to determine the precursor compositions of the altered rocks and is described in detail below. Establishment of fractionation trends The first and perhaps most critical step in the mass change calculations is the establishment of best fit fractionation lines which represent the primary variability imposed by igneous processes amongst the volcanic series. Immobile element pairs are utilized to establish such trends. Igneous incompatible elements such as Zr or Y are often used in studies such as this as a monitor of fractionation. However, Z r and Y are not used in this study because of their inconsistent behaviour and possible compatibility as demonstrated in the previous chapter. T i 0 2 has been chosen instead as the best monitor of fractionation. Although T i 0 2 and A1 20 : , have been shown to be compatible in some units at Seneca, they appear to behave uniformly over the entire range of compositions of the felsic rocks and hence they are useful in monitoring fractionation. MacLean and Kranidiotis (1987) have shown that A 1 2 0 3 and T i 0 2 generally have high degrees of immobility in typical hydrothermal systems and are thus a useful pair of elements with which to establish a regression line or fractionation trend. Mass change effects on binary plots Immobile elements are concentrated by processes involving net mass loss and are diluted by processes involving mass gain (MacLean, 1990). The effects of mass changes on a binary plot of immoble elements are illustrated on Figure 4 .3 . Although these immobile components have not been added to or removed from the system during alteration, the addition or removal of the mobile components such as S i 0 2 changes the overall system size and will make it appear as though the 100 a) 20 CO 3,10 < Net M a s s L o s s (immobile elements are concentrated) Rhyo l i te Dacite Fractionation trend (y=5.78x+11.54) Net M a s s G a i n (immobile elements are diluted) 0 L , . . , , , . ^ — ^ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 T i 0 2 (wt%) b) 20 CO ° 10 < O Net m a s s loss •Ne t .mass ga in I I least altered samples 3 moderately altered samples • strongly altered samples + volcaniclastic rocks 0 0.1 0.2 0.3 0.4 0.5 0.6 T i 0 2 (wt %) 0.7 0.8 0.9 1.0 Figure 4.3. Al 2 GyTi0 2 irnmobile element binary plot, a) illustrates the use of a set of least altered samples to establish a fractionation trend for the felsic rocks (dacites to rhyolites) at Seneca and the effects of mass gain and mass loss on a binary plot of immobile elements, b) is a plot of all felsic samples and illustrates the effects of the dominant mass gains that have diluted the immobile elements. Figure b) is used to determine the precursor Ti0 2 compositions. 101 quantities of immobile elements have changed. This apparent change is due to whole rock geochemical data being totalled to a sum of 100 wt. %; in reality, the amount of the immobile element has remained fairly constant, but the system size has changed by mass loss or mass gain. Determining precursor compositions The effect of closure, a mathematical artifact, is a common problem when dealing with geochemical data that are reported as weight percents and are 'forced' to sum to equal 100 %. However, MacLean (1990) presents a simple procedure which circumvents the effects of closure. An altered sample's precursor composition is calculated by determining the intersection of the alteration line with the best fit fractionation line on a binary plot; in essence an altered sample is moved along an alteration line back to its 'original' position on the fractionation trend. The precursor compositions of a series of samples are determined on an element-by-element basis in which each of the mobile elements are plotted individually against the monitor of fractionation and a separate regression line is fit to the least altered samples on each plot. For example, a regression line can be established for the least altered samples on a S i 0 2 vs. T i 0 2 plot and the mass changes of S i 0 2 for the altered samples are related to their relative displacement from this line. Determining reconstructed compositions T o eliminate the closure effect inherent in the presentation of whole rock geochemical data for altered rocks, MacLean calculates a 'reconstructed composition' which keeps the concentration of the immobile elements constant while adjusting the mobile element concentrations to reflect their changes during alteration. If mass change has occurred, an altered sample would appear to have lost or gained a certain amount of an immobile element due to the fact that whole rock data is totalled to 100%. Since by definition this is not the case, all of the elements need to be readjusted up or down to eliminate this effect. Al l components in the altered sample are adjusted by a mass factor which is the ratio 102 between an immobile element concentration in the precursor and its concentration in the altered rock. The reconstructed composition is calculated as follows: R C = wt. % component (altered rock) * ( IM (precursor) / I M (altered rock)) where RC is the reconstructed composition and IM is the immobile element monitor (e.g. Ti0 2 ) (Maclean, 1990). Thus, the reconstructed composition represents the net mass of a rock that has gained or lost mobile components. If a sample has gained mass during alteration, for example by silicification, then the reconstructed composition will total over 100 wt. %. Conversely, if a rock has lost mass by chloritization or sericitization, then the reconstructed composition will total less than 100 wt. %. A mass change in an individual component is the difference between the precursor composition and the reconstructed composition. 4.4.3.2 MASS CHANGE CALCULA TIONS FOR FELSIC ROCKS A T SENECA The step by step calculation of mass changes for the most strongly altered felsic rocks at Seneca using the method of MacLean (1990) as described above is shown in Table 4.2. The procedure for these calculations is described in the following sections. Fractionation trends The felsic rocks at Seneca form a continuous compositional trend from dacitic to rhyolitic compositions as illustrated in the previous chapter. As such it is possible for the altered samples to have been derived from multiple precursor compositions. T i 0 2 was chosen as the best monitor of fractionation. Al 2 03 was chosen as the immobile element monitor. Figure 4.3a illustrates the derivation of a regression line or fractionation trend for the least altered samples on an A l 2 0 3 - T i 0 2 binary plot. Figure 4.3b relates all of the felsic samples to the same fractionation trend. This plot is used to determine the concentration of T i 0 2 in the precursor for all samples by determining the 103 o © e > c eS T3 O O St <u cu 0> JS u o 13 CU es "5JD c o . o c _© _« u "3 u CU 6D e es J= cj _<u es H 111 £-> vo 00 s 3 NO [— ~ 3 • M O 111 o * l i s o 111 i II * i l l . ; * p : n II 2 aw**; « 111; a ;:;::«::: vo :::**::: = HI ? ea !;©;;:; u 0 0 ^  ro CN CO o ~- o o vo C\ rs PF LL. 3 - C * e^  = £ >, £ -r o O - 3 - r O O r o O O r o O . 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Fractionation trends were derived using a least squares regression line for all major elements versus TiO? for the least altered sample set. Some of these trends are illustrated in Figure 4.4. Precursor compositions Since the precursor T i 0 2 concentrations for all the samples were calculated in the previous step, these values can be substituted for x in the equations for the fractionation lines shown in Figure 4.4. This establishes an altered sample's precursor position on the fractionation trend and allows the precursor concentration for each of the elements to be determined by reading off the ordinate axis on each individual binary plot. Once the precursor concentrations of all elements in a sample were determined, the mass factors were calculated and the precursor compositions were normalized to 100%. Reconstructed compositions and mass change calculations The reconstructed compositions were calculated by multiplying the untreated data by the T i mass factor. Finally the actual mass changes for all samples were calculated by subtracting the precursor concentration from the reconconstructed concentration for each component. The net mass changes were calculated by summing the change in each component for each sample. Negative values indicate mass loss while positive values indicate mass gain. Results Increasing degrees of alteration in the felsic rocks at Seneca correspond to greater negative mass changes or losses of N a 2 0 and CaO, but increasing positive mass changes or gains of K 2 0 . This is due to the conversion of plagioclase to sericite. Changes in other elements such as S i 0 2 and M g O are more variable and wil l be discussed later. 106 107 Mass changes in dacitic to rhyolitic volcanics O CM co 2 1 0 -1 -2 -3 -4 -5 -6 rrr o o CM (0 Z + o ro O -2 -4 8 b .a O . « .Q. . . <t_e . • a 0 ° ' least altered s a m p l e s O O o o O least altered » moderately altered • strongly altered o intense alteration • • f # * \ Fleetwood Zone Vent Zone altered samples altered samples - 2 - 1 0 1 2 3 A K 2 0 (Wt%) Least altered s a m p l e s '*% Fleetwood Zone ! <* <>. altered samples ' %, 1 intense alteration Vent Zone altered samples -2 0 1 2 A M g O (w t%) Figure 4.5. Mass change diagrams. These diagrams plot the mass changes in some of the important elements involved in the hydrothermal alteration of the felsic rocks at Seneca. Both plots demonstrate the increasing loss of NajO and CaO with increasing alteration. Figure b) illustrates the greater gains of MgO that characterize the Vent Zone stockwork samples. 108 Figure 4.5 illustrates some of the results of the mass change calculations. The least altered samples (open symbols) cluster around the origin (zero mass change) whereas most of the strongly altered stockwork samples (solid symbols) plot along an approximately horizontal trend representing a complete destruction of plagioclase and, thus, a maximum loss of N a 2 0 and CaO. Samples cannot plot below this because all of the Na and Ca in the precursor rock has been lost from the system. Figure 4.5a shows that of the intensely altered samples, all have experienced gains in K 2 0 , but the Fleetwood stockwork has experienced greater gains than the Vent Zone stockwork. A similar split in the intensely altered samples is evident in Figure 4.5b. The Vent Zone samples have undergone gains in MgO whereas the Fleetwood samples have experienced losses in MgO. Figure 4.6 summarizes some of the mass changes for these stockwork samples. The bars on the graph are arranged from left to right corresponding to the samples' relative locations on Figure 2.1. The actual mass change values are summarized in the table below the graph. It is clear from this figure that changes in S i0 2 have had the greatest overall effect on the net mass changes. Whereas all but one of the samples have had a strong loss of Na 2 0 and CaO, the samples from the Fleetwood Zone have experienced much greater gains in S i0 2 than the Vent Zone. The Fleetwood Zone, in general, appears to have undergone an overall mass gain, whereas mass losses are more prevalent in the Vent Zone. The mass changes also appear to van' stratigraphically. For example, the Fleetwood samples that have undergone strong gains in S i0 2 and K 2 0 (numbers 1,3,4 and 7 in Figure 4.6) all occur at a higher stratigraphic level (ie. at a shallower depth), perhaps closer to the paleoseafloor. The samples that have lost S i0 2 while gaining MgO and K : 0 (Fig. 4.6. numbers 5 and 6) occur at a lower stratigraphic interval. A similar trend is evident for the Vent Zone samples. Shallower samples (numbers 8 and 10) have had the greatest gains in S i0 2 while the deeper samples have had negative or 109 +40 +30 +20 + 10 0 -10 1 •20 • \ K . , 0 H A S i 0 2 £•« A M g O ' A C a O A N a 2 0 Fleetwood Zone Stockworks 10 11 12 Vent Zone Stockworks Fleetwood Zone Vent Zone 1 2 3 4 5 6 7 8 9 10 11 12 87-12-147 91-10-180 91-10-234 91-16-159 91-18-252 92-28-132 92-31-244 86-28-67 86-28-103 93-VT-02 93-VT-03 93-VT-04 A L T F P A L T F P A L T F P A L T F P A L T F P A L T F L O W A L T F P A L T F P A L T F P A L T F P A L T F P A L T F P A S i 0 2 17.01 16.87 -1 .12 18.69 35.45 -4.73 -16.03 4.80 -1.53 9 .68 -1 .48 1.28 A T i O , -0 .02 -0.01 -0.01 -0.01 -0.02 -0.01 -0.02 -0.01 -0 .02 -0.02 -0 .02 -0.02 A A l j O " , -0 .55 -0 .50 -0 .40 -0 .36 -0 .59 -0 .48 -0 .68 -0 .50 -0 .52 -0 .54 -0 .52 -0.53 A F c O -0 .32 0.93 0.05 -0 .79 1.38 -0.56 -1.06 -0 .32 -0 .86 0.96 -0 .41 0.18 A M n O -0 .07 -0 .10 0.04 -0 .09 -0.11 0.06 -0.03 0.00 0.01 -0.03 0.04 -0.04 A M g O 0.03 -1.21 2.44 -0 .03 -1 .29 0.46 2.06 2.02 2 .73 2 .32 2 .50 1.78 A C a O -0 .85 -0 .63 -0 .38 - 0 . 4 9 -0.90 0.42 -0 .19 -0 .86 -0 .86 -0 .54 -0 .57 -0 .88 A N a 2 0 A K 2 0 -4 .99 -4 .24 -2 .99 -4 .05 -4 .89 0.68 -1.88 -5 .00 -5.11 -4 .92 -5 .12 -5 .12 2.82 5.13 0.46 1.45 3 .07 -1.03 0.45 1.88 1.93 2.03 1.76 2.43 A P 2 0 5 0.01 0.02 0.02 0.05 -0.03 -0.01 0.00 0.01 0.0 1 -0.01 0 .00 -0 .02 T o t a l 13.07 16.24 -1 .89 14.18 32.05 -5 .19 -17 .40 2 .02 -4.21 8.93 -3.81 -0.98 Figure 4.6. Summary of mass changes for strongly altered stockwork samples. Samples 1 through 7 are from the Fleetwood Zone and samples 8 through 12 are from the Vent Zone. The Fleetwood samples show strong gains in S i 0 2 and K 2 0 in their upper portions and overall mass gains. The Vent samples show similar trends, but have experienced MgO mass gains throughout and have had small positive to negative net mass changes. 110 o CM o CO MgO O eg CO < • < < • < a> jE "ca 3 tm c cn c cd X 5 — O CD 1 3 CD to co cn CD c O 8 o ! c _ CO o •r-<D <n CO -O coz.2 c 'to c y) p Q . ™ CD c •fO S. •v - O ) M <o rt . - . . E E 1 Ms (/) 5 to™ E s E o O . tn <B cn c co O (/) w CO o CM + o o oo X Q D O CM •s I * S S « . • s o c c ^ • d o ™ o O <N P oo £; -g K ^ ^ .S U« « cn CU r-CD G f l . S •43 c ca c3 too C 4 = o D ° c J i l l O N c cu | S 8 cn <D o o a ) . t i - r GO o ' a -c O cn rtU •§; 1^  43.2 "2 | fc WO « CD w ^ i-2 s §> 2 IS « o . o r7" - C •*-» 1> M-l W OT 00 111 1 Plate 4.3a: Photomicrograph of Sample 91-10-180, a strongly quartz-sericite altered felsic rock. This alteration assemblage is typical of the upper portion of the Fleetwood Zone. The destruction of feldspar is clearly visible. This sample has experienced an overall mass gain due to the strong addition of silica. (X-Nicols; Field of view = 1.25 mm). Plate 4.3b: Photomicrograph of Sample 91-18-252. a strongly quartz-sericite altered felsic rock from the Fleetwood Zone. This rock has experienced a similar style of alteration as the sample pictured above, and has also undergone net mass gain. (X-Nicols; Field of view = 5 mm). 113 Plate 4.4a: Photomicrograph of Sample 92-31-225. a strongly sericite-chlorite-quartz altered felsic rock. This type of alteration assemblage is typical of the middle to lower parts of the Fleetwood stockwork and parts of the Vent zone stockwork. It is gradational from the assemblage shown in Plate 4.3 and the assemblage pictured below. (X-Nicols; Field of view = 1.25 mm) Plate 4.4b: Photomicrograph of Sample 92-31-244, a strongly sericite chlorite altered felsic rock. This type of alteration is typical of the Vent Zone and the lower portions of the Fleetwood stockworks. Such samples have generally experienced mass gains of K 2 0 and MgO, but net mass losses. (X-Nicols; Field of view = 1.25 mm). 114 only small positive mass changes in S i0 2 while all samples maintain approximately consistent gains in MgO and K 2 0 . This vertical alteration zonation is also illustrated in Figures 4.7 and 4.8 which graphically depict some of the downhole mass changes that have occurred at depth around the Fleetwood and Vent Zone stockworks (see Figure 2.1 for drillhole locations). Drillholes 91-18 and 91-16 on Figure 4.7 have had very strong gains in S i0 2 and K 2 0 in the upper 10 to 20 metres of their stockwork zones corresponding to an alteration assemblage of quartz and sericite (Plate 4.3). These mass gains decrease downhole while mass gains in MgO become larger. This transition corresponds to a change toward an alteration assemblage dominated by sericite and chlorite (Plate 4.4). Although quartz is sometimes present in varying proportions in this lower zone, S i0 2 has generally been lost from these samples. This S i0 2 loss is particularly strong in the lower part of drillhole 91-18 (Figure 4.7). Mass changes in rocks above the stockwork zones are much smaller and are more variable. They reflect mostly the alteration of volcanic glass to chlorite and sericite. However, the mass losses of Na 2 0 are not insignificant and may indicate that hydrothermal activity continued after deposition of these units. Drillhole 91-10 (Figure 4.8) has a similar upper quartz-sericite alteration zone and a lower sericite-chlorite zone. S i0 2 gains are much greater in the hangingwall units of this drillhole, but this may be due to excess primary quartz in the groundmass of these aphanitic intrusions. The stockwork zone intersected in drillhole 86-13 from the Vent Zone differs from the others in that MgO gains occur throughout an upper quartz-sericite-chlorite alteration zone and a lower sericite-chlorite zone. Another common style of alteration present at Seneca is the variable silicification and chloritization that gives the felsic flows their distinctive banding. Although this alteration appears quite strong, it does not result in large mass changes. Despite the amount of chlorite present, the mass changes in MgO are quite small ( <0.75 wt. %). The only significant mass gains that have occurred in these rocks are for S i0 2 which has increased usually by less than 10 weight percent. These samples 115 also have not experienced large losses of N a 2 0 or CaO. This type of alteration occurs in all areas of the Seneca property, even in areas that are relatively distant from the stockwork zones. The widespread distribution, the limited mass change and the relatively constant N a 2 0 and C a O suggest that this alteration is not related to hydrothermal activity. It may reflect the interaction of felsic flows with seawater. 4.5 S U M M A R Y The alteration at Seneca is dominated by an assemblage of quartz and sericite with varying amounts of chlorite. Calculated sericitization indices for the felsic rocks across the property suggest that alteration is stronger and more pervasive to the northwest than in the Pit Area to the southeast. Rocks from the Fleetwood and Vent Zone have undergone an average of over 50 % sericite alteration of feldspar, whereas similar rocks in the Pit Area have undergone an average of less than 35 % sericitization of feldspar. Strong alteration in the Pit Area is confined mostly to the ore zone conglomerate, an extensive coarse grained unit of debris flows and turbidites of varying thickness that has undergone moderate to intense silicification and sericitization. This unit and its characteristic alteration can be traced to almost 1 km from the open pit. The conformable nature of the alteration and the lack of a stockwork feeder zone in the immediate area suggests that hydrothermal fluids flowed laterally through the permeable O Z C unit strongly altering it while the bounding strata remained essentially unaltered and unmineralized. Alteration in the Fleetwood and Vent Zones is related to a roughly linear, northwest-southeast trending series of mineralized stockworks. This alteration is discordant and can be subdivided into two principal alteration zones. The upper zone is up to 25 metres thick, has a typical assemblage of quartz and sericite alteration and has undergone a net mass gain ( S i 0 2 and K 2 0 mass gains; N a 2 0 and CaO 1 16 mass losses). The lower zone varies from 30 to over 100 metres in thickness, has a typical assemblage of sericite-chlorite±quartz and has generally undergone a net mass loss (MgO and K 2 0 mass gains; N a 2 0 , CaO and S i 0 2 mass losses). S i0 2 has been leached from the lower zone while MgO was added. The leached silica may be added to rocks higher in the sequence forming a 'cap' of quartz with additional sericite. It appears that at least a portion of this silica addition which fdls the interstices of the breccia in the stockworks in the Fleetwood Zone has occurred late in the hydrothermal history and is partially contemporaneous with sulphide deposition (i.e. quartz-sulphide veins occur in Fleetwood-Vent Zone; Plate 4.1a). The Vent Zone stockwork displays a similar alteration zonation as described above, but has experienced gains in MgO throughout. This may be due in part to a greater involvement of MgO from seawater in this part of the hydrothermal system. Seawater contains relatively high amounts of Mg (1280 mg Mg/gram seawater, Janecky and Seyfried, 1984). Bischoff and Seyfried (1978) have demonstrated that Mg from seawater is incorporated into precipitating mineral phases once the fluids are heated to over approximately 200° C. Thus, if there is a greater mixing of the seawater with hot hydrothermal fluids then one would expect more Mg to be added to the altered rocks. However, the exposed portion of this stockwork may only represent part of a larger zone that has since been partially eroded, and thus we may be only observing the lowermost portion or perhaps the margins of the overall hydrothermal system. 117 C H A P T E R 5 MINERALIZATION 5.1 INTRODUCTION Previous studies of the Seneca prospect by Pride (1973), Gannicott et al. (1979), Armbaist and Gannicott (1980) and Urabe et al. (1984) were focussed around the discovery site at the small open pit. These authors described the prospect as a small Zn-Cu-Pb-barite massive sulphide deposit that is similar in many aspects to the Kuroko deposits of Japan. Unfortunately, the exposures in the pit that provided the basis for some of their observations have since been obscured. However, more recent diamond drilling in the mid-1980s and early 1990s resulted in discoveries of additional mineralized intersections in the Pit Area and in the Vent and Fleetwood Zones that provided new insights into the genesis of the deposit. This study will subdivide the description of the mineralized zones on the basis of their geographic locations since the different styles of mineralization are generally unique to a particular area of the property. 5.2 PIT AREA Pride (1973) and Urabe at al. (1984) recognized the strong association of mineralization in the Pit Area with a dominantly felsic fragmental footwall unit which is now referred to as the ore zone conglomerate (OZC) (Plate 5. la). Their studies describes a zoned massive sulphide body with a chalcopyrite and pyrite-rich base which is overlain by a sphalente-barite-galena-rich ore. These zones are analagous to the yellow and black ores respectively that occur in the typical Japanese Kuroko deposits. Such a zonation was not as readily discernible in drillcore, although many of the cores examined in this study were 100 to 200 m away from the main sulphide zone in the pit. Pride (1973) also documented fragmental sulphides which suggest that the mineralization formed at or close to the paleoseafloor allowing some some slumping and reworking to occur. 118 Plate 5.1a: Representative drillcore samples of the mineralized ore zone conglomerate (OZC) from the Pit Area. From left to right, a downward succession is represented from massive sphalente-pyrite chalcopyrite-barite, to semi-massive pyrite, to strongly altered felsic conglomerate. The sample to the right is typical of the alteration and texture of most of the OZC. Plate 5.1b: Photomicrograph of massive sulphides and barite in the upper part of the OZC (Sample 85-03-104). An interpreted earlier-formed 'crust' of pyrite-sphalerite-chalcopyrite-barite is shown at the bottom. The remaining space in this sulphide-sulphate framework was filled in by an inferred later bladed barite in the middle of the photo. (Reflected light; Field of view = 5 mm). 1 19 Plate 5.2a: Photomicrograph of massive sulphides and barite from OZC (Sample 85-03-104). Earlier formed colloform sphalerite-pyrite-barite appear to have been partially replaced by chalcopyrite and pynte and then infilled by a later, bladed barite (upper portion of photo). Triple junction intersections between some pyrite grains suggests recrystallization. (Reflected light; Field of view = 1.25 mm). Plate 5.2b: Photomicrograph of Sample 85-03-104 massive sulphides and barite. The sulphides display a bladed texture similar to the barite texture pictured above suggesting that perhaps the sulphides replaced an earlier-formed barite or precipitated on a barite+silica framework. Some lighter grey galena is visible to the right of centre of the photo (Reflected light: Field of view = 2.5 mm). 120 Assav Values Depth Length Zn Cu Pb Ag Au Drillhole (m) (m) % % % rVt & Comments 75-41 141.3 1.0 1.15 0.01 0.85 6.17 0.34 - mineralized OZC 142.3 1.3 6.30 0.05 0.10 44.57 0.34 - mineralized OZC 143.6 0.8 2.60 0.30 0.05 14.06 0.34 - mineralized OZC 83-06 121.4 1.3 17.70 5.11 0.04 144.3 6.96 - massive cpy-sph-py 83-07 61.5 4.0 5.25 1.29 0.08 32.83 1.67 - upper Cu-rich zone 83-11 76.8 4.1 5.20 0.70 0.09 47.60 1.47 - upper Zn-rich zone 85-03 104.2 4.2 1.62 0.06 0.12 45.58 0.98 - semi-massive pyrite 114.0 1.8 0.52 0.012 0.01 2.06 0.17 - typical OZC interval Table 5. 1. Summary of some typical assay results for some mineralized intervals associated with the ore zone conglomerate in the Pit Area. Sulphide mineralization associated with the OZC is dominated by disseminated to semi-massive and stringer pyrite. Massive and semi-massive sulphides are generally restricted to the middle and upper parts of the unit and reach thicknesses of up to 2 metres, but are more commonly around 0.5 metres. These intervals are composed of locally 20 to 50 % sphalerite, up to 75 % pyrite, less than 15 % chalcopyrite, up to 15 % barite and generally at least 10 % felsic fragmental material. Galena is also present in small amounts, but is only discernible in polished thin section. The massive sulphides are discontinuous and usually cannot be correlated between adjacent drillholes and some intersections of the OZC contain nothing more than disseminated pyrite. Some assay data from these mineralized zones are summarized in Table 5.1. More commonly pyrite is the dominant sulphide mineral and occurs as disseminated euhedral grains or as matrix-filling material interstitial to felsic clasts of the OZC. Pyrite also rims the lava clasts in places. Stringers of sulphides (py>cpy>sph) up to 2 cm wide are also observed to crosscut the OZC below the massive sulphides. Some typical assays from such less well mineralized parts of the OZC are the samples from drillhole 85-03 in Table 5.1. 121 Interpretation of sulphide textures In polished thin section, sphalerite, pyrite and chalcopyrite occur both as subhedral to euhedral granular aggregates and colloform-textured intergrowths. The colloform sulphides generally exhibit a zonation consisting of chalcopyrite-pyrite±barite cores surrounded by concentric growth zones of pyrite and sphalerite (Plate 5.1b and Plate 5.2a). Pyrite is often rimmed by sphalerite. Some of the finer micron-scale growth bands contain both pyrite and sphalerite, and in places it appear as though the pyrite is replacing the sphalerite. The colloform sulphides occur as isolated globules, but more commonly as coalesced masses. The more granular sulphide aggregates do not have such a zonation and consist of blebs and irregular masses of sphalerite, pyrite and chalcopyrite with small amounts of galena. Barite occurs in two different forms. Anhedral barite blebs are associated with the granular sulphides. This type of barite is interstitial to the sulphides and can occur at the cores of the colloform spheroids associated with chalcopyrite and pyrite. The barite often appears to be 'corroded' by the surrounding sulphides. Coarser bladed barite is more common and occurs between the masses of granular and colloform sulphides. In places 'bladed' sulphides are also associated with the barite and mimic the texture of the euhadral barite. It is possible that the sulphides have totally replaced the original bladed barite, perhaps first by coating it and then by replacing it from the inside out. Such a scenario may explain the coexistence of barite, a lower temperature mineral with chalcopyrite, a higher temperature mineral. In general, the massive sulphides and interstitial barite form irregular masses and encaistations. The space around these masses has been infilled by coarser, bladed barite. Chalcopyrite commonly appears to be replacing earlier formed sphalerite and pyrite. However, in places chalcopyrite, pyrite and sphalerite±galena appear to have formed together with little or no apparent 122 1 2 3 . BARITE • PYRITE • • • • SPHALERITE • • • • CHALCOPYRITE — GALENA • • • Figure 5. 1. Mineral paragenesis for the massive sulphide zones associated with the ore zone conglomerate at the Pit Area. Solid bars indicate major minerals; broken bars indicate minor minerals. replacement occurring. These observations led to the formation of the mineral paragenesis summarized in Figure 5.1. Stage 1 involved the precipitation of principally barite with smaller amounts of sphalerite, pyrite and galena. The second stage involved the precipitation of the colloform sphalerite and pyrite. The sulphide spheroids coalesced and likely formed a crust around the earlier barite and sulphides. As the hydrothermal system evolved, and perhaps heated up, chalcopyrite began to precipitate and replace some of the early barite 'framework' increasing the permeability in the sulphide layer and enabling the hydrothermal fluids access to the cores of the colloform masses. It appears that sphalerite, pyrite and galena continued to precipitate while this replacement occurred. It is possible that pulses rather than a steady influx of hotter hydrothermal fluids led to this precipitation of chalcopyrite. Finally, as the system waned and cooled stage 3 minerals were precipitated. These late stage minerals consist of principally bladed barite and lesser sulphides that filled in the remaining pore space in the sulphide layer. 5.3 VENT ZONE Mineralization in the Vent Zone consists entirely of stockwork veins hosted by a strongly altered massive dacite porphyry intrusion (Plate 5.3). The veins are 1 to 10 mm wide and are 123 Plate 5.3b: Drillcore samples from DDH 86-28 in the Venl Zone. These samples show the varying intensity of veining throughout the stockwork. The relict feldspar phenocrysts of the host FP rhyodacite are still discernible as darker spots in these rocks. 124 Assav Values Depth Length Zn Cu Pb Ag Au Drillhole (m) (m) % % % Comments 91-16 153.4 1.1 1.54 0.09 0.01 17.0 0.07 - sulphide blebs 154.5 1.1 5.56 0.38 0.37 162.1 2.37 - massive sulphides 155.6 1.0 2.10 0.15 0.04 15.4 0.07 - stockwork sulphides 156.6 1.0 4.74 0.31 0.02 12.7 0.07 - stockwork sulphides 168.0 0.4 19.3 2.34 0.05 20.5 0.07 - massive sulphides 177.0 1.5 2.69 0.36 0.07 8.5 0.04 - stockwork sulphides Average 34.25 2.15 0.30 0.11 13.30 0.14 - from 153 to 184 m Table 5.2. Summary of selected assay data from the main Fleetwood zone stockwork intersected in drillhole 91-16. Mineralization is dominantly disseminated and stockwork vein sulphides (sphalerite, pyrite, chalcopyriteigalena) with patches of semi-massive to massive sulphides hosted by felsic flows and flow breccias. composed of principally quartz, pyrite and sphalerite with scattered blebs of chalcopyrite . Locally, the veins comprise 10 to 15 % of the rock, but more commonly make up less than 5 % of the rock. Although the vein mineralization is relatively extensive, the metal grades are generally quite low. Typical assays for the zone are less than 0.50 % and less than 0.20 % Cu with only trace amounts of precious metals. Higher grade zones reach up to 4 % Zn and 0.75 % Cu over 2 metres, but such zones are sparsely distributed. The basaltic breccias that form the immediate footwall to the main stockwork zone contain disseminated and stringer sulphides and in places contain over 3 % Zn and up to 1 % Cu over a 2 metre interval. 5.4 FLEETWOOD ZONE Mineralization in the Fleetwood Zone in the nortwestern portion of the property is hosted by rhyodacitic flows, flow breccias and synvolcanic intrusions. Sulphides occur as disseminations, veins and as irregular semi-massive patches within the breccias (Plate 5.4). Sphalerite and pyrite are the dominant sulphide minerals with lesser amounts of chalcopyrite and galena variably present. Table 5.2 summarizes some of the assay data from drillhole 91-16 which intersected 34.2 metres of mineralized 125 c S _ o S £ < o C <r c CD g O i l 2 ^ _ a i d CO 4 . E o •PI 511 =) to 3 < *; J 3 >,Q-C3-^ o CO CD <o _-m in J 3 < 0. *" ci C N ) O V c CP E co <o CD J= 5 x J Z X I 8 W C - • D e o CO C CD S'CO 3 i Q - CD ro 5| • T - O T -V V V • - £ <D C e x : 2 \r Q . » -CD co 5 N m O l i -fe ct-s fir"* < O C L ^ •»'<-< C> V o CO 3 2 <B8-I f i rn co ^ " J CD • S o C 51 E C M E a s in m oi m oi 3j i T 1 • E in > o 3 o "5 .Tf CO O Q <D O " O C*"9 CO CD to CO N j*:-o N C \ I o | | £ CO co co O • •650 I s 18-CO o p g o . 0 c u co Ss S o ' c .9 > tS 111 S CO A Cv CO CD >CS M +3 CO CO CD o o u. cn " * J • 126 Plate 5.4: Representative drillcore samples from the Fleetwood Zone stockwork (DDH 91-16). Upper photo: samples are from the main stockwork zone in the upper portion of the mineralized zone; they consist of disseminated to semi-massive sphalerite-pyrite-chalcopyrite hosted by a strongly altered felsic breccia. Lower photo: samples are from the lower stockwork zone which contains only disseminated and stringer sulphides. The brecciation is interpreted to be a primary hydroclastic fragmentation as opposed to a phreatic brecciation. (see Figure 5.2 for locations of these zones). 127 Plate 5.5a: Mineralized flow breccia from a Fleetwood Zone stockwork (DDH 91-18). This stockwork, although less extensive than in DDH 91-16. shows similar features. The host rocks grade downwards into massive felsic flow rocks. Brecciation has preceded alteration and mineralization, and mineralization is strongest where the rock is most brecciated (upper zone). Plate 5.5b: 33-Zone massive sulphides. This zone consists of massive sphalerite and chalcopyrite with lesser amounts of pyrite and barite. The chalcopyrite appears to be replacing the sphalerite. 128 flow breccia grading 2.15 % Zn, 0.30 % Cu, 0.1 1 % Pb and 8.1 g/t Ag. This stockwork interval is illustrated in Figure 5.2. Most of the sulphides occur interstitially to the flow breccia fragments (Plate 5.5a) and are associated with abundant quartz which fills in the remaining space (Plate 5.4). This style of mineralization and the base metal grades are typical for this area of the Seneca property. However, D D H 91-16 represents the thickest such intersection. The stockwork intersected by D D H 91-18 (Figure 2.1) is of comparable thickness, but does not contain as strong mineralization. Other stockwork zones (such as in D D H 87-12) have been intruded by felsic sills. The strong prevalence of synvolcanic intrusions makes it difficult to correlate the stockwork zones between drillholes. As such, it is not clear whether these zones are part of a widespread continuous mineralized interval or if they are each individual stockworks. The similarities in geological associations, stratigraphic position and style of rriineralization and alteration in all the stockwoks in the Fleetwood Zone suggests they are all part of a continuous zone. 33 Zone A zone of massive sulphides, termed the 33 Zone was intersected in drillhole 92-33 located 350 metres to the southwest of drillhole 91-16. This zone is situated at an equivalent stratigraphic position to the mineralization in the rest of the Fleetwood Zone. It consists of 2 metres of massive sphalerite and galena with minor pyrite and galena that is underlain by 1 metre of quartz and chlorite with 1 to 2 % disseminated chalcopyrite and patches of semi-massive pyrite (Plate 5.5b). The massive sulphides are composed of approximately 75 % sphalerite, 5 to 10 % chalcopyrite with the remainder being barite, galena and pyrite. The chalcopyrite appears to be replacing the sphalerite. The massive sulphides are immediately overlain a cherty sulphide layer that is less than 1 metre thick and is composed of massive to finely laminated chert or siliceous fine ash. Up to 5 % sphalerite and 3 % chalcopyrite are present throughout as 3 mm thick 'beds". Overlying this layer is a 129 3 metre thick interval o f strongly chloritized rocks. Alteration has obscured the textures, but this unit appears to have had an original fragmental texture. A 10 centimetre wide layer of massive pyrite occurs at the top of this zone. Less than 5 % sphalerite and chalcopyrite are also present. Two additional holes were drilled to test the 33 Zone. They intersected the same stratigraphic interval 50 metres on either side of D D H 92-33, but failed to intersect any significant mineralization. Therefore, the massive sulphides must either be part of a rather small lens or a part of a larger lens which has been fault-offset or has been disrupted by intrusions. It is also possible, however, that this zone is part of a steeply dipping sulphide vein of zone of replacement. Drillcore observations were not conclusive in determining whether an exhalitive or a vein/replacement origin was more feasible for the 33 Zone mineralization. 5.5 DISCUSSION The different styles of mineralization at Seneca appear to be strongly controlled by the nature of the host lithologies. The most extensive areas of massive and semi-massive sulphides (Pit Area and Fleetwood Zone) are associated with coarse volcaniclastic rocks and breccias whereas vein-type mineralization (Vent Zone) is hosted by massive synvolcanic intrusions and unbrecciated flows. The interpreted paragenesis of the sulphide minerals agrees with the interpretations of other sulphide deposits: an early formation of a barite ± silica framework onto which grows an encrustation of colloform pyrite and sphalerite all of which are recrystallized and/or replaced by later sulphides (cf. Hannington and Scott, 1988; Paradis et al. , 1988). A l l sulphide mineralization at the Seneca deposit is essentially Zn-rich. Lydon (1988) states that the upward and outward increase in Zn:Cu ratio in the massive sulphide lens as well as in the stockwork feeder is one of the most definitive features of volcanogenic massive sulphide deposits. Although the Zn:Cu ratios at Seneca are highly variable they are generally in the order of 10:1 around 130 most mineralized zones in the Pit Area and in the Fleetwood Zone. However, mineralized rocks in the Vent Zone stockwork and the lower portion of the Fleetwood stockwork intersected in D D H 91-16 are more Cu-rich and have Zn:Cu ratios commonly around 6:1 and as low as 2:1. The predominance of Zn over Cu in the mineralized zones is possibly indicative of a cooler hydrothermal system. Halbach et al. (1988) suggest that the formation of galena and sphalerite follows an early stage of deposition of framboidal Fe-sulphides and marks the transition to 'main stage', chalcopyrite-forming mineralization. This transition corresponds to an increase in temperature and a replacement of earlier deposited minerals. Eldridge et al. (1983) document a similar scheme of ore formation for some of the Japanese kuroko deposits with a period of increasing temperature followed by a waning of the hydrothermal system. Framboidal pyrite and sphalerite are abundant in the semi-massive and massive sulphides in the Pit Area. The small amounts of galena and chalcopyrite throughout the property and the high Zn:Cu ratios suggest that the Seneca hydrothermal system never made the transition to 'main stage" mineralization. 131 CHAPTER 6 DISCUSSION AND CONCLUSIONS 6.1 INTRODUCTION The Seneca deposit is hosted by Middle Jurassic age volcanic rocks of the Harrison Terrane. This chapter will discuss the significance of the stratigraphic relationships and geochemical trends of these rocks and compare them with modern and ancient volcanic settings. Patterns within the hydrothermally altered zones will also be compared with those associated with known volcanic-hosted deposits in order to establish a geological and geochemical model for the formation of the Seneca deposit. 6.2 STRATIGRAPHY AND GEOCHEMISTRY 6.2.1 STRATIGRAPHIC SUBDIVISIONS The volcanic stratigraphy at the Seneca property can be subdivided into three major intervals based upon different combinations of lithologies and volcanic facies. These have been termed the Footwall Interval, the Seneca Horizon and the Hangingwall Interval. The Footwall Interval is characterized by basaltic lavas overlain by very coarse, poorly sorted and often heterolithic breccias and mass flows. The Seneca Horizon is a narrower and more discontinuous interval that is composed of felsic flows, breccias and coarse volcaniclastics, and is the host to the mineralization and zones of strongest hydrothermal alteration. The Hangingwall Interval is also composed of felsic flows and volcaniclastics, but the interval is unmineralized and essentially unaltered and the volcaniclastic rocks are dominated by sand and silt/ash-sized detritus; conglomerates and breccias are also less common than in the underlying sequence. 132 6.2.2 FACIES INTERPRETATIONS Felsic flows and breccias are much more common in the Fleetwood and Vent Zones than in the Pit Area where synvolcanic sills are the dominant type of porphyritic rock. Dacitic to rhyolitic sills and dikes are common in almost all areas of the property and at all stratigraphic levels. Mafic intrusions are also ubiquitous, but are less common than felsic intrusions. Andesitic sills are intimately associated with the Seneca Horizon in the Pit Area. The greater prevalence of felsic flows suggests that the Fleetwood and Vent Zones are proximal to an inferred volcanic vent. The Pit Area to the southeast of the property is interpreted to be more distal. The volcaniclastic sequences in both areas are quite similar, consisting of coarse, poorly sorted, felsic lava clast-dominated debris flows overlain by massive to well-bedded volcaniclastic sandstones and ashes of felsic composition. However, this sequence is punctuated by felsic flows in the Fleetwood and Vent Zones, but more commonly by sills in the Pit Area. The section of the Trough Zone that was examined in this study (DDH 91-03), located further to the southeast of the Pit Area, contains a similar fining upward volcaniclastic sequence, but contains no flows or intrusive rocks. As well as lacking extrusive lavas and breccias, this section of the Trough Zone also lacks the very coarse debris flows of the Seneca Horizon. Therefore, it appears that the Trough Zone represents a distal equivalent of the Hangingwall Interval. The uninterrupted volcaniclastic sequence observed in the Trough Zone may represent one continuous eruptive event. The lower massive fine to coarse-grained volcaniclastic sandstone that comprises the lower 70 to 80 metres of the examined drillcore may have been deposited as a single fallback from a pyroclastic eruption which may have originated from a subaerial portion of the volcanic edifice. This eruption also would have likely deposited debris on the more proximal flanks of the edifice. As the volcanic activity waned, this debris would have been washed into the Trough Zone basin as debris flows and turbidites forming the well-bedded, crystal-rich deposits which occur in the upper section of the Trough Zone sequence. This stage of deposition appears to have been punctuated by periods of relative quiescence during which thin dark brown to black argillaceous beds were deposited. These beds also occur at and above the Seneca Horizon in the Pit Area and may reflect normal, non-volcanic sedimentation in this area. A similar depositional history seems to have occurred in the Fleetwood Zone and the Pit Area. However, in these areas the lowermost part of the sequence which represents the initial felsic eruption is characterized by very coarse-grained and poorly sorted volcanic breccias deposited by debris flows or subaqueous lahars. This initial high energy event was followed by lower energy deposition by turbidites and gravity settling forming a similar upper volcaniclastic sequence to the Trough Zone. However, in the Fleetwood and Vent Zones, and to a lesser extent in the Pit Area, this deposition was contemporaneous with the formation of the felsic flows and domes. Thus, in these areas coarser beds of flow-derived hyaloclastite are also common. 6.2.3 G E O C H E M I S T R Y OF THE V O L C A N I C SEQUENCE A T SENECA The volcanic rocks which host the Seneca deposit are bimodal with a basaltic to andesitic composition group of rocks and a dacitic to rhyolitic compositional group. There is a lack of samples at Seneca in the range of 53 to 63 wt. % Si0 2 . Dacitic to rhyolitic composition rocks in the form of flows, breccias, synvolcanic intrusions and volcaniclastic rocks are volumetrically dominant in this part of the stratigraphy. The mafic rocks have flat to slightly light rare earth element enriched patterns and are tholeiitic to transitional in nature (LaNAT>N = 1.2-1.9, Zr/Y < 3.5). The felsic rocks are LR.EE enriched and are transitional to calc-alkaline in nature (La N /Yb N = 2.0-3.7, Zr/Y > 3.5). The mafic lavas referred to as 'fire fountain' rocks are the most primitive rocks in the sequence (low T i 0 2 , Zr and Si0 2 ; high MgO and A1 20 3). The compositions of the more evolved andesitic rocks which were emplaced mostly as synvolcanic sills can be related to these rocks by fractionation of plagioclase, 134 olivine and clinopyroxene. The geochemical trends amongst the dacitic to rhyolitic rocks can be attributed to the fractionation of feldspar, quartz and pyroxene and/or hornblende. 6.2.4 COMPARISONS OF THE SENECA V O L C A N I C ROCKS WITH M O D E R N SETTINGS Previous authors (Mahoney et al., 1995; Arthur et al., 1993) have established that the Harrison Lake Formation represents a Middle Jurassic calc-alkaline volcanic arc, but did not speculate extensively as to the possible paleo-tectonic setting of the area or to the geochemical evolution of the volcanic rocks. The stratigraphic and lithogeochemical observations of this study suggest that the Seneca area has experienced a similar but slightly different evolution than the rest of the Weaver Lake Member. An examination of similar geological settings of Miocene and younger ages where the tectonic regimes are fairly well constrained allows for analogies to be made as to the possible evolution of the Seneca stratigraphy. Lau Basin, southwest Pacific Clift et al. (1995) present a model for volcanism and sedimentation in narrow sub-basins in the Lau Basin in the southwest Pacific based on drillcores recovered during Leg 135 of the Ocean Drilling Program. The Lau Basin, located 500 km to the north of New Zealand, is the back-arc system to the modern Tofua Arc. Volcanic and tectonic activity in this region is related to the west-northwest subduction of the Pacific plate. The Lau Basin has been formed by extensional tectonism and is bounded to the west by the Lau Ridge and to the east by the Tonga Platform and the Tofua Arc. Clift et al. (1995) argue that the remote location of the Lau Basin and Tonga Platform precludes significant volcaniclastic input from any source other than the Lau Ridge or Tofua Arc. Most of the sub-basins that were drilled during ODP Leg 135 were constructed on older rifted arc crust and have similar sedimentary characteristics. Most of the sections recovered consisted of overall fining upward sequences of volcaniclastic conglomerates to coarse sands thick-bedded sands and silts topped by dark brown nanofossil oozes. This sequence overlies basaltic basement rocks. The coarse conglomerates and sandstones contain clasts ranging in composition from rhyolite to basaltic andesite, but are dominated by angular to subrounded dacitic clasts up to 5 cm across. These basal units were deposited by sediment gravity flows, in the case of the massive beds, and as turbidites in the case of the normal-graded coarse sand beds. The finer volcaniclastic sands and silts consist of graded turbidite layers and are dominated by dacitic glass shards. Gift et al. (1995) suggest that the texture and freshness of the glass in these units requires a relatively proximal source such as seamounts within the basin or the flanking volcanic arc. Volcanic glass fragments from the ODP drillcores were analysed for major and trace elements by electron microprobe and were dated by interpreting the sedimentation rates in the sub-basins. Figure 6.1 summarizes the variations in S i0 2 contents of these glasses with time for Site 840 located in the forearc area of the modern Tofua arc. This material corresponds to volcanic activity during the formation of the Lau arc (~ 5-7 Ma) and the construction of the modem Tofua arc on the Tonga Platform (~ 3 Ma to present). There was an intervening period of basin rifting and extension between the formation of these two arcs with a corresponding relative quiescence in volcanic activity (~ 3-5 Ma). The glass compositions reveal a period of bimodal volcanism during the early history of the Tofua arc with a gap in the data set between approximately 60 and 65 wt. % Si0 2 . During this early period of arc development basalts and basaltic andesite lavas were erupted into the rifted sub-basins and were followed by the eruption of felsic lavas. There also appears to be a shorter period of bimodal volcanism during the earliest stages in the formation of the Lau arc during which more primitive basaltic rocks were erupted. The rocks of the more mature arcs are non-bimodal and cover the entire spectrum of compositions from basalts to rhyolites although they lack basaltic compositions of less than 50 wt. % Si0 2 . 136 0 2000 < 4000 6000 •4 oooooooooooooo • ••• > V 8000 data for modern Tofua arc (non-bimodal) (from Ewart et al. , 1973) early bimodal volcanism in Tofua arc Initiation of Tofua arc Lau Basin rifts • early bimodal volcanism Range of data of Harrison Lake • Formation (Mahoney et al. , 1995) - Range of data for Seneca volcanics 50 60 70 Si0 2 (%) 80 Figure 6.1. Diagram showing the variation in total silica content with age for volcanic glasses from the Tonga Platform east of the Lau Basin, SW Pacific. Data is from Clift et al. (1995) and was collected at ODP Site 840 (ODP Leg 135). This diagram illustrates that the early stages of arc volcanism were bimodal in composition in both the older Lau arc (although less pronounced) and the younger, post-rifting Tofua arc. Data ranges for the Harrison Lake Formation, an interpreted Middle Jurassic arc, and the Seneca volcanic rocks within that same belt are shown for comparison. 137 Figure 6.1 also shows the compositional range for data from both the Seneca volcanic rocks (this study) and volcanic rocks from the Weaver Lake Member of the Harrison Lake Formation (Mahoney et al., 1995) of which the Seneca stratigraphy is a part. There is a strong similarity between the compositional ranges of both the Seneca rocks and the rocks from the earliest volcanic activity in the Lau arc. The non-bimodal data set from the rest of the Weaver Lake Member corresponds more closely to the younger parts of the Lau arc (post-6 Ma) or perhaps to the compositions of the modern Tofua arc (Ewart et al., 1973) which are also shown on Figure 6.1. The data and interpretations of Mahoney et al. (1995) for the Middle Jurassic Harrison arc imply that those rocks are representative of a mature arc analogous to the modern Tofua arc. The Seneca volcanic rocks, however, appear to have formed in a slightly different setting within that arc. A strong similarity exists between the stratigraphy of the sub-basins drilled in the Lau Basin and the stratigraphy in the lower parts of the Fleetwood and Vent Zones. In both locations, more primitive basaltic rocks were erupted (the fire fountain rocks in the case of the Fleetwood Zone) and were overlain by very coarse, felsic composition debris flows which were in turn overlain by further felsic volcanic rocks within an overall increasingly well-bedded and finer grained turbiditic volcaniclastic sequence. The stratigraphic similarities and the bimodal compositions of the Seneca rocks suggest that they may have formed in a similar setting to the younger Lau arc and Tofua arc rocks - rifted sub-basins in an intra-arc setting. This comparison is supported by the trace element data for both areas; both are enriched in large ion lithophile elements (LILEs; e.g. K, Rb, Ba)) and have relatively low abundances of incompatible high field strength elements (HFSEs; e.g. Zr, Hf) relative to MOR.B compositions. Figures 6.2a and 6.2b compare the REE trends of the Seneca basalts and basaltic andesites with those of the mafic glasses of different ages from Site 840 on the Tonga Platform. Both data sets have similar flat, MORB-like REE patterns, overall REE abundances and slight L R E E enrichments. The Seneca mafic rocks correspond more closely to the younger latest Miocene rocks a) b) c) 100 t Lau Si te 840 (Late Miocene) B C o x: o 32 o o 10r-—! 1 1-Sample 94-FF-01-O Basalt • Basaltic andesite La Ce Pr Nd Sm Eu (Gd) Tb Dy Ho Er Tm Yb Lu 100 TD C o o o 10 t-- I 1 1 1 1 1 1 1 1 r -Lau Si te 840 (latest Miocene) Sample 94-FF-01 O Basalt • Basaltic andesite _ J 1 L _ La Ce Pr Nd Sm Eu (Gd) Tb Dy Ho Er Tm Yb Lu 100 [ Puertoci tos syn-rift andesi tes CD •*—» •D C o -C o o o 10t-- I 1 1 1- - T 1 1 1 1~ Sample 94-FF-01 y O Basalt • Basaltic andesite La Ce Pr Nd Sm Eu (Gd) Tb Dy Ho Er Tm Yb Lu Figure 6.2. Comparison of the REE patterns of the Seneca basalts and basaltic andesites vv those from the a) Late Miocene and b) uppermost late Miocene of the Lau Basin (Clift and Dixon, 1994) as well as the c) Puertocitos Volcanic Province, Baja, California (Martin-Barajas et al., 1995). These comparative data are shown as the shaded areas. 139 from Site 840 (Figure 6.2b). However, considering that the up to 10 % calcite in the amygdules of the Seneca basaltic rocks would uniformly lower REE patterns, the more primitive rocks may actually correspond to the slightly older Miocene rocks in a less developed rifted sub-basin. In either case, it is clear that the Seneca rocks have experienced a similar history to the Tonga Platform rocks and may be related to intra-arc rifting. Puertecitos Volcanic Province, northeastern Baja California, Mexico To further test the possibility that the Seneca volcanic rocks formed in a rift setting, another rifted arc sequence will be discussed; in this case the comparison will be made with rocks in a continental margin setting as opposed to the oceanic setting of the Lau Basin rocks discussed above. Martin-Barajas et al. (1995) describe the geology and geochemical trends in the Neogene Puertecitos Volcanic Province of Baja California, Mexico. These rocks are divided into three volcanic sequences -a lower interval of arc-related andesitic lavas and two syn-rift rhyolitic sequences which discordantly overlie the arc-related rocks. The transition from arc volcanism to rift volcanism is related to the opening of the Gulf of California at around 1 1-6 Ma (Martin-Barajas et al., 1995). Although all of these rocks are calc-alkaline in nature, trends in the trace element contents are different between the three groups. The arc-related, rocks are light REE-enriched and have greater amounts of the incompatible elements (e.g. Rb, K, Zr, Hf) than syn-rift rocks. The syn-rift volcanic rocks are also light REE-enriched, but to a lesser degree than the arc-related rocks, and have lower overall abundances in the REEs'and the other incompatible elements. The younger of the two syn-rift andesitic sequences is the least evolved lava and, thus, contains lower concentrations of the incompatible elements. The REE trends of the Puertocitos syn-rift andesites are compared with those form the Seneca area in Figure 6.2c. The Puertocitos rocks show a much more negative sloping REE pattern with 140 stronger L R E E enrichments and relatively more depleted HREEs. These trends suggest that the Puertocitos rocks have experienced a relatively strong influence by continental caist. Martin-Barajas et al. (1995) suggest that the extension during rifting created a high geothermal gradient which caused melting of lithospheric, MORB-type mantle. The resulting andesitic melts were contaminated by crustal melts and eventually produced dacitic to rhyolitic rocks. The strong difference in REE trends between the Puertocitos syn-rift rocks and the Seneca rocks, as well as the lower relative L1LE and HFSE abundances of the Seneca rocks, suggest that the composition of the Seneca volcanic rocks are not consistent with formation in a rift setting with a continental crustal influence. However, a possible explanation for the disparate HREE trends in the two areas will be presented below. Medicine Lake Volcanic Center, northern California The results of the investigations of a study of the Medicine Lake volcanic centre in northern California, another example of a bimodal calc-alkaline suite of rocks, by Condie and Hayslip (1975) will be discussed here for the purpose of a drawing further comparison with the Seneca area. The Medicine Lake shield volcano, part of the Cascades volcanic system, is composed mainly of basaltic andesite and andesite flows and was formed during the late Pleistocene and has been active most recently 200-300 years ago. These rocks formed from melts produced as a result of the subduction of the Juan de Fuca Ridge beneath continental North America. Condie and Hayslip (1975) studied a bimodal suite of volcanic rocks that is related to the formation of a caldera prior to 10 000 years ago. Some of the REE trends of their data set are compared with the Seneca REE trends in Figure 6.3. The mafic rocks from the Medicine Lake area are more LREE-enriched and contain slightly greater overall abundances of REEs than the Seneca mafic rocks (Figure 6.3a). Condie and Hayslip (1975) also report a basaltic sample that has a flat, slightly LREE-depleted pattern (not shown here) that is very similar to the REE patterns of the Seneca fire fountain rocks suggesting that a less abundant, more primitive batch of lavas also erupted at Medicine Lake, perhaps in a similar fashion to those at Seneca. 100, -i 1 1 r -i 1 r Sample 94-FF-01 ~ ~o rz o J= o o o 10L-Medicine Lake volcanics Seneca samples O Basalt • Basaltic andesite _i i i i i i i_ _i i i i_ La Ce Pr Nd Sm Eu (Gd) Tb Dy Ho Er Tm Yb Lu 100 -i 1 1 1 1 r -i 1 1 1 r Medicine Lake volcanics CD -*—• * i _ "O fZ o .c o o o 10 S e n e c a s a m p l e s O QFP rhyolites • FP dacites and rhyodacites B Rhyodacitic flows _i i u. _i i i i i i i i La Ce Pr Nd Sm Eu (Gd)Tb Dy Ho Er Tm Yb Lu Figure 6.3. Comparison between the REE trends of the Seneca volcanic rocks and the Medicine Lake volcanics, northern California. The upper diagram compares basaltic andesitic rocks and the lower plot compares dacites and rhyolites from both areas. Data for Medicine Lake volcanics is taken from Condie and Hayslip (! 975). 142 The felsic rocks from Medicine Lake, on the other hand, show a better correlation in REE contents with the rhyodacites and rhyolites from Seneca. Both sets of felsic rocks have similar L R E E enrichments and negative Eu anomalies (Figure 6.3b). However, the Medicine Lake rocks again have H R E E depletions whereas the Seneca rocks have fairly flat H R E E patterns. Condie and Hayslip (1975) propose that the mafic and intermediate composition volcanic rocks at Medicine Lake were derived from the partial melts of the subducting Juan de Fuca Plate which were later modified by magma mixing and fractional crystallization. They suggest that the siliceous lavas were derived by 20 to 50 % partial melting of continental crust of granodiorite composition due to the emplacement of mafic magmas at depth. These interpretations are consistent with the trends in the rare earth element data. Condie and Hayslip (1975) suggest that the dacite lavas were derived by a mixing of rhyolitic and andesitic magmas. This interpretation of magma mixing may have an application for the more intermediate composition rocks at Seneca. The glomerocrysts that are present in the group A and group B dacites and rhyodacites, and that are absent in the group C and D rhyodacites and rhyolites described in Chapter 3, could be part of a cumulate phase from a mafic magma which was incorporated into these rocks upon mixing with a more silicic magma. The texture of these inclusions is similar to the texture of the basaltic andesitic rocks. In summary, the REE trends of the Medicine Lake volcanics are quite similar to those in the rocks at Seneca. As such, a similar complex model of magma evolution involving partial melting of an oceanic slab, fractional crystallization involving plagioclase, melting of continental caist and magma mixing could possibly account for the trace element trends in the rocks at Seneca. A problem with this model is the relatively low abundances of incompatible elements in the Seneca volcanics which would only allow a smaller degree of melting of continental crust. Another problem is the flat, non-depleted 143 H R E E patterns at Seneca. Such a pattern, however, can be explained by fractional crystallization of amphibole which, in some cases, can even create a concave-up H R E E pattern (Cao et al., 1993; Arculus et al., 1995). Hokuroku Basin, northern Honshu, Japan The Miocene volcanic rocks of the Hokuroku Basin in northern Honshu, Japan host perhaps the most famous series of volcanogenic massive sulphide deposits - the Kuroko deposits. These rocks have geochemical similarities to the volcanic rocks at Seneca and, thus, provide an even more relevant comparison. Dudas et al. (1983) describe these rocks as a bimodal suite consisting of tholeiitic to calc-alkaline basalts and calc-alkaline felsic rocks (dacites and rhyolites). The ore-associated stratigraphic interval consists of basal basalts which are overlain predominantly by felsic lavas and tuffs. A comparison of the REE patterns of the Seneca and the Hokuroku volcanic rocks is illustrated in Figure 6.4. Of the various data sets evaluated here, this Kuroko data set appears to match the Seneca data the best for both mafic and felsic compositions. The flat mafic pattern, the LREE-enriched felsic pattern and the flat to enriched HREE patterns are consistent between both data sets indicating that both sets of rocks could have undergone similar styles of magmatic evolution. Dudas et al. (1983) do not ascribe the Kuroko trace element trends to a particular tectonic setting, but they do state that the trace element signatures of the silicic volcanic rocks lie below those of typical calc-alkaline series and most closely resemble silicic lavas of the Tonga-Kermadec arc which includes the Tofua arc and Lau Basin rocks discussed above. Lockwood deposit, western Washington State, U.S. The Lockwood deposit located east of Everett in western Washington State, U.S. is a Cu-rich volcanogenic massive sulphide deposit that is hosted by Jurassic age mafic rocks of predominantly basaltic andesite composition ( C D . Spence, personal communication). A REE plot of one of these 100, -1 1 r CD "v_ "D C o J= o o o 10 Sample 94-FF-01 Lockwood sample Seneca samples Kuroko basalts O Basalt • Basaltic andesite _i i i i i i i i i_ La Ce Pr Nd Sm Eu (Gd) Tb Dy Ho Er Tm Yb Lu 100 -i 1 1 1 1 r ~i 1 1 n r-O sz o o o 10 S e n e c a samples O QFP rhyolites • FP dacites and rhyodacites Q Rhyodacitic flows Kuroko felsic rocks-La Ce Pr Nd Sm Eu (Gd)Tb Dy Ho Er Tm Yb Lu Figure 6.4. Comparison between the REE trends of the Seneca volcanic rocks and Hokuroku Basin volcanic rocks, northern Honshu, Japan. The upper diagram compares mafic rocks and the lower plot compares felsic rocks from both areas. The Lockwood sample indicated on this plot is from the Lockwood deposit in western Washington, U.S. Data for the the Kuroko rocks is from Dudas et al. (1983). 145 basaltic andesites is included in Figure 6.4a. The strong similarity between this pattern and the patterns for the Seneca mafic rocks, although it is only one sample, as well as their similar ages suggests that the Lockwood volcanic rocks may be correctable with the Seneca stratigraphy to the north. 6.2.5 S U M M A R Y The similarities between the major element chemistry (i.e. bimodal suites) and trace element signatures of the Seneca, the Lau Basin/Tonga Platform and the Hokuroku Basin volcanic rocks suggests they may have formed in similar tectonic environments. The geochemistry of these three areas is neither typical of island arcs nor a true rift setting such as a back-arc basin. It appears that these different packages of rocks formed in an environment that was transitional between a calc-alkaline arc and a rift setting. Such a setting could be an intra-arc rift formed in an extensional environment such as described by Gif t et al. (1995). The formation of sub-basins appears to be a prerequisite for the eruption of the more primitive basaltic lavas present in all areas discussed. Perhaps the lack of large amounts of such basalts in these island arc terrains is indicative of the localized nature of the intra-arc sub-basins. It is not clear at what point in the formation of the Harrison Lake arc that the intra-arc rifting occurred. However, the occurrence of the outcrop at Morris Creek of fire fountain basalts which are essentially the same as those intersected by drilling in the Fleetwood-Vent Zone is coincident with the contact between the Weaver Lake Member and the overlying Echo Island Member. The similar geochemical compositions and the relative topographic positions of both occurrences suggests that the fire fountain localities represent contemporaneous volcanism and that, by inference, rifting must have occurred late in the formation of the arc (i.e. within the youngest rocks of the Weaver Lake Member). Alternatively, based on the distribution of the Echo Island Member flanking the volcanic rocks of the 146 Weaver Lake Member (c/"Mahoney et al., 1995), both members may be at least partially lateral facies equivalents. In this case, the timing of the intra-arc rifting becomes more difficult to constrain The geological observations outlined above (unit distributions, volcanic facies changes, etc.), geochemical trends and geochronological data for the volcanic rocks, and comparisons of the Seneca stratigraphy with other volcanic sequences in well constrained tectonic settings (e.g. the Lau Basin) have been combined in the formation of a geological model for the Seneca area (Figures 6.5 a and b). The early history of the Seneca stratigraphy is marked by the formation of intra-arc sub-basins in both the Fleetwood-Vent Zone and the Pit Area. Basalts were erupted into the sub-basins in the Fleetwood-Vent Zone, but apparently not in the Pit Area. Coarse rhyodacite-dominated breccias were deposited next in all areas, perhaps reflecting the onset or resumption of extrusive felsic volcanism in the area or the redeposition of coarse detritus into the sub-basins from the surrounding areas. The subsequent period of volcanism was dominated by the formation of felsic flows and domes in the Fleetwood-Vent Zone, and by the deposition of volcaniclastic material and lesser flows in the Pit Area. It was during this period that the mineralized zones were formed in both areas. Continued volcanic activity resulted in the deposition and reworking of volcanic debris in the form of volcaniclastic turbidites and ash beds in all areas. The entire sequence is intruded by synvolcanic sills and dikes. 6.3 A L T E R A T I O N A N D M I N E R A L I Z A T I O N A model for the formation of the stockwork alteration zones of the Fleetwood and Vent Zones has been devised through a combination of petrographic data and mass change calculations. This model is illustrated in Figure 6.6. Alteration occurs almost entirely within massive to brecciated rhyodacite flows and sills. The underlying mafic lavas are variably altered in some places. The Fleetwood and Vent Zone stockwork zones can essentially be divided into two zones with different alteration assemblages: 1) an upper quartz-sericite zone characterized principally by mass gains in S i 0 2 and K 2 0 , mass losses in Na 20±CaO and overall net mass gains, and 2) a lower sericite-147 r 148 + + + y + + + k + + + + to o -g 'sz a. c " 'co CO o o CD A U N O CO cj C • T3 <D >^ O 3 C •C cr £ <D u £ CL O ») » u £ c T £2 ° Q. cu ra ro <u 15 g 3 u . -•£ .£ JI C m 1 ' } • —' o O N OJ - ri O N U » l -C CCi ' ^ ro cr c <D ro ^  CO CJ .^"0 CD P > C ro c_ o _o O .p o c .2 > o 2 -C co C ° ID „ CT) 2> O aj ro ro CD g.o-0 5i ro S c u y c CD ^ d 1/1 ro.2 - 8 §-. — >••*-• g 6 -5 3 O ' ro 1 ro1 - S o "S £ cj ro p ,o o CD u _C J> CO C/j C ro ro . I I J u. -a ^ 149 CD C o N CD C o N T3 O O CD 1/1 W f- 4-rt V. O o o <u ~ 2 g .S o N -a cr „ o o e 1 / 1 U | 3 .S 5"°:-S T3 O O _ € E _ <D « o E " N RS -a -5 o P ° r3 fi ,„ u (u <u u g r t ^ ° c -c c — o .5 •3-a S w O O « *^  >. "O G tu g *j <« r> g o ° s-— (j ra S-a " P = rt C nj w> <U _ rt 2 3 « O wi rt c c £ .2 ^ w, g « 6 E u c 3 o S " • 2 ^ 2 ? I § >. <2<^ 3 -_ O .2 rt o Vl £ T5 ^ CJ C Js Wl CJ C <U 'P *2 Wl P O cj wi rt fc. "3 ° — ~° rt o >. E wi y j — S m ^ <u rt -o tu -g S o r S t/3 TD •§ O vC vd + 5J ^ on-S ^ G *£ 3>-| g ;5 chlorite±quartz alteration zone characterized by mass gains in K 2 0 and MgO, mass losses in S i0 2 and N a 2 0 and overall net mass losses. Net mass losses have generally occurred throughout the entire Vent Zone stockwork in contrast to the Fleetwood Zone stockworks in which net mass gains are more prevalent. However, since the Vent Zone stockwork is now exposed at surface it is possible that an upper quartz-sericite zone with no MgO gains existed at one time but has since been eroded. Overall, the Vent Zone represents a larger hydrothermal system that may have been more efficient at incorporating Mg in seawater into the alteration minerals (i.e. chlorite). The vertical and lateral zonation of alteration assemblages is similar to other volcanic hosted massive sulphide deposits such as the Millenbach deposit (Knuckey et al., 1982) and the Delbridge deposit (Barrett et al., 1993) in the Noranda camp, Quebec, and some of the Japanese Kuroko deposits such as the Uwamuki deposit (Urabe et al., 1983). Larson (1984) describes a similar alteration system associated with the Bruce deposit in Arizona. Larson (1984) suggests that the more laterally extensive upper sericite alteration formed due to a change in hydrothermal fluid composition. He proposes that chlorite was the early alteration phase, but that as chlorite was produced the composition of the fluids shifted such that sericite became stable in the upper alteration zone; sericite replaced the earlier chlorite in the upper zone while chlorite continued to form in deeper zones. The magnesium content of the chlorites is greatest in the cores of the alteration pipe and decreases gradually outwards. Larson (1984) also demonstrates that a decrease in the activity of water below I greatly expands the stability field of sericite. Larson's model appears to be somewhat applicable to alteration at Seneca. This model would suggest that the predominance of sericite alteration over chlorite alteration at Seneca is indicative of a lower activity of water in the hydrothermal fluids and/or a lesser degree of interaction between the fluids and the rocks. Strong similarities exist between the Furutobe massive sulphide deposit in Japan and the mineralized zones in the Pit Area at Seneca. The Furutobe deposit is a typical Kuroko-style deposit 151 comprising three zones of alteration and mineralization (Kuroda, 1983): 1) a lower siliceous ore zone consisting of disseminated and stringer sulphides hosted by altered dacite, 2) an upper siliceous ore zone consisting of stringer and interstice-filling sulphides hosted by dacitic pyroclastic rocks and 3) a stratiform ore zone consisting of pyrite, sphalerite, chalcopyrite, galena, barite and gypsum which overlies the dacitic pyroclastic rocks. The stratiform and siliceous ore zones have gradational boundaries and the abundance of sulphate minerals increases upwards. The sulphides and sulphates formed by the cooling and mixing of hydrothermal fluids with seawater both within the permeable pyroclastic rocks and at the paleoseafloor (Kuroda, 1983). The existence of massive sulphide bodies at the top of an interval of altered and mineralized felsic pyroclastic rocks, the upward gradation from stringer and disseminated sulphides to massive stratiform sulphides as well as the prevalence of barite and collofonn-textured sulphides in the upper portions of the mineralized zones in both the Pit Area at the Seneca prospect and at the Furutobe deposit suggests that both they may have formed by similar processes at or near the paleoseafloor. Although a stockwork-style feeder zone was not observed directly below the OZC at Seneca, the Vent Zone stockwork or a similar unexposed zone would be analogous to the lower siliceous ore zone of the Furutobe deposit. 6.4 CONCLUSIONS The preceding discussion outlined the geological and geochemical characteristics of the volcanic sequence that hosts the Seneca Zn-Cu-Pb prospect and drew some analogies with modern settings to provide some insight to the possible origins of the Seneca stratigraphy and the mineralization and alteration therein. The following sections will summarize the major conclusions of this study. 152 Volcanic Stratigraphy and Facies Distribution The 300 to 400 metre-thick package of volcanic rocks that host the Seneca deposit comprises a portion of the Middle Jurassic age Weaver Lake Member of the Harrison Lake Formation in the Harrison Terrane located in southwestern British Columbia. The Seneca property is divided into four different areas from northwest to southeast: the Fleetwood Zone, the Vent Zone, the Pit Area and the Trough Zone. The predominance of certain lithologies in each area has facilitated the following volcanological interpretations: • The abundance of mafic and felsic flows and associated breccias in the Fleetwood and Vent Zones is indicative that these areas represent vent to vent-proximal facies. • The Pit Area stratigraphy contains abundant turbiditic volcaniclastic sandstones and siltstones, abundant synvolcanic sills and dikes with lesser amounts of felsic flows indicating that this area represents medial to distal facies. • The Trough Zone sequence, although less well constrained, consists entirely of volcaniclastic turbidites and lesser argillaceous beds indicative of a distal subaqueous volcanic facies. Thus, the volcanic rocks at Seneca correspond to an approximate northwest to southeast transition from vent proximal facies to distal or basinal facies. The volcanic sequence at Seneca is divided into three stratigraphic intervals described from lowermost to uppermost as follows: 1. Footwall Interval. This interval is characterized by the presence of subaqueously-deposited basaltic lavas, which are spatially restricted to the Fleetwood and Vent Zones, and by coarse, poorly-sorted rhyodacite dominated debris flows. 2. Seneca Horizon. This interval is discontinuous and of variable thickness and is characterized by the presence of stockwork and massive sulphide mineralization and/or strong hydrothermal alteration hosted by massive to brecciated rhyodacitic flows, synvolcanic intrusions and coarse volcaniclastics. 3. Hangingwall Interval. This interval, the thickest package examined, consists of felsic flows and breccias interlayered with massive to well bedded volcaniclastic sandstones and siltstones deposited by turbidites and by gravity settling. The lateral discontinuity of the Footwall Interval units suggests that they were deposited within sub-basins. The linear trend and stratigraphic continuity of the mineralized and hydrothermally altered zones of the Seneca Horizon suggest that they formed within a structural zone, possibly a growth fault bounding the sub-basins into which the Footwall units were deposited. Geochemistry The volcanic rocks at Seneca form a bimodal suite comprising basaltic to basaltic andesitic rocks and volumetrically more abundant dacitic to rhyolitic rocks. The low ratios of Zr/Y and LaN/YbN amongst the mafic rocks are indicative of a tholeiitic to transitional affinity. Chemical variations within the mafic rocks can be attributed to the fractional crystallization of plagioclase, olivine and clinopyroxene. The higher Zr/Y and La N /Yb N ratios of the felsic rocks are indicative of a transitional to calc-alkaline affinity. Chemical variations in the major element concentrations amongst the felsic rocks can be explained by the fractionation of dominantly plagioclase with lesser amounts of quartz and hornblende and/or pyroxene. Variations in trace element abundances of the felsic rocks can be attributed to the fractionation of varying amounts of plagioclase, magnetite, hornblende and apatite. Alteration 154 The strongly altered stockwork zones in the Fleetwood and Vent Zones are divided into two alteration zones as follows: 1. Upper quartz-sericite alteration zone. This zone has experienced net mass gain corresponding to strong mass gains of S i0 2 and K 2 0 and mass losses of N a 2 0 and CaO with variable mass changes of MgO. 2. Lower sericite-chlorite ± quartz alteration zone. This zone has experienced net mass loss corresponding to mass losses of N a 2 0 , CaO and variably S i0 2 and mass gains of K 2 0 and MgO. The greater gains of MgO present in the Vent Zone stockwork relative to the Fleetwood Zone suggest that this zone was part of a larger hydrothermal system that was more efficient at incorporating seawater magnesium into the alteration mineral assemblage (i.e. chlorite), or perhaps that overlying volcaniclastic units or flows in the Fleetwood Zone restricted the passage of seawater into the stockwork zones in that area. Min eralization The similarities in the inferred stratigraphic locations of the different mineralized zones at Seneca suggests that the stockwork sulphide veins in the Fleetwood and Vent Zones and the disseminated to massive sulphides hosted by the ore zone conglomerate in the Pit Area formed contemporaneously. The nature of the host rock was an important control on the styles of mineralization observed. Veins and stringers of pyrite, sphalerite, chalcopyrite and quartz are the dominant form of mineralization where the host rocks are massive rhyodacite flows and sills, such as in the Vent Zone, whereas disseminated, semi-massive and massive pyrite, sphalerite, chalopyrite and barite are more prevalent where the host lithologies are flow breccias, such as in parts of the Fleetwood Zone, or coarse volcaniclastic sandstone or conglomerate such as in the Pit Area. Although no potential feeder zone was obsea'ed as a source for the alteration and mineralization in the Pit Area, a stockwork zone such as that in the Vent Zone likely served as a conduit for the hydrothermal fluids. Upon intercepting the permeable OZC unit, these fluids would have likely been dispersed laterally forming the extensive zone of conformable alteration and associated mineralization. The hicli Zn:Cu ratios in all mineralized zones at Seneca are indicative of an overall cooler hydrothermal system, although the greater amount of chlorite alteration and lower Zn:Cu ratios in the Vent Zone suggest that that zone may have been altered and mineralized by slightly hotter fluids with a greater degree of water-rock interaction. The strong net mass gains, particularly in Si0 2 , that characterize the Fleetwood Zone may have 'sealed off the hydrothermal system and as such the stockworks in this zone are lees extensive than in the Vent Zone where mass losses predominate. This interpretation also provides a possible explanation as to the apparent lack of a massive sulphide lens above the Fleetwood stockworks. 6.5 CLOSING REMARKS This study succeeded in outlining spatial, volcanological and geochemical characteristics of the lithologic units at the Seneca providing the basis for the interpretation of the tectonic setting of the volcanic rocks and building a geological framework for the interpretation of the formation of the hydrothermally altered and mineralized zones. Comparing the observations of this study with those from other more recently formed geologic settings proved to be useful by providing a more modern context with which to judge the geological and geochemical characteristics of the volcanic rocks at Seneca. This study revealed that a bimodal suite of volcanic rocks may occur within a sequence of related rocks which on the whole does not have a bimodal geochemical signature. This has important implications for mineral exploration within the Harrison Lake Formation, and perhaps within other volcanic belts, since it is these bimodal rocks which host the mineralization at the Seneca property. The identification of sub-basins within older volcanic arc rocks and the recognition of more primitive composition basalts within these sub-basins also appears to be important since the stockwork -mineralized zones at Seneca are hosted by rocks which directly overlie such basalts. The existence of the sub-basins may indicate that intra-arc rifting or extension has occurred allowing perhaps the tapping of a deeper, more primitive magmatic source and providing the conduits or 'space' required for the upward movement and circulation of the hydrothermal fluids which formed the mineralized zones. 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A reappraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings; Earth and Planetary Science Letters, volume 45, pages 326-336. APPENDIX A LITHOGEOCHEMICAL DATA 164 All of the samples analysed as part of this study were taken from diamond drillcores, except for Sample 94-FF-01 which was sampled from an outcrop at Morris Creek, 5 km to the southeast of the property. As such, most samples are numbered using a combination of their drillhole number and depth. The locations of the sampled drillholes are shown on Figure A . l . Care was taken to avoid sampling vein material in the rocks. All samples were analysed using X-ray flourescence at Geochemical Laboratories, McGill University, Montreal, Quebec. All major elements and Cu, Zn, N i , Cr, V, Sc were analysed using glass beads. The trace elements Zr, Y , Nb, Rb, Sr, Ga and Pb were analysed using pressed pellets. Al l rare earth elements were analysed by induced neutron activation analysis at Activation Laboratories, Ancaster, Ontario. All geochemical data is included in the following tables and has been subdivided on the basis of composition. Detection limits are given at the end of Appendix A 3 . The following abbreviations have been used in the data tables: FP = feldspar porphyry (dacite to rhyodacite) QFP = quartz-feldspar porphyry (rhyodacite to rhyolite) F L = flow F E L D Y K = felsic dike (aphanitic) V C L T = volcaniclastic (siltstone/ash) A L T = altered Figure A . l . Map of the Seneca property showing the locations of the drillholes from which the lithogeochemical samples were taken for this study. Appendix A.l. Chemical composition of felsic volcanic rocks at Seneca. Sample 83-02-67 83-02-186 83-02-277 83-02-320 83-06-50 83-07-27 83-07-54 83-10-91 83-11-24 Hole 83-02 83-02 83-02 83-02 83-06 83-07 83-07 83-10 83-11 Depth 67.0 186.3 277.0 319.6 49.8 26.1 53.0 91.3 28.1 Depth 219.8 611.1 908.8 1048.7 163.4 85.6 173.9 299.6 92.0 Rock Type FP FP FP FP FP FP FP FP FP (Wt. %) S i 0 2 69.43 68.83 70.50 70.06 69.27 72.67 67.51 66.53 69.95 T i 0 2 0.50 0.51 0.43 0.45 0.40 0.36 0.51 0.52 0.47 AI2O s 14.59 14.73 13.95 14.42 14.98 13.47 14.55 15.54 14.22 Fc 2 0 3 3.99 3.90 3.16 3.45 3.77 2.86 4.58 4.22 3.75 MnO 0.09 0.06 0.17 0.07 0.08 0.07 0.11 0.12 0.08 MgO 2.23 2.06 1.84 1.94 1.92 1.86 3.48 3.81 2.83 CaO 0.84 1.13 1.29 0.92 0.78 0.55 0.77 0.68 0.67 Na 20 4.72 5.40 4.44 4.65 5.75 5.31 4.97 3.19 4.43 K 2 0 1.89 1.25 1.50 1.88 1.21 1.12 0.71 2.05 1.31 P 2 O s 0.14 0.15 0.11 0.1 1 0.13 0.08 0.13 0.16 0.12 LOI 1.92 1.78 2.58 2.11 1.93 1.44 2.37 3.20 2.49 Total 100.34 99.80 99.97 100.06 100.22 99.79 99.68 100.03 100.32 (ppm) Cr 106 90 137 193 104 11 18 25 90 Ni 11 8 13 11 6 bd bd 40 6 Co 9 -i j 12 9 10 36 21 5 10 Sc 11 10 8 10 i .1 8 16 10 6 V 57 56 47 51 60 31 82 50 60 Cu 8 3 8 21 21 30 30 91 6 PI) 5 5 4 4 4 bd 1 1 4 Zn 47 55 69 44 82 37 60 91 81 Ba 424 292 263 481 340 468 557 418 463 Rh 24 13 19 24 11 12 8 29 15 Sr 144 126 58 101 108 70 78 56 63 Ga 14 14 12 12 13 13 15 17 13 Nb 6 5 6 6 5 11 10 10 5 Zr 116 115 124 127 110 114 101 104 113 Y 28 26 23 24 2.3 27 31 32 26 Th bd bd bd bd bd bd bd bd bd U bd bd bd bd bd bd bd bd bd Cs na na 0.2 na na na na na 0.6 Hf na na 2.7 na na na na na 2.4 La na na 11.3 na na na na na 9.3 Ce na na 23 na na na na na 24 Nd na na 12 na na na n a na 14 Sm na na 2.76 na na na na na 3.08 Eu na na 0.87 na na na na na 1.03 Tb na na 0.5 na na n a na na 0.6 Yb na na 2.46 na na na na na 2.79 Lu na na 0.35 na na na na na 0.41 Zr/Y 4.1 4.4 5.4 5.3 4.8 4.2 3.3 3.3 4.3 LaN/YbN 3.1 2.2 Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass beads. The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A . l (continued). Chemical composition of felsic volcanic rocks at Seneca. Sample 83-11-40 85-03-70 85-03-126 86-28-67 86-28-103 87-11-75 87-12-69 87-12-147 87-12-301 Hole 83-11 85-03 85-03 86-28 86-28 87-1 1 87-12 87-12 87-12 Depth 85.2 70.5 126.2 67.0 103.0 75.5 69.2 147.4 301.4 Depth 279.4 231.2 414.1 219.8 337.9 247.7 227.1 483.6 988.8 Rock Type FP FP FP ALT FP ALT FP FP FL FP ALT FP FP FL Si02 71.17 69.26 70.35 72.97 70.86 73.94 70.57 75.11 71.50 Ti02 0.45 0.41 0.39 0.38 0.42 0.38 0.37 0.37 0.39 AIzO, 14.57 14.94 14.64 13.09 14.01 12.63 13.73 11.91 12.76 Fe 20 3 3.03 3.89 3.68 2.95 2.68 2.92 3.57 2.92 2.65 MnO 0.07 0.08 0.10 0.11 0.13 0.17 0.10 0.04 0.12 MgO 2.02 2.27 1:54 3.59 4.63 2.04 . 2.96 1.72 2.40 CaO 0.61 0.43 0.74 0.20 0.23 0.55 1.04 0.21 1.50 Na20 4.88 5.97 6.01 0.06 0.00 4.08 3.36 0.14 2.75 K 2 0 1.24 0.49 1.08 3.07 3.25 1.03 1.86 3.43 1.70 P20 5 0.10 0.14 0.12 0.1 1 0.12 O.l 1 0.10 0.11 0.1 1 LOI 2.17 2.05 1.43 3.81 4.05 1.92 2.30 3.61 3.61 Total 100.31 99.93 100.08 100.34 100.38 99.77 99.96 99.57 99.49 Cr 67 48 98 bd bd 28 .32 23 44 Ni 6 7 15 1 bd 6 8 3 9 Co 9 7 6 7 9 bd bd 3 bd Sc 6 8 7 na na na na na na V 43 52 61 44 40 29 49 45 35 Cu 3 7 18 473 12 30 19 65 49 Pb 4 4 4 na na 1 -i .i 7 -i .> Zn 46 57 58 198 164 75 66 2057 86 Ba 143 17 475 557 1400 265 528 873 497 Rb 16 8 8 47 52 15 20 45 21 Sr 65 64 101 8 10 68 137 18 121 Ga 13 14 13 16 14 14 14 17 14 Nb 6 5 5 7 7 9 8 8 7 Zr 134 109 115 108 117 122 120 118 122 Y 29 22 21 26 25 30 26 27 29 Th bd bd bd bd bd bd bd bd bd TJ bd bd bd bd bd bd bd bd bd Cs na na na na na 0.3 na na na Hi na na na na na 2.7 na na na La na na na na na 11.2 na na na Cc na na na na na 24 na na na Nd na na na na na 14 na na na Sm na na na na na 2.95 na na na Eu na na na na na 0.97 na na na Tb na na na na na 0.6 na na na Yb na na na na na 2.46 na na na Lu na na na na na 0.35 na na na Zr/Y 4.6 5.0 5.5 4.2 4.7 4.1 4.6 4.4 4.2 L a N / Y b N Analytical method: X-ray flu ores te nee, induced ncutnin activation analysis The major elements plus C u - Z n - N i - C r - V - S c were analysed using "lass heads. The elements Z r - Y - N b - R b - S r - G a - P h were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A.l (continued). Chemical composition of felsic volcanic rocks at Seneca. Sample 91-02-28 91-02-85 91-02-170 91-03-40 91-10-99 91-10-118 91-10-180 91-10-210 91-10-234 Hole 91-02 91-02 91-02 91-03 91-10 91-10 91-10 91-10 91-10 Depth 28.1 85.2 170.0 39.9 99.1 118.0 180.4 210.1 234.5 Depth 92.0 279.4 557.8 130.9 325.1 387.3 591.8 689.4 769.3 Rock Type Q F P Q F P FP Q F P I-EL DYK l-'LL DYK A L T FP FP F L A L T FP Si02 73.72 73.93 69.13 56.48 74.25 75.04 72.51 75.36 71.48 Ti0 2 0.28 0.28 0.50 0.44 0.28 0.30 0.33 0.39 0.33 Al 2 0 3 13.28 13.61 14.40 19.92 12.57 12.25 11.28 12.81 13.22 Fc 20 3 2.35 2.34 4.27 3.74 2.14 2.16 3.69 1.57 2.85 MnO 0.06 0.06 0.10 0.18 0.08 0.06 0.01 0.01 0.14 MgO 1.88 1.22 2.20 2.86 0.55 0.69 0.48 0.46 3.73 CaO 0.18 0.34 0.73 2.33 1.38 0.85 0.36 1.06 0.63 Na20 3.82 4.94 5.58 4.21 3.48 4.84 0.68 4.22 1.87 K 2 0 2.32 1.94 0.98 3.22 3.29 1.82 5.27 1.53 2.05 P 2 O 5 0.05 0.05 0.1 1 0.09 0.05 0.05 0.10 0.03 0.10 LOI 2.03 1.63 1.83 5.99 1.67 1.25 3.21 2.24 3.57 Total 99.97 100.34 99.83 99.46 99.74 99.31 97.92 99.68 99.97 Cr 129 153 117 0 36 24 0 40 16 Ni 5 6 4 5 5 3 5 10 2 Co 7 6 9 bd bd bd 8 3 5 Sc 2 bd 9 na na na na na na V 26 28 73 58 24 20 20 30 29 Cu 5 11 25 30 29 22 406 67 77 Pb 4 4 8 5 3 1 94 208 3 Zn 57 42 207 90 40 34 13967 1428 239 Ba 445 529 426 5392 1539 1272 6220 2047 665 Rb 18 17 8 48 37 18 49 18 23 Sr 54 73 94 275 104 1 13 40 181 52 Ga 11 11 12 22 13 12 16 14 14 Nb 7 5 6 8 9 9 6 8 8 Zr 141 119 116 193 130 131 106 122 117 Y 26 17 25 38 25 26 30 24 23 Th bd bd bd bd bd bd bd bd bd U bd bd bd bd . bd bd 2.0 bd bd Cs bd na na n a na na na na na Hf 3.1 na na na na na na n a na La 14.2 na na na na na na na na Ce 29 na na na na na na na na Nd 14 na na na na na na na na Sm 2.83 na na na na na na na na Eu 0.72 na na na na na na na na Tb 0.6 na na na na na na na na Yb 2.90 na na na na na na na na Lu 0.45 na na na na na na na na Zr/Y 5.4 7.0 4.6 5.1 5.2 5.0 3.5 5.1 5.1 LaN/YbN 3.3 Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass heads. The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A.l (continued). Chemical composition of felsic volcanic rocks at Seneca. Sample 9116-61 Hole 91-16 Depth 6 1 2 Depth 200.8 Rock Type F P -16-152 91-16-159 91-16 91-16 151.8 159.0 498.0 521.6 F P A L T F P 91-16-161 91-16-3(17 91-16 91-16 161.3 307.5 529.2 1008.9 F P FP 91-18-64 91-18-252 91-18 91-18 64.3 252.5 211.0 828.4 F P F L A L T F P -18-284 92-26-227 91-18 92-26 284.6 227.2 933.7 745.2 F P F P S i 0 2 73.22 73.17 78.18 63.50 67.91 74.06 77.62 64.94 73.26 T i 0 2 0.35 0.35 0.26 0.49 0.52 0.35 0.34 0.54 0.29 A l 2 0 3 13.21 13.41 11.17 13.55 14.48 13.31 10.38 17.30 13.64 Fe 2 0 3 2.71 2.37 1.45 4.22 4.39 2.71 4.10 2.70 2.38 MnO 0.13 0.10 0.01 0.12 0.20 0.11 0.01 0.13 0.08 MgO 2.85 1.60 0.98 4.76 3.95 1.97 0.65 2.94 0.78 CaO 0.51 1.03 0.26 2.50 0.89 0.69 0.16 1.32 1.72 Na 20 2.65 4.75 0.66 2.67 2.05 4.69 0.25 2.36 5.01 K 2 0 1.90 0.66 2.67 1.06 2.09 0.96 3.05 3.52 1.34 P 2 O 5 0.07 0.07 0.10 0.11 0.13 0.08 0.07 0.12 0.06 LOI 2.63 1.97 0.00 6.58 3.54 1.59 3.86 4.27 1.69 Total 100.23 99.48 95.74 99.56 100.15 100.52 100.49 100.14 100.25 Cr 121 438 250 160 68 1 1 bd bd bd Ni 10 10 10 7 4 2 bd 4 bd Co 3 4 2 7 10 17 23 -! 28 Sc 3 2 1 12 5 na na na na V 30 38 36 79 72 32 44 46 40 Cu bd 91 909 23 25 53 870 103 36 Pb 3 3 671 5 4 na na na na Zn 79 150 12525 235 125 110 2592 103 49 Ba 453 4068 24702 1063 374 302 676 1723 725 Rh 27 6 10 15 28 15 43 45 13 Sr 53 242 302 IO0 63 120 18 74 299 Ga 11 10 2 12 13 12 17 15 13 Nb 7 5 bd 5 7 7 6 8 5 Zr 1.35 120 72 108 131 128 103 168 1 14 Y 24 18 16 20 28 23 29 42 17 Th bd bd bd bd bd bd bd bd bd TJ bd 1.0 6.4 bd bd bd bd bd bd Cs na na bd na 0.4 bd bd na na Hf na na 3.2 na 2.9 2.8 2.8 na na La na na 5.8 na 13.3 9.4 11.9 na na Ce na na 12 na 29 22 28 na na Nd na na 8 na 15 1 1 17 na na Sm na na 1.65 na 3.43 2.40 3.16 na na E u na na 0.40 na 1.33 0.61 0.51 na na Tl) na na 0.4 na 0.7 0.6 0.7 na na Yb na na 1.97 na 2.86 2.49 3.09 na na Lu na na 0.32 na 0.44 0.37 0.44 na na Zr/Y 5.6 6.7 4.6 5.4 4.7 5.6 3.6 4.0 6.7 LaN/YbN 2.0 3.1 2.5 2.6 Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass heads. The elements Zr-Y-Nb-Rb-Sr-Ga-Ph were analysed using pressed pellets, (bd = below detection limit; na = not analysed) 170 Appendix A.l (continued). Chemical composition of felsic volcanic rocks at Seneca. Sample 92-27-71 Hole 92-27 Depth 70.9 Depth 232 5 Rock Type Q F P 92-27-85 92-27-177 92-27 92-27 85.2 177.7 279.5 583.0 FP FP 92-28-35 92-28-132 92-28 92-28 35.4 132.5 116.1 434.7 F P F L A L T F L 92-28-185 92-29-140 92-28 92-29 185.0 140.3 606.9 460.1 Q F P Q F P 92-29-227 92-31-281 92-29 92-31 226.8 281.4 744.1 923.0 FP FP S i 0 2 69.59 64.75 68.95 74.13 69.8.3 74.14 75.01 63.72 68.44 T i 0 2 0.39 0.68 0.51 0.34 0.40 0.36 0.27 0.71 0.52 A l 2 0 3 13.97 15.49 14.36 12.78 14.15 12.94 13.04 16.01 14.81 Fe 2 0 3 3.52 6.23 4.43 2.37 2.81 1.95 2.41 5.89 3.93 MnO 0.14 0.20 0.15 0.13 0.18 0.13 0.19 0.20 0.14 MgO 1.45 .3.03 2.86 2.82 2.22 2.27 1.08 3.49 1.61 CaO 2.46 1.61 1.28 0.41 1.52 1.03 1.46 1.71 1.56 Na 20 4.51 5.59 4.31 1.16 5.86 3.25 4.80 5.57 6.19 K 2 0 1.03 0.05 0.73 3.01 0.40 1.67 0.62 0.28 0.43 P 2O s 0.09 0.20 0.1 1 0.07 0.09 O.07 0.06 0.23 0.15 LOI 3.23 2.20 2.39 3.18 2.93 2.67 1.54 2.72 2.12 Total 100.38 100.03 100.08 100.40 100.39 100.48 100.48 100.53 99.90 Cr 156 77 127 bd bd bd 1 bd 17 Ni 18 19 18 1 bd 1 bd bd 5 Co 7 12 9 10 15 9 21 15 2 Sc 6 17 5 na na na na na na V 51 99 78 29 29 29 42 56 70 Cu 46 379 6 73 21 27 61 249 44 Ph 6 6 4 na na na na na 2 Zn 94 184 83 120 100 125 114 171 104 Ba 448 bd 200 1814 139 495 251 146 286 Rb 14 4 11 38 5 24 8 3 5 Sr 179 1 18 141 64 109 64 312 224 162 Ga 12 14 13 12 13 13 13 17 15 Nb 4 5 5 8 8 8 5 4 8 Zr 103 96 114 129 152 147 109 94 133 Y 21 26 26 22 28 33 15 36 32 Th bd bd bd bd bd bd bd bd bd U 1.0 bd bd bd bd bd bd bd bd Cs na na na na na na 0.3 na na Hf na na na na na na 3.0 na na La na na na na na na 9.6 na na Cc na na na na na na 21 na na Nd na na na na na na 9 na na Sm na na na na na na 1.85 na na Eu na na na na na na 0.50 na na Tb na na . na na na na 0.4 na na Yb na na na na na na 1.78 na na Lu na na na na na na 0.26 na na Zr/Y 4.9 3.7 4.4 5.9 5.4 4.5 7.3 2.6 4.2 LaN/YbN 3.6 Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass beads The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A . l (continued). Chemical composition of felsic volcanic rocks at Seneca. Sample 92-31-244 Hole 92-31 Depth 244.5 Depth 802.2 Rock Type ALTFP 92-31-225 92-33-75 92-31 92-33 224.9 75.0 737.9 245.9 FP FP 92-39-71 92-39-200 92-39 92-39 71.1 200.5 233.2 657.9 FP QFP 93VT-01 93-VT-02 86-13 86-13 25.0 50.0 82.0 164.0 FP ALTFP 93-VT-03 93-VT-04 86-13 86-13 65.0 85.0 213.3 278.9 ALTFP ALTFP S i 0 2 61.89 68.41 69.73 72.52 73.53 71.29 71.15 70.22 70.44 T i 0 2 0.60 0.50 0.36 0.35 0:31 0.41 0.38 0.42 0.41 AI 2 0 3 16.77 13.91 15.13 13.77 13.37 14.32 12.28 13.91 13.43 Fe 2 0 3 3.96 4.54 3.28 3.45 2.23 3.48 4.24 3.19 3.72 MnO 0.12 0.17 0.13 0.12 0.10 0.19 0.08 0.16 0.08 MgO 5.29 3.68 2.43 2.24 0.94 2.07 3.76 4.38 3.54 CaO 1.13 0.98 0.95 0.75 1.34 0.54 0.49 0.52 0.20 Na 2 0 4.02 3.57 5.15 3.54 3.97 5.22 0.20 bd bd K 2 0 1.55 1.21 0.83 1.26 2.84 0.54 2.86 3.04 3.57 P i 0 5 0.18 0.13 0.07 0.07 0.06 0.09 0.09 0.1 1 0.09 LOI 4.06 2.95 2.36 2.14 1.52 1.70 4.90 4.28 4.57 Total 99.57 100.05 100.42 100.21 100.21 99.85 100.43 100.23 100.05 Cr 3 26 165 178 376 79 87 84 157 Ni 4 5 9 4 -> 1 1 13 8 45 Co 6 8 7 6 9 -» 7 10 6 Sc na na 3 7 6 10 10 6 5 V 76 80 40 37 29 56 47 34 46 Cu 69 47 37 . 22 609 66 223 28 86 Pb 282 4 5 4 5 3 12 6 14 Zn 338 176 57 48 850 99 430 160 607 Ba 1680 263 261 405 2499 200 546 739 830 Rb 18 16 10 15 20 8 41 36 53 Sr 136 90 173 1 14 219 89 8 8 9 Ga 16 15 12 13 9 12 14 13 14 Nb 8 8 8 7 5 5 6 6 6 Zr 140 113 154 142 120 1 16 117 128 123 Y 40 32 27 26 17 21 24 27 20 Th bd bd bd bd bd bd 2.0 bd 2.0 U bd bd 2.0 1.0 1.0 bd bd bd bd Cs na na bd na bd na na 0.9 0.3 Hf na na 3.2 na 2.8 na na 3.0 2.9 La na na 10.7 na 11.1 na na 11.2 8.6 Ce na na 23 na 24 na na 26 21 Nd na na 12 na 12 na na 14 11 Sm na na 2.73 na 2.21 na na 2.87 2.23 Eu na na 0.64 na 0.72 na na 0.51 0.37 Tb na na 0.5 na 0.4 na na 0.6 0.5 Yb na na 2.63 na 2.04 na na 2.95 2.26 Lu na na 0.41 na 0.30 na na 0.49 0.33 Zr/Y 3.5 3.5 5.7 5.5 7.1 5.5 4.9 4.7 6.2 L a N / Y b N 2.7 3.7 2.6 2.6 Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass beads. The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A.l (continued). Sample 93-SN-44 93-SN-45 93-SN-46 93-I<\V-4f> 93-KW-49 93-FW-51 93-FW-52 Hole 79-08 79-08 79-08 91-06 91-06 91-06 91-06 Depth 200.0 199.0 179.5 22.2 46.5 128.0 204.6 Detection Depth 656.2 652.9 589.0 72.8 152.6 419.9 671.2 Limits Rock Type A L T F P A L T F P FP A L T FP A L T FP FP A L T FP (ppm) Si02 78.57 67.68 72.10 58.00 69.29 74.16 80.87 60 Ti0 2 0.27 0.37 0.35 0.67 0.38 0.31 0.31 .35 Al 2 0 3 10.01 13.86 13.45 15.48 14.33 13.24 8.50 120 Fe 20 3 2.24 3.38 3.11 7.89 4.02 2.46 3.69 30 MnO 0.17 0.29 0.14 0.41 0.20 0.12 0.01 30 MgO 1.02 1.72 2.32 5.08 1.31 1.30 0.56 95 CaO 1.51 3.04 0.92 3.47 2.31 0.83 0.29 15 NazO 2.07 4.44 4.26 4.93 5.13 4.35 bd 75 K 2 0 1.43 1.14 1.00 0.41 0.29 1.80 2.32 25 PiOs 0.06 0.09 0.08 0.13 0.11 0.06 0.09 35 LOI 2.62 3.75 2.31 3.96 2.13 1.52 3.53 100 Total 99.96 99.76 100.05 100.44 99.51 100.16 100.17 100 Cr 7 2 4 40 bd 15 bd 2 Ni bd bd bd 6 bd 37 bd -> Co 23 18 9 30 31 25 22 10 Sc 6 12 9 27 7 3 3 10 V 32 41 45 195 40 39 41 10 Cu 35 19 23 64 23 38 547 15 Pb 6 6 2 4 1 1 49 2 Zn 79 118 69 202 98 57 715 2 Ba 436 400 355 453 1586 1576 1677 50 Rb 19 16 16 4 4 16 34 1 Sr 49 70 70 174 261 165 22 1 Ga 10 13 13 15 14 12 13 1 Nb 11 11 11 8 9 1 1 10 1 Zr 99 131 128 69 1 1 1 126 77 1 Y 23 29 27 26 28 26 19 1 Th bd bd bd bd bd bd bd 0.1 TJ bd bd bd 0.8 bd bd 0.9 • 0.1 Cs na na na na na na na 0.2 Hf na na na na na na . na 0.2 La na na na na na na na 0.1 Ce na na na na na na na 1 Nd na na na na na na na 1 Sm na na na na na na na 0.01 Eu na na na na na na na 0.05 Tb na na na na na na na 0.1 Yb na na na na na na na 0.05 Lu na na na na na na na 0.01 Zr/Y 4.2 4.5 4.7 2.6 4.0 4.8 4.0 LaN/YbN Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass heads. The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A.2. Chemical composition of mafic volcanic rocks at Seneca. Sample 83-02-227 83-06-124 83-10-56 83-17-34 85-03-155 91-08-200 91-16-69 91-16-231 91-16-233 Hole 83-02 83-06 83-10 83-17 85-03 91-08 91-16 91-16 91-16 Depth 227.0 123.9 53.6 34.0 154.8 199.8 69.3 231.1 232.9 Depth 744.8 406.5 175.7 1 11.7 508.0 655.6 227.5 758.3 764.0 Rock Type M A F I C M A F I C M A F I C M A F I C MAFIC MAFIC M A F I C M A F I C M A F I C S i 0 2 47.28 51.41 51.90 51.83 53.66 48.91 50.95 45.56 43.75 T i 0 2 0.68 0.83 0.86 0.83 0.81 0.77 0.77 0.65 0.66 A l 2 0 3 19.93 17.50 18.25 17.51 17.45 20.55 17.16 15.84 16.10 Fc 2 0, 9.40 10.10 9.58 10.11 9.02 9.45 10.67 7.92 9.52 MnO 0.17 0.19 0.21 0.3 1 0.16 0.16 0.58 0.29 0.33 MgO 7.43 6.51 6.16 6.14 5.40 4.17 8.46 6.34 9.04 CaO 4.89 2.97 2.44 2.82 2.93 9.42 3.11 10.45 8.19 Na 20 3.86 4.79 5.92 5.64 5.57 2.92 3.79 2.48 3.08 K 2 0 1.39 0.68 0.10 0.06 0.29 0.36 0.14 1.24 0.20 P 2 O 5 0.13 0.19 0.24 0.22 0.23 0.15 0.1.3 0.14 0.1 1 LOI 5.13 5.54 4.57 4.98 4.87 3.58 5.02 9.09 9.34 Total 100.29 100.71 100.23 100.45 100.39 100.44 100.78 100.00 100.32 Cr 209 75 30 45 55 16 44 150 132 Ni 38 18 21 15 20 19 18 14 23 Co 32 30 34 27 34 25 31 30 36 Sc 29 23 24 22 22 na 29 32 32 V 210 227 215 210 186 280 235 242 229 Cu 45 37 122 69 76 167 1 18 94 93 Pb 9 9 7 8 8 bd 8 bd 10 Zn 66 126 190 204 147 62 286 83 273 Ba 753 747 37 6 148 65 1026 88 Rb 22 18 9 10 10 4 11 7 12 Sr 372 103 119 72 156 352 196 174 177 Ga 15 16 17 16 15 18 14 13 13 Nb 1 4 4 4 4 6 2 7 1 Zr 47 81 77 75 79 49 63 30 44 Y 14 19 21 16 19 19 17 15 14 Th 1.0 1.0 bd 1.0 bd bd 1.0 bd 1.0 U bd bd bd bd bd bd bd bd bd Cs na na na bd bd na na n a 0.4 Hf na na na 1.2 1.4 na na na 0.7 La na na na 3.0 5.2 na na na 2.5 Ce na na na 8 13 na na na 6 Nd na na na 6 8 na na na 5 Sm na na na 1.58 2.09 na na na 1.39 Eu na na na 0.52 0.73 na n a na 0.52 Tb n a na na 0.3 0.4 na na na 0.3 Yb na na na 1.66 1.82 na n a na 1.31 Lu na na na 0.24 0.26 na na na 0.18 Zr/Y 3.4 4.3 3.7 4.7 4.2 2.6 3.7 2.0 3.1 LaN/YbN 1.2 1.9 1.3 Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass heads. The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A.2 (continued). Sample 92-27-333 92-28-374 93-.SN-47 93-IW-48 93-FYV-50 94-IT-01 Hole 92-27 92-28 79-08 91-06 91-06 -Depth 333.5 374.5 161.0 37.3 95.1 - Detection Depth 1094.0 1228.6 528.2 122.4 312.0 - Limits Rock Type MAFIC MAFIC MAFIC ALT MAI- ALT MAF MAFIC (ppm) Si02 45.33 47.89 52.91 43.47 49.65 39.42 60 Ti0 2 0.77 0.58 1.18 0.52 0.82 0.75 35 Al 2 0 3 17.80 20.38 15.18 19.34 17.27 14.97 120 Fc 20 3 10.79 8.71 13.42 11.02 12.29 8.38 30 MnO 0.58 0.41 0.25 0.44 0.82 0.31 30 MgO 13.88 10.45 4.39 0.63 8.60 5.73 95 CaO 1.29 1.07 4.03 21.29 2.51 17.42 15 Na20 3.21 1.30 4.48 0.10 2.33 0.93 75 K 2 0 0.01 3.09 0.29 bd 0.35 0.29 25 P 2 O s 0.13 0.13 0.27 0.11 0.15 0.15 35 LOI 6.92 6.63 4.06 3.46 5.42 11.44 100 Total 100.71 100.64 100.47 100.37 100.21 88.35 100 Cr 133 92 9 144 179 63 2 Ni -i -> 22 bd 1 50 10 3 Co 48 13 28 25 32 24 10 Sc 31 na 29 37 32 34 10 V 264 206 283 279 310 273 10 Cu 151 35 108 22 472 86 15 Ph 6 na 6 7 bd bd 2 Zn 479 317 120 -> -> .1.1 363 86 2 Ba bd 1249 329 134 183 76 50 Rb 9 43 2 bd 2 5 1 Sr 78 78 270 699 181 350 1 Ga 15 16 19 19 18 16 1 Nb 2 4 7 4 8 0 1 Zr 48 37 59 26 40 47 1 Y 16 16 38 12 16 17 1 Th bd bd bd bd bd 0.4 0.1 U . bd bd 4.8 bd bd bd 0.1 Cs 0.2 na bd na na bd 0.2 Hf 0.7 na 1.4 na na 0.6 0.2 La 2.9 na 6.6 na na 3.0 0.1 Cc 8 na 19 na n a 9 1 Nd 6 na 1.3 na na 7 1 Sm 1.79 na 3.81 na na 1.99 0.01 Eu 0.56 na 1.16 na na 0.71 0.05 Tb 0.4 na 0.8 na na 0.4 0.1 Yb 1.53 na 3.25 na na 1.52 0.05 Lu 0.23 na 0.47 na na 0.22 0.01 Zr/Y 3.0 2.3 1.5 2.1 2.5 2.8 LaN/YbN 1.3 1.4 1.3 Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass heads. The elements Zr-Y-Nb-Rb-Sr-Ga-Ph were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A.3. Chemical composition of volcaniclastic rocks at Seneca. Sample 83-10-20 83-17-19 85-03-92 87-11-14-1 87-11-1 fill 87-12-115 91-03-14 91-03-35 91-03-75 Hole 83-10 83-17 85-03 87-11 87-11 87-12 91-03 91-03 91-03 Depth 20.8 19.3 91.8 144.8 160.3 1 14.9 14.0 35.2 75.1 Depth 68.3 63.5 301.2 475.1 525.8 377.0 45.9 1 15.5 246.4 Rock Type V C L T V C L T V C L T V C L T V C L T V C L T V C L T V C L T V C L T S i 0 2 48.22 63.73 73.62 74.98 53.80 72.90 71.71 69.48 72.12 T i 0 2 0.61 0.44 0.26 0.43 0.73 0.32 0.30 0.29 0.41 A l 2 0 3 18.76 13.08 13.25 11.95 16.64 13.22 13.50 13.82 13.42 Fc 2 0 3 8.47 5.90 2.52 2.99 8.45 2.95 2.51 2.96 2.80 MnO 0.20 0.30 0.04 0.15 0.53 0.08 0.08 0.10 0.11 MgO 7.45 5.32 1.82 1.87 7.38 2.10 1.51 1.66 1.48 CaO 3.62 1.91 0.75 0.62 2.50 0.61 1.44 2.03 0.63 Na 20 3.93 2.01 3.29 5.03 5.50 2.58 4.48 4.47 4.27 K 2 0 2.43 2.54 1.82 0.38 0.10 2.11 1.17 1.29 2.86 P 2 O s 0.11 0.08 0.04 0.11 0.16 0.07 0.08 0.07 0.13 LOI 6.24 4.70 3.04 1.44 4.57 2.76 3.51 3.85 1.79 Total 100.05 100.01 100.45 99.95 100.36 99.70 100.29 100.02 100.02 Cr 241 257 32 12 37 21 28 34 25 Ni 38 38 7 4 18 6 2 7 9 Co 28 20 6 2 24 bd bd 1 bd Sc 25 11 5 na na na na na na V 210 113 40 27 228 36 32 29 34 Cu 76 43 10 67 98 30 29 25 56 Ph bd 24 10 2 4 7 2 2 8 Zn 67 198 58 58 303 76 J J 39 53 Ba 1455 1630 88 39 15 690 84 348 639 Rh 20 25 30 5 1 28 15 17 31 Sr 133 67 82 45 1 13 102 66 105 64 Ga 14 11 12 12 15 14 13 14 14 No 8 4 5 9 6 8 8 8 9 Zr 39 90 117 117 61 150 100 112 157 Y 15 21 20 36 26 24 20 22 4.3 Th bd 6.0 bd bd bd bd bd bd bd U bd bd bd bd bd bd bd . bd bd Cs na na na na na na na n a na Hf na na na na na na na na na La na na na na na na na na na Cc na na na na na na na na na Nd na na na na na na na na na Sm na na na na na na na na na Eu na na na na na na na na na Tb na na na na na na na na na Yb na na na na na na na na na Lu na na na na n a na na na n a Zr/Y 2.5 4.3 5.9 -> -i 2.3 6.3 5.0 5.1 3.7 LaN/YbN Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass heads. The elements Zr-Y-Nb-Rb-Sr-Ga-Ph were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A.3 (continued). Chemical composition of volcaniclastic rocks at Seneca. Sample 91-03-116 91-03-150 91-08-167 91-08-184 91-08-194 91-16-94 91-16-153 91-18-181 92-29-164 Hole 91-03 91-03 91-08 91 -08 91-08 91-16 91-16 91-18 92-29 Depth 116.8 150.1 166.9 184.8 194.5 93.9 152.8 181.4 163.9 Depth 383.0 492.4 547.7 606.2 638.1 308.2 501.3 595.1 537.7 Rock Type V C L T V C L T V C L T V C L T V C L T V C L T V C L T V C L T V C L T S i 0 2 73.48 69.60 63.99 73.15 66.00 63.71 79.01 69.60 76.79 T i 0 2 0.34 0.48 0.58 0.29 0.36 0.67 0.21 0.35 0.24 A l 2 0 3 12.11 14.08 17.10 13.98 15.59 15.15 8.46 ' . 13.57 11.95 Fc2Oj 2.53 2.99 4.27 1.78 4.39 6.67 1.26 5.54 1.78 MnO 0.06 0.07 0.15 0.08 0.18 0.23 0.03 0.09 0.04 MjjO 3.38 3.60 3.18 2.44 4.40 5.24 1.37 2.62 3.07 CaO 0.39 0.41 1.10 0.37 0.43 0.92 1.93 0.43 0.25 Na 20 1.66 3.23 4.20 2.15 2.81 3.01 0.00 3.30 0.73 K 2 0 1.91 1.68 2.06 2.73 2.20 1.07 2.23 1.52 2.73 P 2O s 0.09 0.12 0.13 0.07 0.08 0.14 0.01 0.08 0.04 LOI 4.09 3.79 3.30 2.62 3.60 3.64 5.18 3.41 2.95 Total 100.04 100.05 100.06 99.66 100.04 100.45 99.69 100.51 100.57 Cr 20 29 18 25 37 17 264 bd bd Ni 1 -» j 4 -> 13 19 10 -> _> bd Co 5 8 7 5 11 10 6 27 12 Sc na na na na na 14 1 na na V 22 42 60 22 56 123 20 62 27 Cu 33 23 i -) .5 J 19 16 16 7 18 27 Pb 4 j 4 1 3 6 5 na na Zn 51 62 100 49 109 91 52 72 81 Ba 121 372 382 489 439 222 2832 450 627 Rb 22 18 30 39 31 IS 26 20 40 Sr 49 63 126 41 58 67 34 70 37 Ga 14 14 18 16 17 16 5 14 13 Nb 9 9 8 10 9 5 5 7 8 Zr 147 129 129 176 134 102 109 117 140 Y 34 41 35 31 25 34 15 25 21 Th bd bd bd bd bd bd bd bd bd ll bd bd bd bd bd bd bd bd bd Cs na na na na na 0.3 na na na Hf na na na na na 2.4 na na na La na na na na na 6.4 na na na Ce na na na na na 18 na na na Nd na na na na na 13 na na na Sm na na na na na 3.77 na na na Eu na na na na na 0.95 na na na Tb na na na na na 0.7 na na na Yb na na na na na 3.50 na na na Lu na na na na na 0.54 na na na Zr/Y 4.3 3.1 3.7 5.7 5.4 3.0 7.3 4.7 6.7 LaN/YbN 1.2 Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass heads. The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed using pressed pellets, (bd = below detection limit; na = not analysed) Appendix A.3 (continued). Sample 92-39-237 92-33-172 92-33-173 92-33-176 Hole 92-39 92-33 92-33 92-33 Depth 237.4 171.7 173.0 175.4 Detection Depth 778.8 563.2 567.6 575.4 Limits Rock Type V C L T O T H E R O T H E R O T H E R (ppm) Si02 79.51 25.64 93.99 95.49 60 Ti0 2 0.20 0.20 0.06 0.07 35 Al 2 0 3 10.61 13.64 2.42 1.49 120 Fc 20 3 1.05 12.77 2.17 1.15 30 MnO 0.02 0.39 0.01 0.01 30 MgO 0.57 10.40 0.44 0.18 95 CaO 1.10 0.62 0.17 0.23 15 Na20 4.06 8.42 2.20 bd 75 K 2 0 0.63 0.60 0.39 0.29 25 P20 5 0.04 0.10 0.05 bd ' - 35 LOI 2.29 11.63 0.00 1.11 100 Total 100.08 84.41 101.90 100.02 100 Cr 291 0 292 602 2 Ni 9 39 9 8 3 Co 7 31 1 5 10 Sc 4 6 0 -> 10 V 19 118 69 17 10 Cu 81 10029 2294 88 15 Pb 4 94 4450 10 2 Zn 67 166362 42166 300 2 Ba 692 bd bd 1971 50 Rb 5 74 bd 1 1 Sr ' 120 75 106 38 1 Ga 5 0 36 1 1 Nb 5 6 bd 1 1 Zr 125 118 bd 37 1 Y 21 10 3 5 1 Th bd 56.0 bd bd 0.1 U 1.0 bd 24.8 bd 0.1 Cs na na na na 0.2 Hf na na na na 0.2 La na na na na 0.1 Cc na na na na 1 Nd na na na na 1 Sm na na na na 0.01 Eu na na na na 0.05 Tb na na na na 0.1 Yb na na na na 0.05 Lu na na na na 0.01 Zr/Y 6.0 11.8 0.0 7.4 LaN/YbN Analytical method: X-ray fluorescence, induced neutron activation analysis The major elements plus Cu-Zn-Ni-Cr-V-Sc were analysed using glass heads. The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed using pressed pellets, (bd = below detection limit; na = not analysed) A P P E N D I X B M A S S C H A N G E C A L C U L A T I O N S The following tables contain the results of the mass change calculations that are discussed in Chapter 4 of this study. Allsamples in the data set are included in these calculations. A complete description of the theory and methodology of such calculations can be found in MacLean and Kranidiotis (1987) and MacLean (1990). Data in these tables are reported as weigh! percent changes. Negative values correspond to mass loss and positive values correspond to mass gain. 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O N IO O N fN , 00 X o C I - J Lu Tt oo O N ro r _ - vo H O > T oc —< \ n r-CN O N > ro N O O N N O N O r-' N O - - iO - t — r j \0 O "t — i r o O N — - I D IO O Tf IO O — O — 0CI © Cl o o © © o o o © © o r N T f l - - - - , \ O N O N O r N r - - 0 0 © io o O N i o © ro r~ oo © I D O O © ' © ' © ' —' O CN © O « N O — ' r O N O r o f N — ' r o r o f N r ^ -© © T t r N © T t i o r N T t © T t fN © ' © ' O © ' — • ' © ' fN O O © fN —^© lO - tTtoOONNOfN O N — ' © - f © © T f r o O N - t © 0 C -U © ' © ' © ' © ' fN © rN © © <-« T t f N r o O N ' — > r o I — — ^ © 0 0 Tf © I O C— Tt © I O Tf © © ' © ' © ' — < © ' — • © ' © ' © ' © ' r~ — © oo © io ro © >— ro Tf ro rN O N — , fN N O rN •— © u > N 0 0 © 0 © — © Tt I O © NS Tt — O N r - ~ r O N 0 0 0 r O T t r N ,— © ro ro © iO © -t ro © 00 O ©'©'©' O ©'©'©' © io — 1 -t O N •—' ro ro r~- —• 0 0 rN © ro —• © io Tf • — - N O © * -ro © ©' O O © ' © ' — • —< © ci cn — N O io N O ro co —f © Tt r o © r o © © i O N O T f — ' © C N oo © > © ' — • © ' <N ©' C N ©' ©' r~ t- — • — ' C N ' — M IN fN W — ^ C N O N © ro io © ro N O f^ - N O © © N O © ' © ' © ' © ' — - © ' fN © O C \ —1 fN ro Tf — \0 rN N O O N CO I D 0 0 © N O N O © r o r N r ~ N D © © NO' © ' © ' © ' © ' © ' © ' — © ' © ' C N a c >* H s: _< o 3" 37 o K Q Q OS o o o H < O O n O n '" -j c — £ £ I" E" « H ~ -< | e ~ ~ g CAI X Q Q oi 183 OV O V o rs <n OO Cu Lu • r-r- rs rs I OV O N 00 © Cu Lu a "T vo rs vo rs O N O N O vq t Tf' *V oo rs 9 0 — •n cn —i rs r-' >n rs i f r-Ov O N rs >n 00 rs Cu Lu oo cu 0 0 t— .—i <n 9 0 00 O N H 00 <n 0 0 _ J • O ov O N o >n > rn O N T Tf VO d o VO 00 rs 0 0 Lu T Cu ON O N rs in O N Lu r — oo o o rn O ON O N Cu Lu — O N vo rN T f T f v O rn rn Tf — O O rs Tf r s o o v o o i n r n r s r s o o o — ©' © © © ' © ' © © d d rs' ov — r-m o Tf • * ' © ' © © ' © © i n r n T f r ~ - o o r n — oo —i © rs rn >n Tf o >n © © d Tf oo — r s r s — v o o o r - v o v o © m © © ( N v o © i n Tf © d © © © © © © © © © © — O N rs in r - o in rs 0C rn © rn © -f Tf rs rs rs vo © — o o © © — © © © r n -in rs ov oo •—• ov © ov r- m in T f © i n r n — ' r N O v o o © © © in d d —• © — © Tf rn d rs rn i i i i i i 1 ro oo — rs © — r - © T f v o o - f —i © Tf m © —• vo vq o o Tf ©' © rs © —' d — © © ro oo — rn — © Tf —• rs rn o r~ — © m, rn — in o r s © © 0 © 0 0 © vo CN cv © vo rn r-- •—< *— N CV m © v O r s © T f r s m r s © — ' © © © ' © — © rn — © ro oo rn ov rn — in rs — Tf iri —1 © vo — © in <n 'n rs © rs —' d © © ©' rs — rs © © © •n Ov C N 00 Cu Lu rs — O N O N Tf r-' rs rn rs £ ON O V © in in — Tf rs cu Tf © iZ, rs oo H •—• rn oo rs rs O N Cu Lu © 00 V 00 oo rs , 00 oo rs Cu Lu © rs O N r-m r-VO rn , _ ,n H U > cu Lu <y © in © CN in vd oc © — vO cu Lu a © O in r- •> rN Tf' O rn rn —J © — 00 o r- © rs © r- © r- — rn rs © C N — rn O N © © © © © © © d d © C N rs O N ON © O © Tf — © — © © © © rn © — © — Tf Tf oo rs S — o rn o ro m r s o o v o r n v o o v o o i n o o © © V O © © © — 00 Tf © Tf vd © ' © — ' © ' CN © — ' © © ' l~ •—' rn © O rs O ON © 00 © vo in © oo ro r- in © © ' © ' © © © o d d © © Tf oo —i rs O N O O N 'n O Tf V O — rs rn Tf VO © O © n o o o o o o o o o i r i in — o o v T f in c v r s v o — rs — © r n — © T f v O C v © © i n rn — — vO — © rn •— ov vo — rs © rs © r~-© © © © © O O O — — r~ © Tf C v r n v O T f v O r s r s — © © r— © in m © oo in © © — © © © — © © r o rn '— o o v o v o v o r s o o m — ov h O t ' l O ^ T t v O O O -— © >ri © © o o o © ON O N © Tf © vo O N — CU •n rs hv — -n ON O V '3 oo cn to rs — in © T vo — i n vo — > Ov — VO © O N O N rn O N in r - O © V O © © © ©' © © © Tf VD •n oo © — © Tf r -—' © © © ©' d in rn VD O — vo © © 00 © O N C N © rt — © © < © Tf t - ^  S 2 Q 00 rs oc rs ON O V © © co in Cu Lu r - — Tf vq © Tf in d © — rn oc in © Tf O rn 00 —' vo r~ vo © © O — © m — © ( N l r - o O T O v n o o o - l - r -© vo © © rn — O — © <r, I I I i i i i s -C/3 P3 c. •fi JS H a. c. y y o O Q 02 o o ^ o = '7i H < Es, 2 &<2° o o 3 E u z £ d.' H - J : js H i " — a. a. i i W I £ c i " -i' O £«, O O a O •" 184 > N O £ © O n fN 00 rN c-y oo ~- o 1/5 I to ° N ON t~-l« T , 00 ON rN ON in N O ox r~-I- 2 ON 00 o o ON 00 n o •n o O N O N r- oo i—i in CL, LL, 00 00 Cu, L L , S-3 O NO o — O NO o in 00 rN NO O 3 o r-O 00 •n oo o. 00 r- LL, in ro N O — C L L L , t— O - f o o O -t CL m N O u. ON 00 r-1 O rN o o in rN rN oo o -t rN 00 r— in o 00 o N O o O N N O rN in •— 1 00 — * o 00 oo 1 d • d I d rN © d d i -f 1 o rN n 00 -r N O O N O N -t- •— 1 o 00 — N O in © "1 d d d d d d d d d d N O N O o d d d d oo t— in in o —i — — O N - N O © O O — O O O I D O N — in o -t rN -t rN in rN — O — — © —• -t o n © © © © © — rN © © CN 00 rN rN o ri •n 00 © 00 00 oo <N ri -t rN O ro C N — d d d d — d in rN d © 00 -1-rN O rN •n -i-o o in r~ in rN NO © © oe —' d d d d rN d in d ro' oo N O fN O -1-in N O O N n O rN -t ri in rN O N ri O O ro C N ON' d d d d rN d -t rN d 00 rN 00 O O N - f in o r-o N O 'n rN in — oo 00 rN 00 rr ri d d d d d d d d d I D O in -f -I-in in <v - £ & o o rl r~ t—• N O r~ — NO rN M — n £ w r-o 00 ON O N O — © O — ON — — © — -t © tN —• in ro rN O N © ' © • I D r- — © n rN O -t ri O N © d 00 •n © r- N O r i © N O O — © n in rN rN d —' ©' r-' d ©' NO • — ^ © © n o N o o r i N O N o r ~ - t~-00©rNrirN — M \0 O X © ci — ' © ro' i i ro © © O N © ON oo to f O N S <? ON O N O N n n 00 (—' oo n r~ rN r~-> o O N 2 n fi i ON O N n © in O N ©' r~-' © in rN N O L ~ LL, cy o i O N £ c l to i ON O N n n rN C L L L , — - CO N O o o O N rN O N © rN N O © — -r CN (N O ro ©' © ©' © © d d © —' © C N <N • 1 1 • O N N O ri © N O O N C N ri © n CO o CN n oo © © r-i ©' • ©' • © d ©' i © ' © • i ri CN O N O N -t N O ro N O © -t © r- rN -t ri © C N rN © d ©' ©' © © — © d ro' © in NO CN Zl — C L © r-N O rN N O © < © -t © O N — 00 O N "? CN — > N O " + © N O •n in NO o N O rN CL H O N ri © ro © r-- ro ro 00 O N rN © — NO NO ri — © O N O NO 1 © '— © — © in ri © © n NO ri 00 — © O in — — -i- — o o © o © © © © © © O © n N O o © d o ' © O N © N O O N - ) -© ri — O CN o — o — rN I D c g V X X u CAI S Q Q OS d d ° I g § | o ° q 6 ; f 2 £ ~ f i f-E W - - - C I « u 3" S C D x a o a: d o g o | | o § o 0 ; l APPENDIX C ESTIMATES OF PRECISION AND ACCURACY 186 A number of Mineral Deposit Research Unit in-house standards were submitted at different times with the various batches of samples analysed for this study to examine the accuracy of analytical data. Three different standards were used: ALB-1 - Ajax albitite, P- l - Porteau Cove granodiorite and QGRM-100 - gabbro. The MDRU-compiled data from previous analyses of these standards was taken from Fraser (1994). The mean values of this data were used as the accepted values reported in the following tables. It should be noted that these previous analyses were carried out at a different laboratory (X-RAL, Don Mills, Ontario) than that used in this study. As such, these standards only provide an approximate guide to the accuracy of analyses of the samples in this study. The standards were submitted in different batches and were analysed more than once in two of the batches. Several batches of samples from other studies were submitted at the same time as well as between the batches from this study. These other batches included duplicate samples and the same M D R U standards as used in this study and thus provided an additional means of assessing analytical variability both during the analysis of a single batch of samples and between several batches of samples. There was little analytical variation both within a single batch of analyses as well as between batches. Based on the standards, percent errors were low (<5 %, and often < 1.5 %) for the major elements. The trace elements, however, showed a wide range in errors, although the elements most commonly used in this study (e.g. Zr and Y) generally showed relatively low degrees of error (<I0 % error for Zr, 7 to 13 % error for Y). The samples analysed at McGill University usually have smaller associated errors than those analysed at X R A L Labs. Different analytical techniques (e.g. X R F using glass beads versus pressed pellets) can account for some of the disparity in trace element concentrations between the different labs. Comparative accepted versus measured binary plots illustrate the relationship between the various analyses (Figures C. 1 to C.3). Five duplicate samples of rocks collected for this study were also submitted to test precision. This data is included in Appendix C.2 and is summarized in a series of plots in Figures C.4 and C.5. 187 In general, precision was excellent for the major elements (<5 % deviation) except for possibly Na 2 0 and K 2 0 . Some of the trace elements show some scatter, but generally lie within 1 standard deviation of 1:1 line. 188 C eS 00 oo oo 00 00 00 00 00 00 oo 00 oo 00 rs in VO in >n VO r— Tf o 00 vO •n in rn © rS CO rn 00 © Tf 00 © — TT —' Tf d d Tf' d ° d d m 1 S =. o CJ *; CJ on < >-c cs cj s J CS •_! 1 5 <u u in 00 rN 00 VO VO VD VD VD © VO 00 O 00 oo oo rn © — rN •n — • rn •n rN in _ © — vo in o '—1 rn rs © O — O O O O — p o cia><-Sci<-idc>a>c> oo rn Tf in o O N oo o o o oo Tf vo oo O N o 00 o d rn d — • rn m <N d O N O Tf O o O N — d t O O rn ON rN oo O N rn rn vO O N VD rN rn vq Cv CV •n Tf m rn Ov 00 in o o Tf r-—' rN rn <N — -f rN rN rN rN d —' in rN d — o •n d o rN oo m I— rn 795.50 o 16.72 r-oq rn vq m X vq rn 795.50 O N Tf 16.72 00 Tf VO rN Tf 795.50 Tf rN ™T rN rN vo o o o r-- Z - ^ ° ~ >n in in in in >n in in in in in in in »n in in in in in in in in »n in in in O O N — ' 00 ro Tf ov in — — i n - r r N o o r - - o o r N r n r ~ O Tf —' •—< Tf (Ni rs' rN[ rn rs VO •n oo rs rs © Cv in 1.85 r-' 00 o vO — j VO oo 00 vq Tf O N r~ O N in Z 2? S rs g °, oo S oo' 2 <n rn rs o o o o rs O 00 o O N © o © vo •n in vq VO ON o Tf r-rs d d d © d d d d d d d © O N © •n rs rs —, r-. m O0 O rn •n oo rn Tf p Tf vO vO vO Tf Tf rn rs oo © •n _ r-' © © Tf rs Tf Cv Cv © © Cv >n rn o •n o Cv © © Tf rn ©' —' m Tf rs © oooooooooooooooooooo r-r- ra rn Tf rn r~ © rn oo rn rn r- § r~: ie ° in oo r ~ .ri rN • CS © © c _ ^ £ ov r-' 2 °S-<2 rN r n Tf rn oo oo © C N O N Tf VD rn vO rn Tf rn C N © © © — ' rn — © rs © — C N 00 vo in rs ov — — © r-rn ON rs rn '—' p rn OO rn rs rn Tf r— vO VO VO vO VD vO O N 00 00 O C Ov VD 16.53 Ov Tf m in Tf r j vd vd •n © Cv Tf C N Tf 16.53 © rs O N Tf r-Cv — r-- rs '—- Tf in rn — — — © —. © © © © — O © © ' © © ' © © © © © © © rn rn rs © © oo © rn in m vO rn O N in 00 rs rs o o 00 in rn © — o o — ' o rn rN •n d — vd rs © © VD © o m VO Tf o m r-rn rs VO CN 00 o rn © Ov —< © rs © in © d rn 8 ^ £ °°. " i r--7; OV r-o © rs •n vo oo © — Tf rs in in ^ r-. ^ Tf w S ^' 3 ^ o °? rs r- 1 © <=> © °. o o ^ <=> <=>' in o Tf in ©, — — rs rs © d rn Tf vO — OC rs oc rs 00 © •n ON oo — — — vo — rs © p Tf © rn 00 © vD © © © oc •n rN © © © © d d © © © VD rs OV rs VO Tf o rn 00 rs Tf r- rn oo © rn d 00 —' © rs © ' in d © ^ ^ O O O O r v O , - . ' ' ' •n O C Tf % o. — cs — • rn —; C r.i fO Ov rn - 22 2 vD rn © rs vD rN 00 •n Tf O C VO rN Tf vO rn •n vd in rn rn rs r~ rn 00 vO vO rs Tf rs r~- rn Tf o rs Ov • Tf r-' • 00 — ' rs © —1 Tf rn vo vo. oc rs O N 3 O t/) - J u z u 3 £ c a £ ( j a. si co ci •n Tf VO — ' <n vq vq in rn in in —.* vO rs Tf rs © rn in oo rs oc r 5 rs — rs VD rs oc © d r~ o © oo © vo ~f O © © ' 00 ; VO !_ z S) > H 3 189 o in o CS c < a 3 C U •5 — O O v O v O O O I N r o v O O r o o o r o v o o o O i n r o r - ~ p o d d o d d d o d o r - r - r - r - ~ t - - - f r ~ r — Ti-ro fN — vo r~ -n-vq ri vd fN ' O f N v O O O ' O - t O — — O O — O p p f N O o o r- o p! 'n 0 ro O • VO . d o 2 t-' r_i r - o ? oo 'n » Ov I N vo ro O . ro ^ Cv -f p d o 2 ro m' _ r- o ro -t -t- -1- -t- -t -t oo 00 -t (N oo -t 00 O 00 o 'O vO IN -t 'ri •n ro 'ri ro -1- -t 'ri r-r~-r-~ O r~ fN Cv IN f r-ro 00 fN — OC O O o —' Cv — IN 'ri IN ro vd o vd 'ri Z _ f N ~ 2 o v r o O ' r i r ^ c v . O . P — r - v o — O — ro in _' o Cv rN O o ro O < N ~ „ O i n o O r o — o fN — — IN 00 oo rN ro in o vO 00 ro ro r— in ro -t © ' — ' _ -VO •n 00 00 IN Ov ro vO ro o r- o o o r- in — Ov -t -+ ro >n — •n IN •n p -t IN IN — IN -f IN d 00 >n r- in in —" 00 d Ov •ri 00 1 . 00 oo 00 d in IN oo ro vq vO vd fN d ro ro — ro vo ro ro rN o -i- o •n o ro vo Ov -r o Ov ro o rN VD VO VO ro •n VD o Ov IN o IN IN o — — o o o •n o •n oo O 00 00 00 -1- IN IN d d d d d d d d d d IN fN <n rN oc IN >ri -t — 00 oc d 00 d d ro VD 00 >n -f p Ov o ro r-Ov — vq in 00 o rN 00 -1- — in in d in 00 IN — d Cv Ov o 00 ro ro Cv in in o in fN 00 ro -t-vd in in ro IN Ov r-vq LZ' •n in 00 r— o in IN IN in ro IN ro IN vO a ro = r-' ~ Ov •n VO Ov 00 " ^ O O O O O O Q J I ' o 9 ^ £ = M <g ^  o q ^ H ^ i i S S u z i - i a . s q u z — ' n IN ° U > U a . M f f l f i w C ) Z S l > i r - 3 190 Accep ted Concentrat ion (wt %) 0 100 200 Accepted Concentration (ppm) 300 Figure C . l . Comparison of analyses of major and trace element concentrations of in-house standard ALB-1 measured at McGill University versus accepted values (compiled by MDRU) measured at X-Ray Assay Labs. Error bars are +/- 1 standard deviation (App. C. l ) . 191 0 20 40 60 80 Accepted Concentration (wt. %) Accepted Concentration (ppm) Figure C.2. Comparison of analyses of major and trace element concentrations for in-house standard P- l measured at McGill University versus accepted values (compiled by MDRU) measured at X-Ray Labs. Error bars are +/- 1 standard deviation (App. C. I). 192 fZ o cr OJ o fZ o o ~o OJ ZJ CO 03 OJ 10.00 20.00 30.00 40.00 Accepted Concentration (wt. %) 50.00 E C L Q_ fZ o '•*-> CO i _ "c 0J o rz o O T3 0J i _ ZJ CO cc 0J 0 50 100 150 200 Accepted Concentration (ppm) 250 Figure C.3. Comparison of analyses of major and trace element concentrations of in-house standard QGRM-100 measured at McGil l University versus accepted values (compiled by MDRU) measured at X-Ray Assay Labs. Error bars are +/- 1 standard deviation (App. C l ) . 193 Appendix C.2. Duplicate analyses of five samples. Sample 85-03-70 85-03-70 91-02-85 91-02-85 91-16-307 91-16-307 91-16-233 91-16-233 85-03-92 85-03-92 Hole 85-03 91-02 91-16 91-16 85-03 Depth (ni) 70.5 85.2 307.5 232.9 91.8 Det. Depth (ft) 231.2 ' 279.4 1008.9 764.0 301.2 Limits Rock Type FP Duplicate QFP Duplicate FP Duplicate M A F I C Duplicate V C L T Duplicate (ppm) (Wt. %) Si02 69.26 69.17 73.93 73.79 67.91 67.97 43.75 43.91 73.62 73.84 60 Ti0 2 0.41 0.41 0.28 0.27 0.52 0.51 0.66 0.67 0.26 0.26 35 Al 2 0 3 14.94 14.97 13.61 13.70 14.48 14.50 16.10 16.03 13.25 13.33 120 Fc 20 3 3.89 3.87 2.34 2.44 4.39 4.42 9.52 9.56 2.52 2.50 30 MnO 0.08 0.08 0.06 0.06 0.20 0.20 0.33 0.32 0.04 0.05 30 MgO 2.27 2.33 1.22 1.31 3.95 4.01 9.04 9.13 1.82 1.88 95 CaO 0.43 0.47 0.34 0.46 0.89 0.92 8.19 8.26 0.75 0.76 15 Na20 5.97 6.29 4.94 5.01 2.05 2.58 3.08 3.13 3.29 3.68 75 K 2 0 0.49 0.50 1.94 . 1.90 2.09 2.04 0.20 0.20 1.82 1.82 25 P 2 O S 0.14 0.15 0.05 0.06 0.13 0.14 0.11 0.1 1 0.04 0.06 35 LOI 2.05 1.91 1.63 1.35 3.54 3.37 9.34 9.37 3.04 2.63 100 Total 99.93 100.20 100.34 100.44 100.15 100.74 100.32 100.79 100.45 100.84 100 (ppm) Cr 48 57 153 34 68 77 132 118 32 37 2 Ni 7 3 6 16 4 19 23 22 7 4 3 Co 7 5 6 0 10 4 36 28 6 0 10 V 52 52 28 43 72 68 229 271 40 28 10 Cu 7 94 11 33 25 43 93 100 10 39 15 Ph 4 0 4 0 4 0 10 3 10 6 2 Zn 57 135 42 82 125 174 273 31 3 58 108 2 Ba 17 55 529 495 374 380 88 160 88 112 50 Rb 8 7 17 18 28 27 12 1 30 31 1 Sr 64 66 73 77 63 65 177 187 82 86 1 Ga 14 15 1 1 13 13 14 13 14 12 14 1 Nb 5 5 . 5 6 7 7 1 2 5 7 1 Zr 109- 99 119 112 131 123 44 37 117 1 11 1 Y 22 24 17 18 28 31 14 17 20 22 1 Analytical method: X-ray fluorescence, induced neutron activation analysis Thc major elements plus Cu-Zn-Ni-Cr -V-Sc were analysed using glass beads. The elements Zr-Y-Nb-Rb-Sr-Ga-Pb were analysed usin <; pressed pellets. (bd = below detection limit; na = not analysed) 194 Figure C.5. Plots of selected trace elements for five duplicate samples. Data is included in Appendix C.2. Error bars are approximated by standard deviations calculated in Appendix C.J. A l l concentrations are in ppm. 

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