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Geology, alteration, mineralization and metal zonation of the Mt. Milligan porphyry copper-gold deposits Delong, R. Campbell 1996

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G E O L O G Y , ALTERATION, MINERALIZATION AND M E T A L ZONATION OF T H E M T . MILLIGAN PORPHYRY C O P P E R - G O L D DEPOSITS by R. CAMPBELL DELONG B. Sc. (Honours), Memorial Universty Of Newfoundland, 1976 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA May 1996 © R. Campbell DeLong, 1996  In  presenting  degree  at  this  the  thesis  in  partial  fulfilment  University  of  British  Columbia,  freely available for copying  of  department publication  this or of  reference  thesis by  this  his  for  and study. scholarly  or  thesis for  her  Department The University of British Columbia Vancouver, Canada  DE-6 (2/88)  I further  purposes  the  requirements  I agree  that  agree  may  representatives.  financial  permission.  of  It  gain shall not  that  the  an  advanced  Library shall make  permission  be  granted  is  understood  be  for  by  the that  allowed without  for  it  extensive  head  of  my  copying  or  my  written  ABSTRACT  The Mt. Milligan porphyry copper-gold deposits are in central British Columbia, 155 km northwest of Prince George. They were discovered in 1987 after extensive soil and lithogeochemical sampling, geological mapping, ground and airborne geophysical surveys, trenching and diamond drilling. An indicated resource of 299 million tonnes grading 0.45 g/t Au and 0.22% Cu has been delineated in two deposits: Mt. Milligan Main, which includes the 66 zone, and the Southern Star. The Mt. Milligan deposits are hosted by porphyritic monzonite stocks and adjacent volcanic rocks of the Witch Lake formation of the Takla Group. All are within the Early Mesozoic Quesnel Terrane. Potassic and propylitic alteration characterize the Mt. Milligan deposits. Potassic alteration is most intensely developed around the contacts of the MBX and Southern Star stocks. An inner biotite subzone is spatially related to the core of copper-gold mineralization. Propylitic alteration occurs peripheral to, and locally superimposed on, the outer borders of the potassic alteration. Central parts of most of the deposits are: (i) gold and copper rich, (ii) mineralized with chalcopyrite, and minor bornite and pyrite, and (iii) associated with potassic alteration. Peripheral mineralization, including the gold rich but copper poor 66 zone, is marked by minor lead-zinc-silver mineralization (mainly in sparse veins) associated with propylitic alteration. Higher gold concentrations in the 66 zone can be accounted for by overprinting of two periods of gold deposition from AuCl" and Au(HS) " bearing solutions. Other characteristics consistent with this two stage model of gold deposition are: (1) two populations of gold, one correlating with copper (gold transport as AuCl" similar to the higher temperature copper chloride complexes), and one independent of copper (transport as Au(HS)2"), (2) two periods of alteration: (i) potassic (consistent with gold being transported as AuCl" ), and (ii) a propylitic retrograde overprint of an assemblage of epidote-pyrite associated with alteration of biotite to chlorite, and (3) elevated zinc, lead and silver concentrations, indicative of lower temperature solutions that would favour gold transport as Au(HS)2~. 1  2  1  1  Geology, Alteration, and Metal Zoning at the Mt. Milligan Copper-Gold deposits  Table of contents ABSTRACT  ii  T A B L E OF CONTENTS  iii  LIST OF FIGURES  v  LIST OF TABLES  ix  ACKNOWLEDGEMENTS  x  CHAPTER 1  1  INTRODUCTION AND SETTING  l  1.0 Introduction 1.1 Location, Access and Physiography  1 3 5  1.2 Exploration History  8  1.3 Scope and Purpose of this Study  CHAPTER 2  9  REGIONAL AND DEPOSIT GEOLOGY  9  2.0 Regional Geology  9  2.1 Property Geology  13  2.2 Deposit Geology  14  2.3 Structure  14  2.4 Lithologies  18  2.5 Alteration  33  iii  2.6 Hypogene Copper and Gold Mineralization  41  CHAPTER 3  49  M E T A L ZONATION A N D ALTERATION MINERAL DISTRIBUTION  49  3.1 Introduction  49  3.2 Methods and Data Collection  49  3.3 Results 3.4 Zonation  : :  50 63  3.5. Discussion  74  3.6 Conclusions  79  3.7 Exploration Parameters  80  CHAPTER 4  83  VARIATIONS IN BIOTITE COMPOSITION  83  4.1 Reasoning and Methods of investigation  83  4.2 Methods  84  4.3 Igneous versus Hydrothermal Biotite  84  4.4 Results  86  4.5 Discussion  90  4.6 Hydrothermal biotite  96  4.7 Conclusions  CHAPTER 5  105  '.  109  SUMMARY OF CONCLUSIONS  109  REFERENCES  115  APPENDIX 1  122  APPENDIX II  125  iv  List of Figures Figure 1.1. Map of British Columbia showing Quesnel trough with the locations of porphyry copper-gold deposits including Mt. Milligan. page 2 Figure 1.2. Map showing access and infrastructure to the Mt. Milligan deposits. page 3 Figure 1.3. View looking northwest at the peak of Mount Milligan. page 6 Figure 2.1. Regional geology and deposits from Nelson et al., 1991. page 10 Figure 2.2. General geolgy of the property showing local features. page 12 Figure 2.3. Generalized geology of the Mt. Milligan deposits at the 1000 metre elevation from plans of Placer Dome and Continental Gold. page 15 Figure 2.4. Cross-section of the Mt. Milligan Main deposit looking north at the 9600 north line (Figure 2.3) page 16 Figure 2.5. Cross-section of the Southern Star deposit looking north at the 8600 north line (figure 2.3) page 17 Figure 2.6. Examples of Takla Group andesitic to basaltic volcanic rocks of the Witch Lake formation from the Mt. Milligan area. page 20 Figure 2.7. Photomicrographs of augite porphyritic volcanic rocks of the Witch Lake formation from the Mt. Milligan area that were classified as latites in the field. page 20 Figure 2.8. Examples of different varieties of trachyte in samples of NQ core (4.76 cm in diameter) from the Mt. Milligan area.. page 25 Figure 2.9. Monzonite and monzonite intrusion breccia samples of NQ core (4.76 cm in diameter) from the MBX and Southern Star stocks. page 28 v  Figure 2.10. Diorite from a post-mineral dike. Sample is of NQ core (4.76 cm in diameter). page 29 Figure 2.11 .Different types of alteration found at the Mt. Milligan Main deposit illustrated by four pieces of NQ core. page 36 Figure 2.12. Propylitic alteration overprinting potassic alteration in the Mt. Milligan deposit illustrated in NQ core. page 36 Figure 2.13. Photomicrograph of hydrothermal biotite with clear K-feldspar in groundmass of a latite. Figure 2.14. Fine-grained K-feldspar development illustrated in stained NQ core. page 39 Figure 2.15. Variations in potassic alteration in monzonite from the Mt. Milligan deposit. page 39 Figure 2.16. Epidote-albite-pyrite propylitic alteration in the 66 zone illustrated by NQ core. page 39 Figure 2.17. Photomicrographs of chlorite occurrences Mt. Milligan deposit. page 43 Figure 2.18. Irregular chlorite development in propylitically altered trachytes, Mt. Milligan deposit as illustrated by NQ core. page 43 Figure 2.19. Habits of chalcopyrite occurrence, Mt. Milligan deposit as illustrated by NQ core. page 43 Figure 2.20. Chalcopyrite habits in the MBX zone illustrated by photomicrographs of polished thin sections. page 44 Figure 2.21. Photomicrograph of gold grain in polished thin section. page 44 Figure 3.1. Tree diagram illustrating correlation characteristics among elements and minerals at Mt. Milligan. page 51  vi  Figure 3.2. Alteration mineral abundances page 53 Figure 3.3. Metal distribution box-plots. page 60 Figure 3.4. Metal abundances page 65 Figure 3.5. Metal zoning as originally postulated by Emmons (1927). page 71 Figure 3.6. Gold plotted versus copper for the Mt. Milligan deposits. page 73 Figure 3.7. Pyrite halo around the Mt. Milligan and Southern Star deposits. page 75 Figure 3.8. Oxygen fugacity plotted against temperature to illustrate gold solubility. page 77 Figure 3.9. Generalized oxygen fugacity-temperature diagram illustrating the evolution of a fluid. page 78 Figure 3.10. Element ratio plots. page 81 Figure 4.1. Location of drill core samples prepared and analysed for biotite using the electron microprobe. page 85 Figure 4.2. Titanium plotted against fluorine. page 87 Figure 4.3. SEM analytical traverses across igneous biotite grains. page 89 Figure 4.4. Titanium concentration expressed as atoms per unit formula plotted against the Fe# (Fe/Fe+Mg). page 92 Figure 4.5. Plot of major element variation used in classification of biotites. page 92  vn  Figure 4.6. Sorted samples illustrating the subtlely higher Fe# in the 66 zone compared to the MBX zone of the Main Deposit. page 97 Figure 4.7. Temperatures of OH - F exchange at constant f(H 0)/f(HF) that would account for observed biotite compositions at both DDH 123 and 168. page 97 2  Figure 4.8. Scatterplots showing major element variation among the hydrothermal biotites at the Mt. Milligan Main deposit. page 99 Figure 4.9. Plot of the plagioclase component (Table 4.4) versus the tschermak component for Mt. Milligan Main deposit biotites. page 102 Figure 4.10.Plot of the ferro-magnesian component (Table 4.4) versus the tschermak component for biotites of the Mt. Milligan Main deposit. page 102 Figure 4.11. Plot of the soda-potassic component (Table 4.4) versus tschermak component for biotites of the Mt. Milligan Main deposits. page 103 Figure 4.12. Plot of the edenite component (Table 4.4) versus the tschermak component for biotites of the Mt. Milligan Main deposit. page 103 Figure 4.13. Plot of the magnesio-calcic component (Table 4.4) versus the tschermak component for biotites of the Mt. Milligan Main deposit. page 104 Figure 4.14. Plot demonstrating the average relative size of the tschermak component at each of the sample sites for hydrothermal biotites at the Mt. Milligan Main deposit. page 106  viii  List of Tables Table 2.1. Whole Rock Analyses from Mt. Milligan. page 22 Table 4.1. Average biotite compositions from the igneous biotites from the two locations. page 86 Table 4.2. Average igneous and hydrothermal biotite cation compositions from all locations. page 88 Table 4.3. Equilibrium constants for the exchange reaction OH-mica + HF <=> F-mica + H 0. page 93 2  Table 4.4. Thompson exchange components used to derive Figures 4.9 to 4.13. page 101  ix  Acknowledgements  This thesis would not have been completed without the kindness, patience, and encouragement of my supervisor, Dr. Colin Godwin. Instrumental in the formation of many of the ideas that appear in this work are the thoughts, comments and suggestions of members of the faculty, the Mineral Deposits Research Unit and collegues at the University of British Columbia. Of particular importance was Dr. Cliff Stanley at U. B. C. who helped me enormously with the chapter on biotites, although I take full responsibility for any faulty reasoning in using the techniques he suggested to me. Also very helpful to me were discussions with Dr. Kelly Russell, Dr. Tom Brown, Dr. Greg Dipple and Dr. John Thompson. Mark Rebagliati was responsible for hiring me to work at the Mt. Milligan site and continued with his encouragement and insightful suggestions as Exploration Manager with the Hunter-Dickinson Group. Fellow employees of Continental Gold Corporation at the Mt. Milligan site were both fun to work with and sharing with their ideas and concepts. Many of my thoughts and conclusions originated with them. Particularly I would like to mention Brian Bower, Mike Harris, Jim Oliver, Nadia Caira, and Sylvia Heinrich. I would also like to express my gratitude to Bob Dickinson, Bob Hunter, and Dave Copeland for their encouragement and the funding provided by Continental Gold. Placer Dome Ltd. and the B.C. Science Council also provided funding. Finally, I would like to express my appreciation and heartfelt thanks to my partner Irene Smith.  x  Chapter 1  Introduction and Setting  1.0 Introduction  The Mt. Milligan and Southern Star deposits are recently discovered large-tonnage, lowgrade, potentially open pittable, gold-copper discoveries in north-central British Columbia. They have many characteristics of the alkaline suite porphyry deposits described by Barr et al. (1976), and some of the characteristics of the diorite model of Hollister (1975). Alkalic porphyry copper deposits are an exploration target of particular interest because they are often, as are the deposits at Mt. Milligan, enriched in gold. Other examples (Figure 1.1) of this style of deposit are the mines at Copper Mountain and Ingerbelle (Fahrni et al., 1976; Stanley et al, 1995) near Princeton, and Afton and Ajax (Carr and Reed, 1976; Ross et al., 1995 ) near Kamloops. Prospects include Mount Polley, formerly known as Cariboo-Bell (Hodgson et al., 1976; Fraser et al., 1995), Lorraine (Wilkinson et al., 1976; Bishop et al., 1995) and Galore Creek (Allen et al., 1976; Enns et al., 1995). British Columbia examples of these deposits are directly associated with alkalic (quartz deficient) intermediate intrusions emplaced in roughly coeval rocks of island arc affinity. They are Triassic to Early Jurassic in age (Mortimer, 1987; Mortensen et al., 1995).  Alkalic porphyry deposits of this style are found in a regionally extensive, 1,200 km long, northwesterly trending belt of rocks in central and southern British Columbia (Figure 1.1) known as the Quesnel trough (Garnett, 1978; McMillan et al., 1995). In northern British  1  100  LEGEND — — czr *  Road Railway Quesnel Trough Open Pit Porphyry Mines & Projects  0  MILES 200  300  100 200 300 400 500 KILOMETRES  FIGURE 1.1. M a p o f British Columbia showing the Quesnel trough with the locations of porphyry copper-gold deposits including M t . M i l l i g a n . 2  Columbia they also occur within the Stikine Terrane. The Quesnel Terrane is defined by the early Mesozoic Takla Group and its southern equivalent the Nicola Group (Monger et al., 1990). It forms the eastern part of the Super Terrane known as the Intentiontane Belt (Figure 1.1). 1.1 Location, Access and  Physiography  The Mt. Milligan and Southern Star deposits are on the eastern side of the Intermontane Belt (Figure 1.1) and occur within the Quesnel Terrane (Monger et al., 1990).  These deposits, located near latitude 55° 08' north and longitude 124° 04' west at an elevation of 1,100 metres, are in the Omineca Mining Division approximately 150 km northwest of Prince George in north-central British Columbia (Figures 1.1 and 1.2). They are about 100 km west of Mackenzie and 60 km north of Fort St. James.  Access to the property from Prince George or Mackenzie is via Highway 97 to Windy Point (Figure 1.2). From there an all-weather main-line logging haulage road provides excellent access to the property. A 10 km extension from the Omineca Mining Road through relatively flat terrain will ultimately provide a route to Fort St. James and Vanderhoof. The British Columbia Railway services Mackenzie and Fort St. James and the Canadian National Railway services Prince George and Vanderhoof.  The property containing the Mt. Milligan deposits covers a 15 km long northwest-trending ridge. The deposits sit immediately east of the ridge 10 km southeast of the summit of Mt. Milligan (Figure 1.3). Local relief is in the order of 300 metres with an average elevation of approximately 1200 m. The Mt. Milligan and Southern Star deposits are situated in an area of gentle relief at an elevation of 1100 metres.  3  ^Stronsay  Cheni  I Fort Ware  Ingenika.  Fort St. John 29  4  Germansen Landing rakla Landing Mackenzie  Bell  Bullmoose  Mt. Milligan  |  Kennedy Substation  Granisle  Smithers  (27)  Fort St. James  Equity Prince George  Endako 16)  LEGEND Highway Year Round Industrial Road -H-  Railway  g  Mines And Mine Development Projects  25  MILES 50  75  '50  KILOMETRES  F I G U R E 1.2. M a p showing access and infrastructure to the M t . M i l l i g a n deposits.  Drainage from the property is to the Nation River, which is part of the Peace River drainage system that flows to Williston Lake and then on to the Arctic Ocean (Figure 1.2). Vegetation consists mainly of lodgepole pine in gravel covered areas and spruce and balsam in areas of till. 1.2 Exploration  History  The first exploration recorded activity in the area was by prospector George Snell. In 1937 he found gold bearing float on the southwestern flank of Mt. Milligan. In 1945 he staked 10 two-post claims west of Mitzi lake (Figure 2.2.). Assays from five pyritic andesite float samples ranged from trace to 148.8 grams per tonne gold. The source of this float was not found.  Pechiney Development Ltd. staked 10 two-post claims on the western flank of Mt. Milligan just north of Heidi Lake in 1972. Subsequent exploration work identified induced polarization and copper soil geochemical anomalies. Pechiney drilled five holes, but did not encounter significant copper mineralization. They allowed the claims to lapse.  The Mt. Milligan region remained inactive until 1983 when Selco Inc., on the recommendation of C. M. Rebagliati, initiated a regional lithogeochemical survey to explore for auriferous porphyry copper deposits. Claims were staked to cover a regionally significant gold-arsenic anomaly located east of Heidi Lake. In early 1984, Selco amalgamated with BP Resources Canada Limited (BP).  In April of 1984 prospector Richard Haslinger staked mineral claims adjacent to those staked by Selco. BP optioned these claims in August of 1984 and in early 1985 staked some additional claims in the area. The Haslinger claims cover part of the Milligan Main and  5  Figure 1.3. V i e w looking northwest at the peak of Mount Milligan. The M t . Milligan M a i n deposit is about 10 k m to the southwest of the peak about 2 k m east of where the picture was taken (on the North Slope stock, Figure 2.1).  6  Southern Star deposits. BP completed extensive geological, soil geochemical, magnetic and induced polarization surveys and a trenching program during 1984 and 1985 on the Haslinger and adjacent claims to the south of the Mt. Milligan Main deposit. This work identified two polymetallic-auriferous vein systems and two areas of low-grade copper-gold porphyry mineralization.  When BP discontinued exploration for auriferous porphyry deposits in early 1986, C. M. Rebagliati introduced the property to David Copeland of Lincoln Resources Inc. who optioned the property and continued exploration. Work resumed on the property and on September 25, 1987 diamond drill hole 87-12 intersected significant copper-gold mineralization in the MBX zone of the Mt. Milligan Main deposit (Figure 2.3). Lincoln reorganized to become United Lincoln Resources Inc. in 1988 and continued exploration and delineation drilling. Continental Gold Corp. acquired 64% of United Lincoln and in August of 1988 the two companies merged. On July 12, 1989 diamond drill hole 89-199, drilled on a induced polarization target, discovered the Southern Star deposit.  On October 22, 1990 Placer Dome exercised a previous agreement to purchase all of Continental Gold's interest in the Mt. Milligan and Southern Star deposits. Prior to this Placer Dome had acquired BP's shares in the project. Consequently, Placer Dome currently are the outright owners of the property.  Placer Dome concluded that the Mt. Milligan property was uneconomic in 1992 and wrote off their investment. However, they are currently re-assessing the economic potential.  _ Current estimated reserves of the Mt. Milligan Main and Southern Star deposits are 299 million tonnes of 0.22% copper and 0.45 grams per tonne gold in estimated geological reserves (Sketchley et al., 1995). 7  1.3 Scope and Purpose of this Study  The Mt. Milligan deposits have significant economic potential and have been extensively drilled. Therefore, an investigation of the patterns of alteration and the relationship of these patterns to the concentration and zonation of metals in these deposits is of great interest to the explorationist.  This study is designed to document the distribution and characteristics of the alteration mineral assemblages. It attempts to establish the relationship between these patterns and the occurrence of economic metals. It also describes the general geologic framework of the Mt. Milligan and Southern Star deposits in the context of alkali porphyry copper-gold mineralization in the Canadian Cordillera (Barr et al., 1976). Detailed examination of the composition of the alteration biotite and the relative abundance of biotite and other alteration minerals is used to determine the nature of possible hydrothermal processes that could account for the alteration assemblages and metal deposition. This is used to better understand the genetic interpretations for the Mt. Milligan Main and Southern Star deposits.  A comprehensive data base is provided by more than 800 diamond drill holes. The present study is focused on the 1000 metres (above sea level) plan because it is well mineralized and mineralogically zoned. It characterizes the deposit in a general sense. The data collected from drill hole piercing points at the 1000 metre level were treated with the methodology described by DeLong et al., (1991).  8  \  Chapter 2 Regional and Deposit Geology  2.0 Regional  Geology  The M B X , Southern Star and other associated monzonitic bodies (Figure 2.1) are intruded into Triassic to Lower Jurassic volcanic rocks of the Takla Group that may be, at least in part, consanguineous.  The Takla Group is part of a hook-shaped belt of subalkaline to alkaline volcanic and sedimentary rocks that is about 35 k m wide and over 1100 k m long. Along with its southern equivalent, the Nicola Group, it forms the dominant lithologic unit of the Quesnel Terrane. The Quesnel Terrane is separated from the Slide Mountain Terrane and the Wolverine Metamorphic Complex to the east by the Manson fault zone and from the Cache Creek Terrane to the west by the Pinchi fault zone (Nelson et al., 1991; Garnett, 1978). The Quesnel Terrane is interpreted to be an island arc assemblage (Mortimer, 1987; Monger et al., 1990). The volcanic rocks are derived from shoshonitic to mildly alkalic magmas (Mortimer, 1987; de Rosen-Spence, 1985).  The Takla Group in the M t . Milligan region consists of an Early Late Triassic sedimentary unit that interfingers with, and is overlain by, voluminous volcanic, pyroclastic and epiclastic rocks up to Lower Jurassic in age. Augite phyric rocks predominate, although plagioclase and hornblende are present and can be dominant in places (Nelson et al. 1991). Takla Group volcanics tend to be unusually potassium rich (shoshonitic) and are transitional to alkalic in their major element chemistry (Rebagliati, 1990; Ferri, 1991). Nelson et al.  9  ^C%j'-:. l-irij^TJjft JJ liifejii: '-.1  Mount Milligan Mit2i  Lake!  V.V.':. ftJJiJJiJJiJJftJJiJJV'-"-' - ' '• i T i J J 3 i i =i J J i J J =! 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JJ 3 JJ dtCO;J - JJ 3 JJ ft JJ 3 J . j J 3 J 13 J J 3J*3 JJ ft J J =i ii =; J J =i JJ 3 J J 3 J J = J J 3 JJ 3 J J 3 Bci J J 3 JJ 3 JTSJ J 3 il - J J 3 i ,3jj3JJ3JjftjJiijJ  ^ 3Hi 3  ftJJ3JjagJ3JJ3JJ  =  J J 3 J J 3 ii 3 J J 3 J J =J J J =i J J 3 J J =i J J 3 J jjftjjiijftjj^jjftjjft jji-jj3jjftjJ3jjftjj = 3JjftJj5>U3JJ3JJ3JJftii3JjftJJ3JjftJJ=ii JJftJJ =J JJ 3JJ ^ JJ =i JJ 3 JJ =J JJ =J JJ ^ JJ =i JJ ^ JJ =  =j J JftJ J  =i I r a  J J =J J J  =i J JftJ J =i J JftJ J =i J J  ^  JJ  =i  JJft.JJftJJft.JJftJJft.JJftJJft'iliJJftJJUJJftJJz  J.  LEGEND  fc^d l*+ * +1 ['•••.''•~:•'.'{ I I 1-1 |-"ft-"=j  TERTIARY Sedimentary & Volcanic Rocks TRIASSIC - JURASSIC Hornblende monzonite Crowded feldspar porphyry m o n z o n i t e Takla Group Sedimentary Rocks Takla Group Volcanic Rocks UPPER PALAEOZOIC Slide Mountain Group PROTEROZOIC Wolverine Complex Major Regional  Mount Milligan Intrusive Complex 2 MBX Stock 3 Southern Star Stock 4 Goldmark Stock 5 North Slope Stock 6 Heidi Lake Stock •XS Snell Prospect •XP Pechiney Prospect 1  Fault  MLES 2  3  Figure 2.1. Regional geology and deposits from Nelson et al., 1991  10  (1991) have informally subdivided the Takla Group into the Rainbow Creek, Inzana Lake, Witch Lake and Chuchi Lake formations. These units interfinger and are in part, facies equivalents. However, stratigraphically the lowermost unit is usually the Rainbow Creek formation overlain in sequence by the Inzana Lake, Witch Lake and Chuchi Lake formations. The deposits at Mt. Milligan are within the Witch Lake formation.  The Rainbow Creek lithofacies is dominated by fine-grained sedimentary rocks with a lesser abundance of volcanic rocks. The Inzana Lake formation is transitional between the underlying Rainbow Creek and overlying Witch Lake formations. It consists of epiclastic and sedimentary rocks with minor pyroclastic rocks. The Witch Lake formation is dominated by submarine, augite porphyry agglomerates and flows that are andesitic to basaltic in composition. The volcanic rocks of the Chuchi Lake formation are andesitic to latite-andesite in composition.. The phenocryst assemblage is dominated by plagioclase with variable amounts of augite and hornblende. Some of the Chuchi Lake formation is subaerial in origin.  Two U-Pb zircon ages, 183 ± 3 Ma for the North Slope stock (Figure 2.2)and 182.5 ± 4.3 Ma for the Southern Star stock (Mortensen et al., 1995), indicate a Bajocian age for these intrusions. Fossils (Nelson et al., 1991) suggest a Toarcian to Pleisbachian (187 to 193 Ma) age for the Chuchi Lake formation. Nelson et al. (1991) also report clasts of MBX-like monzonites in the Chuchi Lake formation. This data is consistent with Middle Jurassic intrusion into Early Jurassic stratigraphy.  To the north of the Mt. Milligan region, the Hogem batholith occurs along the western margin of the Quesnel Terrane. A subaqueous to subaerial volcanic complex is present between Mount Milligan and Chuchi Lake (Nelson et al., 1992); the Mt. Milligan intrusive complex occurs southeast of the Hogem batholith. This alignment of major intrusions from 11  N  o o o o Q OJ  Mt. Miltigar)  55 10'00"  LEGEND Mitzi  x x > X X x x >  + +  + + ^ + + II  =  II  n  =  =  II  W W  y'Y//A /////  L.  BIOTITE MONZONITE LEUCOQABBRO QUARTZ  MONZONITE  X  DIORITE MONZONITE PORPHYRY TAKLA GROUP VOLCANICS  Km 0  1  CMt, Milligan Southern Star  2  Scale  and DepositsJ  3 ("Outline of Joint  CFrom Company  Venture  Claims)  Plans)  Figure 2.2 General geology of the property showing local topographic features  the western to the eastern side of the Quesnel Terrane, probably reflect a major cross-arc structure.  Crowded feldspar porphyritic monzonite stocks and dikes that are associated with porphyrystyle copper-gold mineralization are related to the intrusive complexes and occur within and around the volcanic complex. These stocks are expressed by small magnetic highs (Sketchly et al., 1995) on the flanks of the large composite regional magnetic high (G.S.C., 1963). Examples of these monzonite stocks in the Mount Milligan area are the M B X , Southern Star, North Slope and Goldmark stocks (Figure 2.1).  2.1 Property  Geology  The M t . Milligan property (Figure 2.1) is underlain mostly by volcanic rocks of the Witch Lake formation of the Takla Group (Nelson et al., 1991). Minor sedimentary rocks of the Rainbow Creek formation, and Early Tertiary volcanic and sedimentary rocks are present also. Andesitic rocks of the Witch Lake formation generally trend north-northwesterly and dip moderately to steeply to the east.  The Witch Lake formation is intruded by coeval plutonic rocks that are known as Takla Group intrusions. M i n o r post Takla Group intrusive rocks are also present. Most of the Mount Milligan intrusive complex are coeval with Takla volcanic rocks. This intrusive complex  consists  dominantly  of  monzonite  with minor  diorite/monzodiorite  and  gabbro/monzogabbro. The M B X , Southern Star, Goldmark and North Slope stocks, which host the known  mineralization, are composed of monzonitic rocks. Post Takla Group  intrusions comprise granitic rocks, which constitute a minor portion of the Mount Milligan intrusive complex. Trachyte, monzonite and diorite post-mineral dikes are common. K - A r ages of 96 M a and 108 M a (unpublished data from the U . B . C . Geochronology Laboratory)  13  from hydrothermal biotite from the M t . Milligan M a i n deposit suggest a Cretaceous age for at least one set of these dikes.  2.2 Deposit Geology The M t . Milligan deposits are divided into the M t . Milligan M a i n and Southern Star deposits. The M t . Milligan M a i n deposit consists of four (Figures 2.3 and 2.4) coalescing zones: (i) the Magnetite Breccia zone ( M B X ) , (ii) the 66 zone, (iii) the West Breccia zone ( W B X ) , and (iv) the Deep West Breccia zone ( D W B X ) . The M t . Milligan M a i n deposit occurs within the M B X stock and adjacent latitic, high K-basaltic, and trachytic volcanic rocks of Witch Lake formation. The Great Eastern fault, a moderately east dipping, northerly to northwesterly trending structure, truncates the 66 zone portion of the M t . Milligan M a i n deposit, separating it from sedimentary rocks of Rainbow Creek formation and Early Tertiary volcanic and sedimentary rocks. The M t . Milligan M a i n deposit is separated from the Southern deposit by the steeply dipping northwesterly trending Divide fault. The Sputhern Star deposit (Figure 2.3) occurs within the Southern Star stock and adjacent andesitic to high K-basaltic volcanic rocks of the Witch Lake formation.  Rocks of the Witch Lake formation generally trend north-northwesterly and dip moderately to steeply east (Figures 2.3 and 2.4). However, north of the M t . Milligan M a i n deposit, these strata dip steeply west. In the southeastern portion of the M t . Milligan deposits, the stratigraphy trends northerly to northeasterly. Graded bedding and cross-bedding in tuffaceous rocks indicate that the stratigraphy faces east and is right side up.  2.3 Structure A t least four episodes of faulting affected the area containing the M t . Milligan deposits. Relative ages are defined by cross-cutting relationships, especially as related to intrusive  14  LEGEND TERTIARY m  SEDIMENTARY AND VOLCANIC ROCKS  CRETACEOUS—EOCENE ™ , POST MINERAL DYKES BSSS (DIORITIC, MONZONTTIC, TRACHYTIC)  TRIAS SIC-JURASSIC FIvl  MONZONTTIC STOCKS AND DYKES  iiaH  TRACHYTIC VOLCANIC ROCKS  I  MT MILLIGAN MAIN DEPOSIT  I ANDESITIC— LATTTIC VOLCANIC ROCKS (53  '  GENERALIZED GEOLOGY OF THE MT MILLIGAN COPPER - GOLD PORPHYRY DEPOSITS  FAULT ZONE ALTERATION  K  POTASSIC (FELDSPAR)  B  POTASSIC (BIOTITE)  P  PROPYLITIC  • •• POLYWETALUC VEINS 9SOON  8500N  SOUTHERN BOUNDARY ZONE  SOUTHERN STAR STOCK  0  FEET  100 200  200 400 500  METRES  Figure 2.3. Generalized geology of the Mt. Milligan deposits at the 1000 metre elevation is from plans of Placer Dome and Continental Gold.  15  LL!  OVERBURDEN TERTIARY UESOZOIC SEDIMENTARY A N D VOLCANIC CRETACEOUS POST  S3  •  MINERAL  ROCKS  EOCENE DIKES  (DioRmaiioNZOHmc.  TRACHTTIC;  TRIASSIC - JURASSIC UONZONmC S T O C K S ANO DIKES  mi  TRACKTDC VOLCANIC R C O S  i—i 1_!  ANOESffiC -  LATJTIC VOLCANIC  ROCXS  FAULT ITERATION POTASSiC-SIOTTTE SUBZCNE 3 X POTASSIC- K-FELDSPAR SERICflTC S p PROPYLtnC Au. EQUIVALENT O IS Cu SYMBOL Au gromi/<onns O.000 - 0.2 49 0.250 - 0.399 0.4O0 - 0.999 > 1.000  1g/t Au  i  Figure 2.4. Cross-section of the Mt. Milligan Main deposit looking north at the 9600 north line  (Figure 2.3).  Taken from company  sections of Placer Dome  Continental Gold.  16  and  3 3 0 M  GENERALIZED GEOLOGY SOUTHERN  STAR  DEPOSIT  1200m  Hi UOOm  OVERBURDEN TERTIARY - WESOZOIC SEOIUENTASY MiO VOLCANIC RCCXS CRETACEOUS - E O C E N E PCST UINESAL CUKES (DIORmCMONZONmC. TRACKYTIC; TRIASSC - .'URASSiC UONZOWTIC STOCKS AND CIKES  1000m  900m  —  PROPTUT1C  Ml. EOUrVALENT 1 I S D J • Iq/t Au SYMBOL Mi s r a m s / t o n r *  i '  700m  LATTTK: VOLCANIC R O C K S  ALTERATION 3 POTASSC-aWTITE SUBZONE < POTASSIC-K-r-tLOSPAR ?  300m  ANOEsmc -  o.ooo - o.;<9  O - 0.299 0.400 - C.999 >l.000  Figure 2.5. Cross-section of the Southern Star deposit looking north at the 8600 north line (Figure 2.3). Taken from company sections of Placer Dome and Continental Gold.  17  activity or mineralization .The earliest episode gave rise to the northerly trending, shallowly easterly dipping, bedding parallel Rainbow fault (Figures 2.3 and 2.4).  The second episode is marked by prominent east-northeasterly trending cross-faults that cut the M t . Milligan deposits. These structures include the Oliver fault, the Caira faults and some of the Southern Star cross-faults (Figure 2.3). The Caira faults are intruded by monzonite dikes and may belong to an early episode of faulting, which was reactivated during development of the Oliver fault.  The third episode is reflected by northwesterly trending, steeply easterly dipping faults, represented by the Divide fault. It occurs within the Southern Star deposit and in the western portion of the M t . Milligan M a i n deposit (Figures 2.3 and 2.4).  The fourth episode is marked by major regional structures such as the northerly to northwesterly striking, moderately easterly dipping Great Eastern fault. This fault truncates the extreme southeastern portion (66 zone) of the M t . Milligan M a i n deposit and juxtaposes Takla Group rocks with Early Tertiary rocks lying to the east (Figure 2.3). Another important northerly trending fault is the steep easterly dipping Harris fault (Figures 2.3 and 2.4), which separates the W B X zone from the D W B X zone.  2.4  Lithologies  2.4.1 Andesitic Rocks Rocks mapped in the field as andesitic volcanic rocks are andesite to basalt based on compositions in Table 2.1 (Figure 2.6). These rocks from the Witch Lake formation (Nelson et a l , 1991) underlie most of the area around the Southern Star and to the west and south of the M B X stocks (Figure 2.3).  18  Figure 2.6 (on following page). Examples of Takla Group andesitic to basaltic volcanic rocks of the Witch Lake formation from the Mt. Milligan area. Samples are 4.76 cm diameter NQ core (A and B). (A) Monolithic augite porphyritic lapilli tuff. Note that augite phenocrysts are actinolite altered and the groundmass is hydrothermally altered to K-feldspar indicated by yellow staining. (B) Heterolithic debris flow from the Witch Lake formation. (C ) Photomicrograph of a augite porphyritic andesite to basalt with single actinolite crystal perfectly pseudomorphing an augite phenocryst (cpx). Figure also illustrates dusty, slightly altered plagioclase (pi) microphenocry sts. Figure 2.7 (on following page). Photomicrographs of augite porphyritic volcanic rocks of the Witch Lake formation from the Mt. Milligan area that were classified as latites in the field. (A) Augite (cpx) porphyritic basalt of the Witch Lake formation. Note very fine-grained biotite interstitial to the plagioclase microlites. Also present are blebs of green biotite (bi) replacing mafic grains. (A) was photographed in plane polarized light. (B) is same view as (A) with crossed nicols to emphasize the very fine-grained biotite in groundmass.  19  20  Monolithic fragmental varieties of porphyritic andesite constitute most of the unit and are characterized by actinolite-altered augite porphyritic lapilli tuff (Figure 2.6A) and minor augite crystal and lithic tuff (Rebagliati et al., 1990). Minor augite porphyritic flows and heterolithic debris flows (Figure 2.6B) are interbedded with the fragmental rocks. Plagioclase and/or hornblende phenocrysts are locally present in the flows, individual lapilli or crystal tuffs. In places, subordinate, discontinuous, heterolithic, coarse fragmental units have finer grained upper parts, which could represent caps to turbiditic submarine debris flows.  Augite porphyry andesite flows are characterized by 10 to 30% euhedral to subhedral phenocrysts pseudomorphed by dark green augite-shaped actinolite. The actinolite altered phenocrysts occur singly (Figure 2.6C) or are glomeroporphyritic. The phenocrysts range in size from 0.5 to 3 mm across, but some units have crystals up to 8 mm across. In some of the coarser grained flows, plagioclase is a microphenocryst phase in addition to being present in the groundmass (Figure 2.6C). In these flows, the plagioclase comprises up to 40% of the rock. Plagioclase crystals are pale cream, lath-like to stubby, and range from 0.5 to 1 mm in length. The groundmass of more altered flows can contain varying amounts of microgranular K-feldspar, anhedral mosaics of plagioclase, coarse anhedral epidote, finer grained intergrowths of albite and epidote, micron sized sphene, chloritic material and carbonate. Minor fine-grained felted green biotite and intergrowths of biotite and K-feldspar are present locally (Figures 2.6D and 2.6E). The microgranular K-feldspar of the groundmass could be magmatic or hydrothermal in origin.  Crystal tuffs and lapilli tuffs, which can resemble flows, are dark green and contain 5 to 40% actinolite pseudomorphs after augite. A fragmental texture is recognizable in thin section. The groundmass consists of irregularly shaped fragments, generally 0.2 to 0.5 mm across, fibrous grains of actinolite and a few broken grains of plagioclase. Some larger 21  actinolite crystals, up to 8 mm across, have euhedral faces preserved. Other groundmass components, which are mostly hydrothermal, are felted green biotite, sphene, rutile, sericite, carbonate and epidote.  Table 2.1 Whole Rock Analyses from Mt. Milligan. 1  Rock  SiQ  2  TiQ A1 0 Fe 0 MnO MgO CaO Na Q K Q 2  2  3  2  3  2  2  P O L.O.I. Total 2  s  2  1.3*  49.73 0.96  12.60 10.88 0.25  8.50  7.97  3.00  1.92  0.33  3.33 99.47  1.3*  51.32 0.69  11.98 8.66  0.12  10.19 7.36  2.19  2.19  0.26  4.38 99.34  3.3*  50.62 0.90  12.48 9.88  0.12  7.61  6.49  3.36  3.26  0.30  4.19 99.21  4.1  55.49 0.54  17.30 4.46  0.03  1.74  3.52  2.81  8.31  0.22  5.00 99.42  4.1  56.76 0.45  17.49 4.67  0.06  1.66  3.15  4.02  6.55  0.21  4.70 99.72  4.1  56.42 0.52  17.81 4.93  0.08  2.15  3.93  4.97  4.52  0.24  3.30 98.87  4.1  55.21 0.46  17.43 4.59  0.13  1.65  4.71  4.07  4.69  0.20  6.60 99.74  4.1*  56.79 0.49  17.83 5.47  0.04  2.87  3.35  5.16  3.50  0.36  3.49 99.35  4.1  57.78 0.46  18.01 4.86  0.07  2.11  3.44  4.75  4.14  0.25  3.80 99.67  6.2  64.69 0.38  15.84 3.79  0.08  1.29  3.88  3.04  3.95  0.17  2.50 99.61  6.2*  63.51 0.40  15.10 4.56  0.06  1.45  3.54  2.24  3.53  0.19  5.24 99.82  6.3  51.75 0.76  16.91 8.43  0.17  3.92  5.97  3.07  5.23  0.45  3.00 99.66  1. Most whole rock analyses are from Continental Gold data. (ICP analysis, Min-En Laboratory, Vancouver, B.C.). 2. Andesite = 1.3. Latite = 3.3. Monzonite from MBX stock = 4.1. Diorite variety of postmineral intrusion = 6.2. Monzonite variety of post-mineral intrusion = 6.3. Those marked with an * are from Faulkner et al. (1989), ICP analysis from Acme Laboratory, Vancouver, B.C.  22  2.4.2 Latitic Rocks  (Figure 2.7)  Rocks mapped as latitic volcanic rocks are texturally similar to the andesitic volcanic rocks and are characterized by 15 to 40% actinolite altered augite phenocrysts. These rocks are probably potassically altered alkali basalts (Table 2.1). They underlie most of the area around the MBX stock and less commonly the areas adjacent to the Southern Star stock (Figure 2.3). The latitic volcanic rocks are distinguished from the andesitic volcanic rocks by these characteristics: darker colour, general absence of megascopic hornblende, presence of biotite, shown as very fine-grained material interstitial to the plagioclase microlites in Figure 2.1 A, and greater K-feldspar content, based on staining. The darker colour is due to the presence of biotite, which is related to potassic alteration. The general absence of hornblende could be the result of destruction during potassic alteration. 2.4.3 Trachytic Rocks  (Figure 2.8)  Rocks mapped as trachytic volcanic rocks are interbedded with the latitic volcanic rocks in the eastern portion of the Mt. Milligan Main deposit (Figures 2.3 and 2.4). They are the only marker units in the area of the deposits (Rebagliati, 1990). The trachytic rocks are characterized by a high K-feldspar content and a general lack of mafic minerals except for fine-grained biotite and chlorite. Minor fine-grained plagioclase is also present.  Massive and bedded (Figures 2.8A and B) varieties of trachytic rocks occur on the property. Massive varieties contain curvilinear pyrite-chlorite partings (Figures 2.8A and C). These partings may mimic flow banding. Brecciation is evident locally in these massive to banded rocks (Figure 2.8D). Concentric clots of chlorite, calcite (Figure 2.19B) and lesser epidote occur around pyrite, which locally have magnetite cores that occur as texture destructive spots up to 2 cm across (Rebagliati et al., 1990). K-feldspar occurs as microlites or as finegrained mosaics. Bedded varieties locally exhibit cross-bedding and graded bedding (Figures 2.8A and B). The bedded rocks are generally discontinuous, occurring above and  23  Figure 2.8 (on following page). Examples of different varieties of trachyte in samples of NQ core (4.76 cm in diameter) from the Mt. Milligan area. (A) shows both massive trachyte with parting or banding defined by pyrite (top of photograph) and bedded trachytic tuff. Yellow staining in one sample illustrates the abundance of K-feldspar. (B) Example of bedded variety illustrating the development of pyrite along bedding planes. (C) Massive trachyte with variably spaced partings or bands. Some parting is defined by dark green chlorite and some by pyrite. (D) Brecciated trachyte. Matrix is fine-grained dark green chlorite and coarse pyrite aggregates. Concentric clots of chlorite, calcite and lessor epidote around pyrite cores are also found in massive trachytes.  2  5  below the massive trachytic volcanic rocks and extending laterally from them. Pyrite and chlorite are common along bedding planes (Figure 2.8B) and are disseminated throughout the unit. Trachytic volcanic rocks are either porous tuffaceous rocks of andesitic composition that underwent intense potassic alteration, or true rhyolite to trachyte flows and tuffs. 2.4.4 Syn-mineral Intrusive Rocks  (Figure 2.9)  The MBX stock is a moderately westerly dipping monzonite body approximately 400 metres in diameter (Figures 2.3 and 2.4). In the southeastern portion of the Mt. Milligan Main deposit, the Rainbow dike, which is up to 50 metres thick, protrudes from the footwall of the MBX stock as an elongate bowl shaped body with gently dipping sides open to the southeast. The western side of the dike is subparallel to stratigraphy as defined by interbedded trachytic volcanic rocks, and occupies the plane of the Rainbow fault. The northeastern side steeply cross-cuts stratigraphy (Figures 2.3 and 2.4).  The Southern Star stock is a moderately westerly dipping, north-northwesterly striking, tabular body of monzonite, which forks at its northern end (Figure 2.3). In plan view, its margins are more irregular and undulose than those of the MBX stock. The stock is approximately 800 metres long by 300 metres wide.  The MBX and Southern Star stocks are typically crowded plagioclase porphyries (Figures 2.9A, B, and C) containing up to 70% locally aligned plagioclase phenocrysts from 1 to 10 mm long,. Some parts of the MBX stock show rare K-spar phenocrysts up to 2.5 mm long. Phenocrysts occur in a fine-grained to aphanitic greyish-pink to dark grey groundmass composed mostly of K-feldspar with lesser plagioclase, and minor quartz, hornblende and biotite. Accessory minerals include apatite, chlorite, rutile and sphene. Where alteration is not strong, plagioclase phenocrysts display polysynthetic albite twinning and often Karlsbad 26  Figure 2.9 (on following two pages). Monzonite and monzonite intrusion breccia samples of NQ core (4.76 cm in diameter) from the MBX and Southern Star stocks. (A) Monzonite with salmon-pink veinlets of Kfeldspar that is cross-cut by thin chalcopyrite rich veinlet. (B) Monzonite intrusion breccia from the Southern Star Stock. (C) Brecciated monzonite in aphanitic salmon-pink K-feldspar matrix (stained yellow). (D) Photomicrograph of monzonite showing a single igneous biotite grain about 2 mm long in plane polarized light that is mantled by very fine-grained biotite and scattered grains of opaque magnetite. (E) Same view as (D) with crossed nicols showing a groundmass of abundant K-feldspar. Edge of plagioclase phenocryst altering to sericite appears in lower right corner of (D) and (E). (F) Hornblende grain completely altered to a mosaic of hydrothermal biotite. (G) Xenolithic monzonite containing angular fragments of biotite altered volcanic rocks. (H) Angular monzonite fragments in a more K-feldspar rich monzonitic matrix. (I) Rounded monzonite fragments, indicative of milling, in a brecciated monzonitic matrix. A, B, C, G, H and I are samples of NQ core (4.76 cm in diameter). D, E and F are photomicrographs. Figure 2.10 (on second following page). Diorite from a post-mineral dike. Sample is of NQ core (4.76 cm in diameter). Note zoning of plagioclase in larger phenocrysts.  28  20  twinning. Composition derived from twinning (Michel-Levy method) is oligoclaseandesine. The plagioclase grains generally show clear outer rims that may be more albitic and represent late stage crystallization. Occasional subhedral to anhedral but well-crystallized grains of biotite (Figures 2.9D and E) are usually less than 3% of the rock. They range up to 1.5 mm across but are usually smaller. Most of the biotite is found in fine-grained, felted patches and as very fine-grained rims to homogeneous, optically continuous biotite grains. These rims are interpreted to be the result of hydrothermal breakdown of small phenocrysts of primary biotite.  Hornblende is rare but can occur as relict prismatic and occasionally equant grains up to 0.5 mm across (Figure 2.9F). Hornblende can be partially altered to carbonate, greenish biotite and chlorite. Because these alteration minerals commonly are not formed by replacement of hornblende, it is difficult to estimate the original hornblende' content. Hornblende is not observed in most thin sections; where it is observed it constitutes less than 3% of the rock volume.  Apatite occurs as fine-grained euhedral laths and sometimes as relatively coarse, stubby, euhedral microphenocrysts.  Pyrite, chalcopyrite and magnetite occur as scattered, irregular grains. Biotite is commonly mantled by secondary magnetite. Except in well mineralized zones magnetite is the dominant opaque phase. Rare fine-grained rutile was noted by Harris (1989).  30  The groundmass is almost always an interlocking to felsitic microgranular mosaic of Kfeldspar of grain size 20-100 microns. It occasionally includes microphenocrysts of plagioclase, small grains and pockets of quartz with rare myrmetitic textures.  Some stock margins, particularly those of the Southern Star stock, are characterized by a plagioclase-hornblende porphyritic monzonite border phase. Rafts of volcanic rocks are most common in the Southern Star stock. Localized areas of monzonite intrusion breccia comprise xenoliths of volcanic rock and/or earlier monzonite (Figures 2.9G, H and I).  Along the contact of the stocks, particularly the MBX stock, a hybrid monzonite-volcanic rock is present. This rock has gradational boundaries and textures that are characteristic of both monzonitic and volcanic rocks. The hybrid contains ghosts of plagioclase phenocrysts and remnants of augite phenocrysts. It could be an intrusion breccia with primary textures that are obscured by intense pervasive hydrothermal alteration and magmatic assimilation.  Hydrothermal breccia occurs extensively throughout the Southern Star stock (Figures 2.9B and C), and less commonly in adjacent volcanic rocks and along the margins of the MBX stock. The breccia is characterized by K-feldspar flooding (Figures 2.9C and H) and veinlets (Figure 2.9A) that vary in thickness and intensity. There appears to be a gradation from massive relatively unaltered monzonite through a crackle breccia to well-developed breccia with a K-feldspar-rich matrix. In areas of well-developed brecciation and intense K-feldspar alteration, the matrix is aplitic. In areas of abundant veining, brecciation and K-feldspar flooding (Figure 2C), clasts of brecciated volcanic rocks appear rotated (Figure 2.9G). The breccia clasts are generally of the same composition as the host rock, although less commonly, exotic lithologies are present. Veinlets are composed dominantly of K-feldspar  31  with subordinate to minor quartz, magnetite, pyrite and chalcopyrite, minor molybdenite and bornite.  Locally, along the eastern margins of the MBX stock (Figure 2.3), magnetite comprises over 50% of the breccia matrix. Discovery drill hole 12 intersected one of these magnetite-rich breccias in the area now referred to as the MBX (magnetite breccia) zone.  Non-mineralized late intermineral plagioclase-hornblende porphyritic monzonite dikes are common throughout the Southern Star stock. Locally they cause significant dilution of grades. 2.4.5 Post-mineral Intrusive Rocks (Figure 2.10)  Three suites of post-mineral dikes that cut the Mt. Milligan Main and Southern Star deposits, from oldest to youngest, consist of trachyte, monzonite and diorite. These dikes are generally unaltered to weakly altered and lack sulfide mineralization.  The trachyte dikes are the earliest and are most common in the south-southwestern portion of the Mt. Milligan Main deposit (Figure 2.3) and the adjacent northern portion of the Southern Star deposit. They are 1 to 15 metres wide, strike northeasterly and dip moderately northwesterly. Trachyte dikes are grey, massive, fine-grained and non-porphyritic. Kfeldspar is the dominant constituent. Most trachyte dikes contain accessory magnetite; a few contain traces of chalcopyrite. The monzonite (Table 2.1) dikes intruded after the trachyte dikes and occur throughout the Mt. Milligan Main and Southern Star deposits. Monzonite dikes are up to 10 metres wide, strike northeasterly and dip moderately northwesterly. They are characterized by abundant plagioclase phenocrysts, up to 2 mm long, and may contain augite and hornblende phenocrysts, up to 5 mm across. The phenocrysts occur in a fine-grained K-feldspar bearing 32  matrix. Accessory magnetite is always present. Some monzonite dikes are weakly propylitized.  The diorite dikes, which are compositionally (Table 2.1) granodiorite (Lang 1992), are youngest. Although present in both deposits, they are most common in the northeastern portion of the Main deposit (Figure 2.3). These dikes are up to .5 metres wide, strike northwesterly and dip steeply northeasterly. Diorite dikes (Figure 2.10) are characterized by scattered, commonly glomeroporphyritic zoned plagioclase phenocrysts, up to 10 mm long, in a fine-grained matrix. Subsidiary hornblende phenocrysts, up to 2 mm long, and minor quartz eyes up to 1 cm across are present also. Some dioritic dikes are weakly carbonatealtered.  2.5 Alteration  Potassic and propylitic alteration characterizes the Mt. Milligan Main and Southern Star deposits (DeLong et al., 1991). Copper and gold mineralization is associated mainly with the potassic alteration (Figure 2.3), except in the 66 zone where gold rich, but copper poor mineralization is associated with propylitic alteration. Sericitic alteration, affecting plagioclase phenocrysts in the stocks and related dikes, overprinted the potassic alteration. Early in the alteration history, most augite was replaced by pseudomorphic actinolite. Late carbonate alteration is present and locally abundant.  Potassic alteration is most intensely developed around the contacts of the MBX and Southern Star stocks (Figure 2.3). It decreases in intensity towards the cores of the intrusions and outwards into the country rocks (Figure 2.11). Potassic alteration is less widespread around the Southern Star stock than the MBX stock. Where faults and fractures are abundant, potassic alteration extends farther beyond the stock contacts. Propylitic  33  alteration is best developed away from the stocks, beyond the potassic zone (Figures 2.3 and 2.4). The propylitic assemblage locally overprinted the potassic alteration, demonstrated in Figure 2.12. Less commonly potassic alteration overprinted propylitic alteration. These relationships can be interpreted to mean that the two alteration types are, in part, contemporaneous. Propylitic alteration is longer lived, however, as evidenced by the more frequent overprinting of propylitic alteration on potassic alteration. Details of these two dominant alteration types follow. 2.5.1 Potassic  Alteration  Potassic alteration is characterized by hydrothermal K-feldspar that is present throughout the potassic zone. In areas of more intense potassic alteration, hydrothermal biotite, chalcopyrite, lesser magnetite and minor bornite occur in addition to K-feldspar. This is referred to as the biotite subzone (Figure 2.3).  Pervasive hydrothermal K-feldspar alteration is responsible for the field classification of volcanic andesite to basalt as latitic and trachytic volcanic rocks in the Mt. Milligan Main deposit area. K-feldspar, although more widespread than biotite, is developed erratically. It is associated with copper and gold, but less clearly than biotite (cf Chapter 3, Figure 3.1). In latitic rocks, K-feldspar occurs as veinlets and envelopes along fractures, and as a patchy to pervasive fine-grained dark greyish to pinkish aphanitic groundmass that obliterates magmatic textures (Figure 2.14A). Veinlets commonly contain accessory quartz and minor calcite. Relict augite phenocrysts, replaced by actinolite and calcite, are present locally. In trachytic rocks, intense pervasive K-feldspar alteration occurs in the more permeable tuff beds demonstrated in Figure 2.14B. In andesitic rocks, which host the Southern Star stock, K-feldspar alteration is mostly fracture controlled. It occurs around fractures and in veinlets that are generally close to intrusive contacts. Pervasive K-feldspar replacement of the groundmass is uncommon but occurs in strongly altered monzonites (Figure 2.15).  34  Figure 2.11 (on following page). Different types of alteration found at the Mt. Milligan Main deposit illustrated by four pieces of NQ core (4.76 cm in diameter). From left to right: (a) intense dark grey fine-grained biotite alteration of latitic volcanic rock, (b) K-feldspar (light grey) and biotite pervasive to patchy alteration, (c) Coarser grained patchy to pervasive epidote-albite-pyrite alteration, (d) Intense epidote-pyrite alteration from 66 zone. Propylitic alteration of this intensity is restricted to the 66 zone. Figure 2.12. Propylitic alteration overprinting potassic alteration in the Mt. Milligan deposit illustrated in NQ core. (A) Pyrite vein with minor epidotealbite cross-cutting K-feldspar-biotite altered augite porphyritic latite. Kfeldspar is more intensely developed in the envelope of the vein perhaps due to earlier potassic fluid flow along this fracture. (B) Epidote-pyrite overprinting dark grey biotite altered latite in the 66 zone. Figure 2.13. Photomicrograph of hydrothermal biotite with clear K-feldspar in groundmass of a latite. View is in plane polarized light.  35  36  K-feldspar alteration is restricted generally to the outer margins of the monzonitic rocks and is similar to that in the volcanic rocks except for its greyish pink color. The most intense areas of alteration occur in the Mt. Milligan Main deposit, particularly at the contact of the Rainbow dike with the MBX stock (Figure 2.3).  2.5.1.1 Biotite Subzone  The biotite subzone contains the highest copper and gold concentrations. Consequently, the characteristics of biotite in this subzone are detailed below.  Hydrothermal biotite is developed best in latitic volcanic rocks in the biotite subzone. It occurs as fine-grained felted patches that comprise up to 60% or more of the volcanic rocks (Figure 2.13) adjacent to monzonite intrusions, and decreases away from them (Figure 2.11). Pervasive biotite alteration is usually restricted to the groundmass, however alteration in the biotite subzone leaves distinct, often euhedral, pyroxene-shaped pseudomorphs of actinolite and calcite after augite phenocrysts. Locally, the alteration is intense enough to be texturally destructive, and the rock appears as fine-grained, patchy to swirled aggregates of biotite and K-feldspar, usually with magnetite and microscopic acicular actinolite disseminated throughout the groundmass. This type of intense alteration often hosts the highest grade copper and gold mineralization. Thus it forms the core of the MBX zone.  Hydrothermal biotite also occurs along the margins of K-feldspar veinlets in latitic and trachytic volcanic rocks. In andesitic rocks, which host the Southern Star stock, hydrothermal biotite occurs only adjacent to K-feldspar veinlets. Actinolite-augite is sometimes partially to completely replaced by calcite or calcite and pyrite.  37  Figure 2.14 (following page). Fine-grained K-feldspar development illustrated in stained NQ core (4.76 cm in diameter). (A) K-feldspar altered pyritic volcanic rock from the 66 zone. (B) Bedded trachytes illustrating Kfeldspar flooding along bedding planes. Figure 2.15. Variations in potassic alteration in monzonite from the Mt. Milligan deposit. Upper pale grey sample of NQ core illustrates K-feldspar flooding. Lower sample contains more hydrothermal biotite. Figure 2.16. Epidote-albite-pyrite propylitic alteration in the 66 zone illustrated by NQ core. This style of alteration is frequently, but not always, associated with high gold assays.  38  39  Hydrothermal biotite in monzonite occurs near its contacts with volcanic rocks. However, it is less abundant in the monzonite than in the more mafic volcanic rocks.  2.5.2 Sericitic  Alteration  Plagioclase phenocrysts are pale green and sericitic in the M B X and Southern Star stocks and the Rainbow dike. The plagioclase is replaced by saussurite, aggregates of sericite, epidote, chlorite and calcite. Minor sericite occurs in K-feldspar and quartz veinlet selvages in the Esker zone. In general, however, the sericitization of plagioclase alteration appears to be pervasive and is not related obviously to fracture permeability.  2.5.3 Propylitic  Alteration  Propylitic alteration occurs as a widespread zone that is peripheral to, but locally cross-cuts and can overlap, potassically altered rocks. It is characterized by epidote, albite, calcite, chlorite and pyrite (Figures 2.16, 2.17, and 2.18), and is developed best in andesitic and latitic volcanic rocks. Most areas that did not undergo potassic alteration are characterized by minerals of the propylitic assemblage. Propylitization extends up to 2 k m from the monzonite stocks.  Epidote, the most common and widespread propylitic mineral (Figure 2.16), is generally associated with pyrite. It occurs in narrow alteration envelopes around fractures and pyritecalcite veins, or as veinlets with chlorite and calcite. Epidote also is present as irregular aggregates in the groundmass and medium-grained clots that nucleate commonly on mafic phenocrysts. The clots are roughly circular and are zoned concentrically with pyrite and minor chalcopyrite, epidote and epidote intergrown with albite and calcite. Magnetite is present in the cores of some clots. Where epidote occurs with potassic alteration, it generally overprints and cross-cuts the potassic minerals.  40  Chlorite, although most common in the propylitic zone, also occurs sporadically in the potassic zone. It occurs on the edges of pyrite grains (Figure 2.17A), replacing mafic grains after primary biotite and hornblende (Figure 2.9F), in the groundmass of monzonite and in tectonically disrupted zones. Chlorite appears as patchy alteration (Figure 2.17B) in the groundmass and in the bands of banded trachytes in the propylitic alteration zone (Figure 2.18).  Albite forms fine-grained, creamy white irregular patches in the groundmass of volcanic rocks. It is present in the 66 zone with potassic and propylitic alteration, and in the MBX zone with potassic alteration. Albite is sometimes intergrown with minor epidote, although generally the two do not occur together in trachytic volcanic rocks.  Calcite occurs in propylitic alteration mainly as a replacement of the groundmass in volcanic rocks or as replacement of actinolite, which is a pseudomorph of augite. It also occurs in clots and veinlets with epidote, chlorite and pyrite in latitic and andesitic volcanic rocks. In trachytic volcanic rocks, calcite occurs in clots with chlorite and pyrite. 2.5.4 Carbonate  Alteration  Late calcite and ankerite veinlets cut all rock types and alteration assemblages. It is post mineralization. The ankerite veins are commonly associated with major structures that locally contain tectonic breccias with a carbonate matrix.  2.6 Hypogene Copper and Gold  Mineralization  Mineralization in the Mt. Milligan Main deposit comprises four coalescing zones: MBX, 66, WBX and DWBX (Figures 2.3 and 2.4). The MBX zone is in the central portion of the deposit where the Rainbow dike protrudes from the footwall of the MBX stock.  41  Figure 2.17 (on following two pages). Photomicrographs of chlorite occurrences Mt. Milligan deposit. (A) Thin pyrite veinlet showing either chlorite selvages or thin chloritic envelopes. Biotite is altered to chlorite along this veinlet margin, illustrating on a larger scale the process postulated in Chapter 4 that accounts for iron-magnesium ratios observed in analyses of biotites. Photographed in plane-polarized light. (B) Chlorite-biotite clot with carbonate in trachyte from 66 zone. Crossed nicols illustrates anomalous blue-green pleochroism fine-grained chlorite. Figure 2.18. Irregular chlorite development in propylitically altered trachytes, Mt. Milligan deposit as illustrated by NQ core (4.76 cm in diameter). Parting or banding is also defined by chlorite. Figure 2.19. Habits of chalcopyrite occurrence, Mt. Milligan deposit as illustrated by NQ core. (A) Quartz veins with fine-grained chalcopyrite lining small cross-cutting fractures. Other thin fine-grained chalcopyrite occurrences are aligned parallel to quartz vein margins. Quartz stockworks, although important in some parts of the WBX zone, are not common in the Mt. Milligan Main deposit (B) Disseminated chalcopyrite in potassically altered monzonite from the MBX zone. Figure 2.20. Chalcopyrite habits in the MBX zone illustrated by photomicrographs of polished thin sections. (A) Disseminated chalcopyrite in K-feldspar-biotite altered latite. One grain of pyrite is present. Both reflected and transmitted light were used in this photomicrograph making the chalcopyrite less bright than in (B) where only reflected light is present. (B) Chalcopyrite veinlet. Figure 2.21. Photomicrograph of gold grain in polished thin section. Bright yellow gold grain (Au) is in a fracture in pyrite (py). Also present are chalcopyrite (cp) and bornite (bo). Sample is from the 66 zone. Gold grain is 10 microns across.  42  43  The MBX zone contains gold and copper mineralization that grades laterally southeastwards into the gold rich and copper poor 66 zone, which surrounds the southern half of the Rainbow dike. The WBX zone and its down-faulted extension, the DWBX zone, are located in the northwestern portion of the deposit. Both occur along the hanging wall of the MBX stock, and contain copper and gold associated with quartz veinlets displayed in Figure 2.19. The WBX and DWBX zones contain higher concentrations of quartz veins and less disseminated chalcopyrite relative to other zones of the deposit.  The copper and gold mineralization in the Southern Star deposit occurs in the hanging wall and footwall of the Southern Star stock and in the adjacent volcanic rocks (Figure 2.3). Sulfides, in particular pyrite, are less abundant in the Southern Star deposit than in the Main deposit. Mineralization in both deposits consists mostly of pyrite, chalcopyrite, lesser magnetite, minor bornite and traces of molybdenite in potassic alteration, and pyrite in propylitic alteration. In potassic alteration, the best mineralization is developed in monzonite and volcanic rocks adjacent to the footwall, and to a lesser extent, the hanging wall contacts of the stocks. The Rainbow dike and the enclosing volcanic rocks are mineralized also. Copper and gold grades generally decrease with distance from the intrusions, closely paralleling the relative intensity of potassic alteration. In areas of propylitic alteration, pyrite concentrations are highest adjacent to the potassic zone and generally decrease outward from the stocks. 2.6.1  Chalcopyrite  Chalcopyrite is an integral component of potassic alteration and is most abundant in the biotite subzone where it is present with pyrite, bornite and gold in K-feldspar and quartz veinlets. A decrease in chalcopyrite content occurs with increasing distance from intrusive contacts.  45  Chalcopyrite occurs mostly as fine-grained disseminated grains (Figure 2.19B) and fracture fillings (Figure 2.20B) and less commonly as veinlets (Figure 2.9B) and in other veinlets of calcite, pyrite and lesser quartz and K-feldspar with chalcopyrite selvages. Disseminated grains (Figure 2.20A) commonly are present in biotite-rich envelopes around veins. Adjacent to the MBX stock, chalcopyrite is accompanied locally by pyrite, forming coarse sulphide aggregates. Chalcopyrite-bearing veins contain pyrite and magnetite in a gangue of K-feldspar, quartz and calcite. In massive trachytic volcanic rocks, chalcopyrite accompanies pyrite along curvilinear partings and as disseminated grains.  Chalcopyrite also occurs in gold-rich quartz veinlets (Figure 2.19A) in the WBX and DWBX zones of the Mt. Milligan Main deposit, and in the Southern Star deposit. Minor Kfeldspar envelopes are associated with these veins, which contain more quartz than other veins on the property. In the Southern Star deposit fracture controlled mineralization is more important than at the Mt. Milligan Main deposit. 2.6.2  Magnetite  Magnetite is present throughout the potassic alteration zone and is less common in propylitic alteration. It occurs as disseminated grains, patches, and in veinlets and hydrothermal breccia matrix. A magnetite breccia that occurs in the MBX zone of the Mt. Milligan Main deposit comprises 50% massive magnetite as stockwork veins along the footwall contact of the MBX stock (Figure 2.3). Disseminated magnetite is most common in biotite rich rocks. Veinlets with magnetite contain subordinate pyrite, chalcopyrite, Kfeldspar, calcite and native gold. In the core of the MBX zone, some of the massive magnetite veins carry gold contents up to 237 g/t. Trachytic rocks contain magnetite-rich laminae. In all but the 66 zone, relative magnetite abundance is positively correlated with gold and copper concentrations.  46  2.6.3 Bornite  Bornite is present exclusively in potassically altered volcanic rocks as blebs and disseminated grains in lens-shaped zones close to the footwall contacts of the MBX and Southern Star stocks. K-feldspar veinlets are common in these areas. Bornite is associated with higher gold concentrations. In contrast, pyrite is rarely associated with bornite. 2.6.4 Pyrite  Pyrite is the most widespread sulfide in and around the deposits. Pyrite content increases sharply from potassic to propylitic alteration, where it is most abundant and forms a crude halo adjoining the potassic zone (DeLong, 1993). Pyrite occurs as disseminated grains, veinlets (Figure 2.8B), veins (Figure 2.12A), vein selvages, large clots, patches (Figure 2.8D), blebby aggregates, and as a partial to complete replacement of mafic minerals such as actinolite and/or augite. It is commonly found with calcite. In the 66 zone, gold is associated with 5 to 20% coarse pyrite. Several generations of pyrite veinlets are indicated by cross-cutting relationships. Pyrite occurs also as a minor constituent of veins in potassically altered rocks. In trachytic rocks, pyrite occurs with chalcopyrite in curvilinear partings (Figure 2.8A). 2.6.5 Gold  Gold is present as grains ranging in size from less than 5 microns up to 100 microns. The grains fill microfractures in sulfides, adhere to imperfections on chalcopyrite and pyrite grains (Figure 2.21), and occur as inclusions in pyrite, chalcopyrite and magnetite. Gold that is visible in hand samples is associated with bornite and, less commonly, massive magnetite.  47  2.6.6 Polymetallic  Veins  Gold-silver-bearing veins occur in volcanic rocks adjacent to the MBX and Southern Star stocks and cross-cut previously developed propylitic alteration. These polymetallic veins are developed at the boundary of the potassic alteration zone and within the propylitic alteration zone. Most of the veins occur in the Creek and Esker zones, but they are widely distributed around the entire periphery of the Mt. Milligan deposits. The veins comprise sulfide rich and carbonate-quartz rich types; most are structurally controlled and generally strike eastnortheast.  The sulfide rich veins are hosted by andesitic volcanic rocks. They contain mostly pyrite with lesser chalcopyrite, sphalerite, galena, molybdenite, arsenopyrite, tetrahedritetennantite and gold, and minor amounts of quartz, K-feldspar and carbonate gangue. Kfeldspar alteration envelopes are well developed around some of the polymetallic veins that in turn are enclosed by intense propylitization.  The carbonate-quartz rich veins occur in propylitized latitic volcanic rocks northwest and northeast of the MBX stock. They contain pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, tetrahedrite, magnetite, ilmenite and hematite in a gangue of quartz, carbonate, chlorite and K-feldspar. Scanning electron microscope work with an energy dispersive analyzer (Mcintosh, 1990) showed that the tetrahedrite is silver-bearing.  48  Chapter 3  Metal Zonation and Alteration Mineral Distribution 3.1 Introduction  Hypogene mineralogical zoning is recognized at the Mt. Milligan and Southern Star deposits. The alteration assemblages and economic zones are described in Chapter 2. On a macroscopic scale two distinct mineralogical assemblage zones were reported early in the history of the exploration of this deposit (Rebagliati et al., 1989). A potassic assemblage associated with the chalcopyrite mineralization is surrounded by a peripheral propylitic assemblage. DeLong et al. (1991) recognized an inner biotite subzone defining further the zonation within the potassic assemblage (Figure 2.3).  The mineralization is divided into the Mt. Milligan Main (cf. Sketchley et al., 1995), and the Southern Star deposits. The Mt. Milligan Main deposit is divided into gradational zones (Figure 2.3) called the Magnetite Breccia zone (MBX), the West Breccia zone (WBX), the Deep West Breccia zone (DWBX), and the 66 zone. The DWBX zone is encountered below the 1,000 metre level elevation (Figure 2.4). The purpose of this chapter is to describe the distribution of metals across the Mt. Milligan deposits and to relate this distribution to the hydrothermal alteration assemblages. 3.2 Methods and Data  Collection  The samples for metal analyses were collected from the drill core where the hole intersected the 1000 metre elevation level. Overall hydrothermal alteration, described in Chapter 2 (Figure 2.3) uses the same level and sample set.  49  Concentrations of metals obtained from analyses of pulps from the vertical 10 metre interval between 995 and 1005 metres, above and below the pierce point for each hole, were averaged and assigned to the location of the pierce point. The averaged assay data for gold and copper assigned to each 10 metre section surrounding the pierce point are averages of 2 metre interval assay data. The magnesium, iron, sulphur and trace metal concentrations were obtained from composited samples analyzed by Corninco Exploration Research Laboratory, Vancouver, B.C., of Corninco Resources International Limited. These samples were prepared by mixing 50 gram pulverized splits from each of the 2 metre sample interval pulps covering the 10 metre section centered on the 1000 metre elevation level. The proportion of K-feldspar, biotite, magnetite, bornite, epidote, calcite and albite was visually estimated as a percentage of the core for each 10 metre intersection of drill core. Assays of post-mineral dikes have been excluded. About 114 pierce points on the 1000 metre elevation were examined. All data were collated into a computer file for statistical examination and computer plotting.  3.3 Results  Most of the copper and gold mineralization found in the MBX and WBX zones of the Mt. Milligan  Main deposit and in the Southern Star deposit is associated with potassic  alteration. The inner biotite subzone (Figure 2.3) is spatially related to the core of coppergold mineralization. The 66 zone has gold mineralization, but low copper mineralization. It is associated with mixed alteration (Figure 2.3).  50  AVERAGE LINKAGE METHOD TREE DIAGRAM DISSIMILARITIES -1.000  1.000  BI AU  cu MT  1  BO AG PB ZN PY F.P  Figure 3.1. Tree diagram illustrating correlation characteristics among elements and minerals at Mt. Milligan. Closely linked components are adjacent and are linked near the lefthand side of the diagram (see text). Alteration minerals: B O = bornite, B I .= biotite, E P = epidote, M T = magnetite, P Y = pyrite.  51  Two distinct mineral assemblages that define the two prominent alteration zones are illustrated graphically by the tree diagram in Figure 3.1. The length of the line connecting adjoining minerals and elements is inversely proportional to a statistical measure describing the degree of similarity of distribution for the pair. The estimated abundance of individual minerals on the 1000 metre level are illustrated in Figure 3.2. Figure 3.1 defines both the potassic (Figures 3.2B to 3.2D) and propylitic (Figures 3.2E and 3.2F) alteration assemblage minerals. The inner biotite rich zone displayed on Figure 2.3 was defined by biotite occurrence displayed in Figure 3.2B.  A set of box plots (Figure 3.3) graphically displays the distribution characteristics of five metallic elements for each of the mineralized zones. Those samples found in the periphery of the deposits or outside the zones are included in the Outside "zone". Several samples come from the poorly mineralized core of the MBX stock, between the MBX zone and the WBX zone, and are classified here as barren "zone". In the box plots the horizontal line inside the box represents the median. The horizontal ends of the box represent the lower and upper hinges (the 25th and 75th percentiles). In addition, asterisks represent outside values, which are data values just outside the inner fences defined by the upper and lower values predicted by normal distribution. Open circles represent far outside values, which are data values farther outside the fences.  The distribution of the five metallic elements (gold, copper, lead, zinc and silver) are examined in more detail, below.  52  10500  9500h North 8500h  7500+ 11500  12500  13500  14500  East  Figure 3.2A. Index plan showing the approximate locations of the mineralized zones on 1,000 m elevation level at Mt. Milligan.  10500  O Oo  0  • •o  • •» ° ° O^D)  9500  • • •o  cc O  OS •  o  • •• •• °°^>  t3  •  c o  8500  7500 1 1500  12500  13500  14500  EAST  Figure 3.2B. Bubble plot of visually estimated biotite abundance at each of the sample locations on the 1000 metre elevation level. Largest bubble represents a visual estimate of 30% hydrothermal biotite: small dots are stations where no biotite was observed. The inner biotite subzone of the MBX and 66 Zones have the highest biotite abundance that characterizes potassic alteration (Figures 2.3 and 3.1).  54  10500  i  0  o  O 0  o  o  Qim o o •  to •  a :CQB °  •  ©  0  O  9500 • o IE I— CC  :  o  '• •  O  o •  o  Q O  o  • « .>  o :  o  o ;  Q.o  o  O  •  •  8500 •  :  O  •  0 o  ;  7500 11500  o  o  o  i  12500  13500  14500  EAST  Figure 3.2C. Bubble plot of visually estimated magnetite abundance at each of the sample locations on the 1000 metre elevation level. Largest bubble represents a visual estimate of 10% magnetite: small dots are stations where no magnetite was observed. The MBX Zone has the highest magnetite abundance. The single large concentration of magnetite in the center of the MBX Zone is a result of the 10 m drill intersection sampling abundant breccia with a magnetite matrix. High magnetite is associated with potassic alteration (Figures 2.3 and 3.1).  55  10500  CCD  9500  O 8500  o  7500 1 1500  •  12500  13500  14500  EAST  Figure 3.2D. Bubble plot of visually estimated bornite abundance at each of the sample locations on the 1000 metre elevation level. Largest bubble represents a visual estimate of 0.3% bornite: small dots are stations where no bornite was observed. The two' northern occurrences are within the inner biotite subzone that is part of the potassic alteration (Figures 2.3 and 3.1).  56  10500  O  9500  _  o  o  o • •  x i-  O  •  A  cc o  o 8500  O  °@  •.•.•.•SO« C  ?  o  ooo OO °o o  o  ° o O  7500 11500  12500  13500  14500  EAST  Figure 3.2E. Bubble plot of visually estimated epidote abundance at each of the sample locations on the 1000 metre elevation level plan. Largest bubble represents a visual estimate of 15% epidote. The small dots represent stations where no epidote was observed. Epidote, concentrated peripheral to the main deposit and in the 66 zone, is characteristic of propylitic alteration (Figures 2.3 and 3.1).  57  10500  0° O o 9500  o  GD  QP  j oo  aQ O O  zz i— cc  o  o  z  8500  11500  oc§5 ; o o o o O oo • o Oi o • <Q  O  : ° OO o  O  j  i  O o oOO  -  -o  12500  13500  14500  EAST  Figure 3.2F. Bubble plot of visually estimated pyrite abundance at each of the sample locations on the 1000 metre elevation level plan. Largest bubble represents a visual estimate of 10% pyrite. Pyrite, concentrated peripheral to the main deposit and in the 66 zone, is characteristic of propylitic alteration (Figures 2.3 and 3.1).  58  3.3.1 Gold  Distribution  Gold concentration at the individual sampling sites is indicated in Figure 3.4B. The size of the circle is proportional to the assay of the 10 metre composite that has a pierce point at thcentre of that circle. Gold is associated with potassic alteration near the contact of the Takla volcanic and volcaniclastic rocks with the MBX stock (MBX zone and WBX Zone) and the Southern Star stock (Southern Star deposit). Figure 3.3A illustrates the distribution characteristics of gold by zone. The highest gold concentrations are closely associated with the inner biotite zone in the MBX zone. Gold is also found to the southeast of the MBX stock near and to the east of the Rainbow Fault in the 66 Zone. Here it is associated with biotite but also with epidote and pyrite. Locally the epidote and pyrite veins and veinlets crosscut earlier pervasive biotite. As shown in Figure 3.3B and 3.4C the 66 Zone has lower copper concentrations.  3.3.2 Copper  Distribution  Copper distribution demonstrated by the bubble plot of Figure 3.4C, is also associated with potassic alteration, particularly the inner biotite subzone and is similar to gold in that respect. The MBX and WBX zones of the Mt. Milligan Main deposit and the Southern Star deposit contain most of the significant copper (Figure 3.3B). 3.3.3 Lead  Distribution  Lead distribution is demonstrated as a bubble plot on the 1000 metre level in Figure 3.4D and in the box plot of Figure 3.3C. Only six samples have lead concentrations over 20 ppm. All single point high lead analyses are from in samples either on the periphery of the main deposit ("outside") or in the 66 zone.  59  ZONE  Figure 3.3A. Box plot illustrating distribution of gold (Au) by major zones as indicated in the index figure and Figure 2.3. The highest gold concentrations are in the MBX and 66 Zones. The WBX Zone and the Southern Star deposit have moderate gold concentrations. 10000  8000  6000  o 4000  2000  ZONE  Figure 3.3B. Box plot illustrating distribution of copper (Cu) by major zones as indicated in the index figure and Figure 2.3. The highest copper concentrations are in the MBX and WBX Zones and the Southern Star deposit. The 66 Zone (Z66) is generally low in copper.  60  150  100 m  ZONE  Figure 3.3C. Box plot illustrating distribution of lead (Pb) by major zones as indicated in the index figure and Figure 2.3. Isolated, relatively high lead concentrations occur outside the main deposit area and in the 66 Zone (Z66). 400  300  N  200  100  _l  I  I  L_  ZONE  Figure 3.3D. Box plot illustrating distribution of zinc (Zn) by major zones as indicated in the index figure and Figure 2.3. Isolated, relatively high zinc concentrations occur outside the main deposit area and in the 66 Zone (Z66).  61  1  1  1  i  1  o o o o o -  *  o  o  1 1  T T  *  i  i  0  |  6*6 i  i  1  ZONE  Figure 3.3E. Box plot illustrating distribution of silver (Ag) by major zones as indicated in the index figure and Figure 2.3. Generally the same as for lead and zinc but also isolated, local high silver concentrations occur in the Southern Star deposit and the WBX Zone. 3.3.4 Zinc  Distribution  Zinc distribution is demonstrated as a bubble plot on the 1000 metre level in Figure 3.4E and in the box plot of Figure 3.3D. The distribution of zinc is similar to that of lead. All high concentrations of zinc occur outside the main deposit or in the 66 zone. 3.3.5 Silver  Distribution  Silver distribution is demonstrated as a bubble plot on the 1000 metre level in Figure 3.4F and in the box plot of Figure 3.3D. Silver occurrence is similar to the distribution of lead and zinc. High silver concentrations are either outside the main deposit or in the 66 zone. Isolated relatively high concentrations are also located in the WBX zone and the Southern Star deposit.  62  3.4  Zonation  The distribution of copper, gold, zinc, lead and silver at the Mt. Milligan and Southern star deposits reflect, in a general sense, patterns (Figure 3.5) that have been recognized in the porphyry environment (Emmons, 1927; Jones, 1992). More specifically gold and copper are located in the central parts of deposits whereas lead, zinc and silver are more peripherally located. Gold, as at Mt. Milligan, in addition to being central can be more peripherally located (Emmons, 1927).  Mt. Milligan and Southern Star deposits are characterized by the following five subzones (cf. Jones, 1992), from the inside of the deposit to the outer edges (Figure 2.3) based on metal and alteration mineral distribution: 1. "barren" core, 2. inner bornite subzone, 3. chalcopyrite-gold subzone, 4. pyrite halo, 5. lead-zinc-silver subzone.  3.4.1 "Barren"  Core  The poorly mineralized " barren" core is located between the MBX and WBX zones, generally in the central part of the stock. Alteration is generally weak and ranges from sericitic to potassic. Pyrite is often present but in minor amounts. Structurally controlled  63  zones of montmorillonite development are present. At the Mt. Milligan Main deposit the barren core occurs in the central parts of the MBX stock.  3.4.2 Bornite Subzone  An inner bornite subzone exhibiting gold enrichment in the central parts of the mineralization (not indicated on Figure 2.3) is found in the inner part of the MBX zone (Figure 3.2D) and Southern Star deposit. There are three occurrences of bornite, all in the central to inner part of the biotite alteration zone (Figure 2.3). Bornite occurrence correlates generally with relatively high copper and gold grades. Gold is commonly associated with bornite in a potassic assemblage dominated by secondary biotite (Figure 3.2B) and magnetite (Figure 3.2C) (cf. Sillitoe, 1979; cf. Cox and Singer, 1988). The sporadically developed inner bornite zone occurs in biotite alteration interpreted as the most intense potassic alteration.  3.4.3 Chalcopyrite-Gold  Subzone  The chalcopyrite-gold subzone is entirely within potassic alteration. This subzone is represented by the MBX and WBX zones and the Southern Star deposit outside the inner bornite subzone (Figures 3.4B and 3.4C). Gold and copper concentrations co-vary (Figure 3.6). Similar correlation between gold and copper grades in the potassic alteration zone of many porphyry copper deposits has been well documented (Sillitoe, 1979, 1988; Perello and Cabello, 1989). Examples in the Canadian Cordillera include Bell and Granisle (Dirom et al., 1995), Kemess (Rebagliati et al., 1995), Mt Polley (Fraser et al., 1995), Ajax (Ross et al., 1995), Copper Mountain (Stanley et al., 1995) and Island Copper (Perello et al., 1995).  64  10500  *  , WBX 9500 North  Southern Star *  MBX 66  8500  7500+^ 11500  12500  13500  14500  East  Figure 3.4A. Index plan indicating the approximate locations of the mineralized zones on the 1000 metre elevation level at M t . Milligan  65  10500  O CO  9500  cc O  Q •  o  •o  o  •  o °  .O  O o • o o  8500  o  o  •  .0  o o  O • o O O  7500 1 1500  o CD o  o • • • o  12500  13500  14500  EAST  Figure 3.4B. G o l d distribution on the 1000 metre elevation level plan. The largest bubble represents 1.71 g/t and the smallest 0.02 g/t.  66  10500  i  :  O  O Q  C®  ° °> o  o • •:•>•©•  • -oOOO  9500  -o.Q  •o  Q 0 0  o  ' • • a-  • .  O  .  •n  •  0  •: O • • . : o •  8500 •  : O  o  :  ' • ' .  o o  -  •  i 11500  12500  13500  14500  EAST  Figure 3.4C. Copper distribution on the 1000 metre elevation level plan. The largest bubble represents 0.87% copper and the smallest 8 ppm copper.  67  10500  9500  •o ••• • • ° 0 •  8500  •  o  c  )  11500  12500  13500  14500  EAST  Figure 3.4D. Lead distribution on the 1,000 metre elevation level plan. The largest bubble represents 124 ppm lead and the smallest 4 ppm.  68  10500  o o O  Q  °  9500 °O  °  °° ° ° o cc OP  cc O  o  Q  •  8500  o  7500 11500  •  •  o  O  o  .  o  a  12500  13500  14500  EAST  Figure 3.4E. Zinc distribution on the 1,000 metre elevation level plan. The largest bubble represents 301 ppm zinc and the smallest 10 ppm.  69  10500  o  °0  0 ®  ° •  o O ° 0 °  ° <© O  9500  O  °  cc O z  O  O ° Zf  8500  _  :. .6 0 O  7500 1 1500  O  o  o  °o ° ° ° °  0°°  0  •>  "  •> „  % O 0  O o ° °Q° o o o o o 0  :.. • • o o O . Q  .  Q  0  •  o  12500  .  13500  14500  EAST  Figure 3.4F. Silver distribution on the 1000 metre elevation level plan. The largest bubble represents 4.6 silver and the smallest 0.4 ppm. 3.4.4 Pyrite Halo  The pyrite halo (Figures 3.2F and 3.7) surrounds potassic alteration of both the Mt. Milligan Main and Southern Star deposits and occurs within propylitic alteration (Figure 2.3). Polymetallic gold bearing veins occur as isolated veins and swarms (Esker zone and Creek  70  Figure 3.5. Metal zoning as originally postulated by Emmons (1927). A peripheral gold zone that overlaps lead, zinc and silver was documented. The 66 Zone may be analogous to this outer gold zone.  zone in Figure 2.3). These veins are located in the pyrite halo and to a lesser degree in the potassic-chalcopyrite-gold zone. The 66 zone is situated, in part, in the pyrite halo. The pyrite halo at Mt Milligan varies in width from 60 to 200 metres except in the 66 zone, where the area of increased pyrite abundance widens to almost 500 metres.  3.4.5 Lead-zinc-silver  Subzone  The lead-zinc-silver subzone (Figures 3.4D to 3.4F) is weakly developed peripheral to higher grade mineralization. This subzone is contained within propylitic alteration.  71  Increased concentrations of lead, zinc and silver, occur sporadicallyat the gradational contact between the potassic and propylitic alteration and in the 66 zone associated with hydrothermal minerals of both the potassic and propylitic alteration assemblages. The lead-zinc-silver subzone partly overlaps the pyrite halo. In the 66 zone, as observed in the pyrite halo, the lead-zincsilver zone markedly overlaps areas of biotite development. All three elements exhibit spotty elevated concentrations that are restricted to the periphery of the deposits and the 66 zone. Lead, zinc and silver bearing minerals are not often observed at Mt. Milligan but galena, sphalerite and silver-bearing tetrahedrite occur in the polymetallic veins (Mcintosh, 1990). Small veinlets similar to the larger polymetallic veins, although not common, are documented in drill logs in the periphery of the Mt. Milligan deposits.  The outer extremities of both the pyrite halo (Figures 3.2F and 3.7) and the lead-zinc-silver (Figures 3.4D to 3.4F) enrichment zone (4 and 5, above) are poorly defined because of a lack of definition drilling and sampling beyond the extent of the high grade limits of the deposits.  Alteration assemblage distribution (Figure 2.3, cf. DeLong et al., 1991 and DeLong, 1992) has a similar geometry to the pattern of metal distribution observed on the 1,000 metre level. Gold and copper enrichment generally coincide with the potassic alteration assemblages, particularly where biotite is important.  72  Mt. Milligan Deposits Gold vs Copper  o  0.2  0.4  0.6  0.8  Copper %  Figure 3.6. Gold plotted versus copper for the Mt. Milligan deposits. The 66 zone is represented by the squares and all other zones and peripheral samples are shown as X's. Copper is more closely correlated with gold in the other zones than it is in the 66 Zone. A lead-zinc-silver subzone is sporadically developed at the periphery of potassic alteration and in the surrounding propylitic alteration.  The patterns of alteration zonation and metal distribution at Mt. Milligan are similar to other porphyry systems. Here, at the Mt. Milligan deposits, the following subzones are identified:  73  (i) "barren core", (iii) bornite-gold, (iv) chalcopyrite-gold, (v) pyrite halo and (vi) lead-zincsilver.  This zonation is similar for the case of an idealized gold-enriched porphyry system as described by Jones (1992) and the zoning pattern of metals around a felsic intrusive body as described in 1927 by Emmons. From the centre outward the composite zoning common to gold-enriched porphyry deposits is: (i) barren (or subeconomic) core; (ii) molybdenum; (iii) bornite-gold; (iv) chalcopyrite; (v) pyrite halo (with or without gold in shear zones, quartzsericite-pyrite stockworks, and distal skarns); (vi) lead-zinc-silver; (commonly as veins;) (vii) distal epithermal gold, (generally in veins). This pattern is usually modified by the influence of host rocks, structure and other factors. It is most commonly recognized in those deposits related to felsic and intermediate intrusive rocks.  The gold enriched 66 zone does not fit easily with this idealized model of metal distribution. Gold in the 66 zone is associated with both potassic and propylitic alteration, the pyrite halo and the lead-zinc-silver subzone zone. Furthermore, the gold is not as strongly correlated with copper in the 66 zone as it is in the MBX and WBX zones and the Southern Star deposit (Figure 3.5). 3.5.  Discussion  Gold occurs in two different alteration assemblages and in different subzone environments at the Mt. Milligan deposits. Thus two different modes of gold transportation and precipitation are implied. Possibly comparable dual populations of gold have been described  74  10500  9500 x hcc  o  8500  7500 11500  13500  12500  14500  EAST 3.7. Pyrite halo around the Mt. Milligan and Southern Star deposits. The hachured line approximately encloses areas of over 3% pyrite. The dashed line indicates limit of data. The pyrite halo is discontinuous, perhaps truncated in the north by the Oliver fault (Figure 2.3). Figure  75  from volcanogenic massive sulphide deposits (Huston and Large, 1989). Gold is associated with copper in the base of the massive sulphide body and with pyrite, sphalerite and galena at the top and fringes of the massive sulphide body.  A mechanism for gold being observed in two different types of assemblages may be derived from studying solubility data of gold (Figure 3.8) as chlorocomplexes and thio-complexes (Jones, 1992; Huston and Large, 1989). Summaries of the evolution of fluid chemistry (Meinert, 1982; Large et al., 1988; Huston and Large, 1989) in porphyry systems suggest that gold is transported as a chlorocomplex in early higher temperature hydrothermal fluids and is precipitated by the following reaction: 4AuCl "+2H 0 <^> 4Au°+4H + 8Cl~+0 +  2  2  2  Equation 3.1  Copper and iron are also transported in similar chlorocomplexes. Gold and copper traveling together is suggested by the correlation between copper and gold in the inner bomite and chalcopyrite-gold subzones (MBX and WBX zones and the Southern Star Deposit, Figure 3.17). High gold grades associated with copper are promoted by higher temperatures (275°C to 350°C) and relatively oxidized fluids as indicated by the presence of bornite (Huston and Large, 1989) as in the inner bornite subzone (Figure 3.2D).  Gold may continue to deposit beyond the chalcopyrite-gold zone into the pyrite halo from a thio-sulphide complex if the gold budget of the system allows, as it appears to do in the 66 zone at Mt. Milligan. This is comparable to the second occurrence in massive sulphides  76  aiS - 1 0 "  2 5  .  pH - 4.5 Figure 3.8. Oxygen fugacity plotted against temperature to illustrate gold solubility ([ones, 1992). (a) Demonstrates gold solubility as Au(HS)2" and AuClf ; it also shows the stable iron-bearing phases: hm = hematite, py = pyrite, mt = magnetite, po = pyrrhotite. (b) Is similar but introduces the stability of galena (gn), sphalerite (si) and chalcopyrite (cp). This diagram illustrates: (i) the similar stability of lead (galena) and zinc (sphalerite) against higher gold solubility as Au(HS)2', and (ii) copper (chalcopyrite) concentrations with higher gold solubility as AuCb'.  77  ZOO  3C0  -*00  500  500  Figure 3.9. Generalized oxygen fugacity-temperature diagram Qones, 1992) illustrating the evolution of a fluid from one that could cause high temperature skarn-like alteration (1), to a fluid that could cause potaassic alteration in a porphyry environment (2), to a cooler fluid where Au(HS)2' becomes the more important transportation species of gold (3). GA = garnet, PX = pyroxene, ACT = actinolite, QZ = quartz, CA = calcite.  78  (Huston and Large, 1989). The relevant reaction for gold precipitation from the thiocomplex [Au(HS)2~) is :  4Au(HS) "+2H 0+4H o4Au°+8H S+0 +  2  2  2  2  Equation 3.2  Gold is precipitated by this reaction from cooler (120-250°), near neutral pH, relatively oxidized fluids. At constant pH the solubility gold as Au(HS) " actually increases with 2  decreasing temperature (Jones, 1992). The reduction of the activity of sulphur by the precipitation of sulphide, in this case pyrite, is an effective method of depositing gold from this type of solution. Iron precipitation evolving from the magnetite stability field to the pyrite stability field in the 66 zone is indicated by unique textures in the 66 zone where magnetite grains are surrounded concentrically by pyrite and epidote.  3.6 Conclusions  Gold, therefore, can be transported and deposited at both higher (>300° C) and lower (150° to 275°C) temperatures if both gold-chloride complex and later gold thio-complex bearing fluids are considered. If a structure, such as the Rainbow fault, were to act as a long lived conduit for evolving, cooling fluids then overprinting zonal characteristics, such as observed in the 66 zone, can be explained. Propylitic alteration overprinting potassic alteration over a broad area, an expanded pyrite halo, and a broader occurrence of lead, zinc and silver all suggest that the 66 zone was exposed to a hydrothermal fluids that evolved from saline fluids that caused potassic alteration to cooler, near neutral pH fluids that were responsible for propylitic alteration and minor lead, zinc and silver enrichment. This overprinting is also marked by the breakdown of earlier precipitated biotite, which is described in detail in Chapter 4.  79  3.7 Exploration Parameters  The number of samples taken outside the mineralized area is not large enough to effectively show the use of trace metal distribution, particularly the ratios of certain elements as exploration vectors. The ratio of copper to lead in Figure 3.10A, however, increases toward the centre of the deposit. The ratio of gold to copper in Figure 3.1 OB emphasizes the 66 zone.  80  EAST  Figure 3.10A. Cu-Pb ratios [Cu/(Cu+(Pb x 10))]  at the 1,000 m elevation level.The deposits are outlined. Some sharpness of deposit definition is due to a lack of information around the periphery of mineralization. The 66 Zone is shown as Cu-Pb ratio low because of higher Pb concentrations and lower Cu concentraions. Thus Pb and Cu concentraions resemble the characteristics of the inner part of the propylitic alteration that surrounds the deposit.  81  11500  12500  13500  14500  EAST  Figure 3.10B. A u - C u ratios [Au(ppm)/(Au(ppm)+Cu%] at the 1,000 m elevation level showing the gold enhanced 66 Zone. Absolute values o f gold are low on the western and northern periphery o f the deposits but the A u - C u ratios are similar to the 66 Zone. Biotite data (Chapter 4) suggests that the fluids that deposited the gold in the 66 Zone are propylitic fluids similar to the fluids causing alteration and metal deposition on the periphery o f the main deposits.  H2  Chapter 4  Variations in Biotite Composition  4.1 Reasoning and Methods of investigation  Biotite is intimately associated with much of the copper and gold mineralization at M t . Milligan (cf. Chapters 2 and 3, Figures 2.3 and 3.1). Biotite displays a strong spatial association with the inner high grade core of the deposit (Figure 2.3).  Biotite is a trioctahedral mica with an ideal formula K2(Fe,Mg)6Al Si602o(OH,F,Cl)4 In 2  porphyry deposits  most biotite is formed during igneous crystallization or during  hydrothermal alteration related to the metal deposition event. Potassic alteration, which includes biotite, hosts large amounts of economic mineralization in many porphyry copper deposits. The composition of biotites has been studied in the context of mineral deposits (Kesler et al., 1975; Jacobs and Parry, 1979; Gunow et al., 1980; Chivas, 1981; and Taylor, 1983) and more widely in mineralogical, metamorphic and tectonic studies (Ague and Brimhall, 1988; Conrad et al., 1988; Parsons et al., 1993). These studies have proved useful in defining regional gradients of parameters such as the oxidation state of the batholith and local discontinuities in batholith parameters using biotite composition.  Concentrations of iron, magnesium, aluminum, titanium, silicon, fluorine and chlorine vary in biotite because of substitution or coupled substitution. The other important minerals associated with biotite in the high grade zone at M t . Milligan (bornite, chalcopyrite, pyrite, magnetite and potassium feldspar) have a relatively fixed stoichiometry. Variation of thermodynamic parameters related to the hydrothermal fluids, such as temperature and composition, are therefore, more strongly reflected in the composition of biotite than in  83  most other minerals. Consequently, biotite compositions at Mt. Milligan were examined to provide more information on the spatial distribution of these controls.  4.2 Methods  Polished thin sections were prepared from 26 locations on the 1,000 metre elevation plan of the Mt. Milligan deposit (Figures 2.3 and 4.1). These sections were carbon coated. A Cameca SX-50 Electron Microprobe in the Department of Geological Sciences was used to analyze grains optically classified as biotite. A total of 270 analyses on 225 grains were obtained. The data were recalculated to atoms per formula unit based on a total anion charge of 44 (20 oxygens and 4 OH).  4.3 Igneous versus Hydrothermal Biotite  Biotite is present in the Mt. Milligan deposit both as small phenocrysts (cf. Chapter 2) in the intrusive monzonites and as a fine-grained hydrothermal alteration product in both the monzonites and the surrounding volcanic rocks illustrated in figures 2.9D and 2.13 in Chapter 2. The igneous biotites are not always preserved because of later superimposed alteration; they never exceed 5% of the rock volume.  Hydrothermal biotite is an important part of the potassic alteration assemblage in the surrounding volcanic rocks where near the monzonite intrusions it can reach 60% of the rock volume. Fine-grained biotite, thought to be hydrothermal because of its shreddy texture and because it locally can be found related to hairline fractures, is also present in the intrusive rocks themselves. This type of biotite is usually less than 5% of the rock by volume.  84  10500  10000  • • cr O  9500  .. \. •- 168  • • V  2  308  9000  8500 12000  12500  13000  J  13500  14000  E A S T  Figure 4.1. Location of drill core samples prepared and analyzed for biotite using the electron microprobe. The MBX zone and 66 zone of the Mt. Milligan Main deposit are outlined in dashed and solid lines respectively.  85  4.4 Results  Classification of the biotite at Mt. Milligan is shown Figure 4.5. Twenty-two of the 26 sample locations have (Figure 4.1) compositions characteristic of true biotite. 4.4.1 Igneous Biotites  Primary igneous biotite occurs as a subhedral to rarely euhedral phenocryst phase up to 3 mm long illustrated in Figures 2.9D and C in Chapter 2. It is characterized by deep brown to orange-brown pleochroism. Table 4.1. Average biotite compositions from the igneous biotites from the two locations. Std = Standard Deviation Anion /Cation  DDH 168 (n=31)  Std  DDH 123 (n=29)  Std  2.17  0.10  2.30  0.10  0.01  0.01  0.02  0.01  5.54  0.04  5.55  0.05  2.44  0.08  2.51  0.11  2.91  0.14  2.82  0.15  1.91  0.09  1.97  0.04  0.53  0.04  0.48  0.06  0.01  0.01  0.00  0.00  0.03  0.01  0.03  0.01  F"  0.42  0.09  0.18  0.03  c r  0.04  0.01  0.04  0.01  Fe  2+  Mn  2+  Si  4+  Al  3+  Mg K Ti  2+  +  4+  Ca  2+  Na  +  Biotite phenocrysts and microphenocrysts having subhedral primary igneous textures, from core in two drillholes, were analyzed with the SEM. A total of 53 analyses on 9 grains were recorded and studied. Locations of the drillholes are shown in Figure 4.1. Complete 86  analytical data are listed in Appendix JJ. Table 4.1 presents the average composition of the biotite grains from the two locations.  Mt. Milligan Biotite Composition #Ti cations vs. #F anions 0.7 .. L S I . .  0.6  •  HCII-I  •  0.5  I  CO  •  0.4  : DDH 123  • .q q  DDH 168  - • E3  •  O p  n  0.3  -H-  +  *v:&----v-:-  0.2  1*  0.1 0  -t-  0  + 0.2  • Igneous  0.4 #F A n i o n s  0.6  0.8  + Hydrothermal  Figure 4.2. Titanium plotted against fluorine, both expressed as atoms per unit formula. The higher titanium concentrations in igneous biotites and the two sets of fluorine concentrations associated with the two locations are exhibited clearly. Data are from Appendix 2.  The standard deviation for each element is very close to that expected from analytical error, thus demonstrating little internal variation. The halogen fluorine content of biotite is an exception (Table 4.1). The higher fluorine concentrations come from grains in fresher looking monzonite (DDH 168) of the MBX stock, whereas the lower fluorine  87  concentrations are from a more altered monzonite (DDH 123) in the outer part of the MBX stock (Figure 2.3).  Table 4.2. Average igneous and hydrothermal biotite cation compositions from all locations  Anion/Cation  Hydrothermal (n = 208)  Igneous (n = 53)  Fe  2+  2.07  2.23  Mn  2+  0.01  0.01  Si  4+  5.75  5.54  Al  3+  2.61  2.47  3.08  2.87  +  1.94  1.93  4+  0.18  0.51  0.02  0.01  0.03  0.03  F  0.22  0.32  Cl"  0.03  0.04  Mg K Ti  Ca  2+  2+  Na  +  Analytical traverses across individual microphenocrysts of igneous biotite display little variation or zoning except for minor magnesium enrichment towards the core of most grains (Figure 4.3a). Traverses across grains from the more altered monzonite display similar patterns but zoning may have been obscured by alteration near the rims of the grains.  88  Biotite C o m p o s i t i o n s - D D H 168 Fe - Mg Variations across grains 0.48  0.38 Rim to Rim Traverse Across Grain  Figure 4.3a. SEM analytical traverses across 7 igneous biotite grains from DDH 168. Data are from Appendix U. The iron/magnesium variation is illustrated. Biotite Compositions - D D H 123 Fe - Mg Variation across grains 0.5  0.48  3  0.46  + CD LU  £  0.44  0.42  0.4 Rim to Rim Traverse Across Grain  Figure 4.3b. SEM analytical traverses across 3 igneous grains from DDH 123. Data are from Appendix U. The iron/magnesium variation is illustrated.  89  4.4.2 Hydrothermal Biotites  Secondary, hydrothermal biotites are recognized in thin section by their shreddy, felted microcrystalline texture (Figures 2D and E). They replace original groundmass. This biotite is commonly the major groundmass component in potassically altered volcanic rocks. Biotite also replaces primary mafic minerals. Occasionally fine-grained felted aggregates of biotite will pseudomorph igneous hornblende (Figure 2.F). Secondary biotite, often associated with very fine-grained Ti-oxides and pyrite, is locally mixed with chlorite. Secondary biotite also can be distinguished from igneous biotite by lower relative titanium abundance (Table 4.2, Figure 4.4). Table 4.2 is the average composition of biotites from the 22 locations.  Hydrothermal biotite exhibits pale brown to, more commonly, an olive to pale green pleochroism. Figure 4.5 displays a relatively small variation in cation composition. A subtle difference in the Fe# (Fe/(Fe+Mg)) exists spatially. Biotites from the 66 zone and those peripheral to the MBX zone exhibit slightly higher values (Figure 4.6). 4.5 Discussion  Igneous biotites are texturally distinct from the hydrothermal biotite (Cf. Chapter 2, figures 29D and 2.13). Biotite grains thought to be igneous phenocrysts are coarser grained (up to 3 mm long) than hydrothermal biotite and are generally subhedral. In contrast, hydrothermal biotites are characteristically shreddy, felted and microcrystalline in texture. Colour is also different. Igneous biotite exhibits deeper brown to orange-brown pleochroism, in contrast with hydrothermal biotite that has pale brown or, more commonly, olive to pale green pleochroism.  90  The igneous biotites are distinct compositionally from the secondary biotites. Figure 4.4 and Table 4.2 indicates that in almost all cases the igneous biotite contains more titanium than hydrothermal biotite. The exceptions are analyses obtained from the rims of biotite phenocrysts. It is likely that these rims have incipient alteration.  =Colour variation in biotites at Mt. Milligan might be related to composition. Hayama (1959) concluded that the colour in biotites was controlled by its Ti0 content and 2  Fe203/FeO ratio. A variation in biotite colour similar to the Mt. Milligan biotites, was noted in biotites of differing histories in the Wopmay orogen in the Northwest Territories (Lalonde and Bernard, 1993). The green pleochroism observed in that study occurred in biotites having higher Fe as determined by Mossbauer investigations (Rancourt et al., 3+  1992) and lesser titanium. Lalonde and Bernard (1993) also observed more titanium with red-brown pleochroism. Similarly studies of biotite by Guidotti (1985) in from metamorphic rocks demonstrated that biotite becomes progressively darker in shades of orange to reddish-brown with an increase in titanium content. 4.5.1 Halogens in igneous biotites  The behaviour of halogens is important in ore-forming processes (Munoz, 1984). Thus, because of the significance of fluorine and chlorine in the metal transporting process the halogens are examined below. The concentration of fluorine varies little among grains at each of the two sites (note the small standard deviation of fluorine in Table 4.1 for DDH 168 and DDH 123) but greater differences are exhibited between the two sites (Table 4.1 and Figure 4.2).  Halogens occur in biotite by substituting for hydroxyl (OH") anions. In natural biotites the molal F/(OH+F+Cl) can be high, and in some cases, can approach unity. The amount of  91  Mt. Milligan Biotite Compositions #Ti Cations vs. Fe/(Fe+Mg) 0.7  •  0.6 in  n  0.5  CO  o  0.4  J  0.3  *  0.2  • DDH  0.1  -++-  0.3  284  -  0.35  0.4  0.5  0.45  0.6  0.55  0.65  Fe/(Fe+Mg)  n  Igneous  +  Hydrothermal  Figure 4.4. Titanium concentration expressed as atoms per unit formula plotted against the Fe# (Fe/Fe+Mg). Data are from Appendix U. Hydrothermal biotite exhibits a much larger range of values for the Fe#. Mt Milligan Hydrothermal Biotites Major Element Composition  Siderophyllite  2.7  2.3 Si/AI 1.9  "Q  D D H 228  1.5 + 0.35 r— annite  0.45  a  "  D D H 116  4,  0.55 X phlogopite  M B X - W B X - Central  0.65  0.75 phlogopite — |  66 Z o n e  Figure 4.5. Plot of major element variation used in classification of biotites. Data are from Appendix JT. With the exception of biotites from DDH 116, the 66 zone biotites are slightly shifted towards the annite endmember.  92  chlorine substitution in the OH site is subordinate to F. Thus, molal Cl/(OH+F+Cl) is commonly less than 0.05. The amount of hydroxyl replacement by halogen in biotite is controlled by a number of independent factors. Munoz (1984) lists the most important as: (i) the activity of halogen ion or halogen acid (HF or HC1) present during crystallization, (ii) the cation population of the octahedral sheet in biotite (particularly iron and magnesium), (iii) the temperature of hydroxyl-halogen exchange, usually the temperature of crystallization from an igneous melt or temperature of precipitation from hydrothermal fluids, and (iv) the effects of postcrystallization leaching or enrichment due to hydrothermal fluids or groundwater. Table 4.3. Equilibrium constants for the exchange reaction OH-mica + HF<=> F-mica + H 0 2  Composition  Log K  Phlogopite  2100/T+1.52  Annite  2100/T + 0.41  Siderophyllite  2100/T + 0.20  Muscovite  2100/T + 0.11  1  1. Munoz and Ludington, 1974, 1977; Ludington and Munoz, 1975. Fluorine concentrations in biotite are affected by the cation population of the octahedral sheet and the temperature of hydroxyl - halogen exchange (Munoz, 1985). Therefore, measured fluorine concentration in the crystallizing magma or the cooling hydrothermal  93  fluids, is only an indication of relative fluorine enrichment in biotites with similar Mg/Fe ratios and similar temperatures of final hydroxyl-halogen equilibrium.  Using the experimentally derived equilibrium constants in Table 4.3 a single numerical value can be defined to express relative fluorine enrichment that considers both Mg/Fe and aluminum. This value, referred to as the fluorine intercept value [IV(F)] by Munoz (1984), is: IV(F)bio = 1  5 2 x  M g + °-  42  X  A n  + 0.20 X  s i d  -log(Xp/X H) 0  Equation 4.1  Combining the equation for the fluorine intercept [IV(F)] with equilibrium constants from Table 4.3 the following linear equation is derived: 7(H 0)/ log 2100/T + IV(F) //(HF) 2  Equation 4.2  where / = fugacity and T = absolute temperature (°K).  The intercept value can be used to calculate temperature if fugacities are known. Conversely, if the temperature of hydroxyl - halogen exchange equilibrium can be measured, the ratio of the fugacities of H 0 to HF can be calculated. The igneous biotites 2  from two locations have similar compositions except for their fluorine concentration. Thus, two IV(F) can be calculated from Equation 4.1 for the average compositions of biotite grains from DDH 123 and DDH 168 (Table 4.1): 1. rV(F)  168 = l-52(.52) + 0.42(.28) + 0.20(.20) -log(0.10/.89)  DDH  = 1.9 2. IV(F)  DDH 1 2  3 = 1.52(.50) + 0.42(.27) + 0.20(.22) - log(0.04/.96)  = 2.25 These two fluorine intercepts are markedly different. It is apparent even without calculating this intercept that the difference in fluorine concentration is not caused by the cationic  94  composition of the octahedral sheet (e.g. Fe-F avoidance or the presence of octahedral aluminum) because of the similarity of octahedral site occupation between the samples. The determination of the intercept values also suggests that the difference in fluorine is probably due to one or more of the following: (i) temperature variations; (ii) changes in the ratio of the fugacities of H2O and HF in the melt from which the respective biotites crystallized, or (iii) the effects of post-crystallization leaching or enrichment due to hydrothermal fluids  Assuming that the ratios of fugacities did not vary significantly throughout the crystallizing MBX stock, the differences in temperature of crystallization (OH-F exchange) that allow for difference in fluorine concentrations at the two locations can be plotted (Figure 4.7) from Equation 4.2: 2100/T 1/T  168  168  +IV(F)  168  = 2100/T  = [2100 / T + IV(F) 123  123  123  +rV(F)  123  and  - IV(F) ] / 2100 168  Figure 4.7 shows the difference in temperature of the OH-F exchange for the two locations. For example, a difference in the temperature of about 100°C is required between the two locations when the temperature for OH-F exchange for DDH 123 is 900°C, if the ratio of fugacities of H2O and F are the same at both locations.  Conversely, if the temperature of crystallization (i.e. the temperature of OH-F exchange) is assumed to be the same for both locations then the difference in the log of the ratio of the fugacity of H2O to the fugacity of HF of the melts from which the two biotites formed can be calculated from evaluating Equations 4.1 and 4.2 as follows:  = 0.35  95  Thus, if the biotites reached equilibrium at the same temperature, the ratio of the fugacity of H 0 to the fugacity of HF was higher in DDH 123 than in DDH 168. 2  In conclusion, either the temperature of fluorine - hydroxyl exchange equilibrium was higher for biotites at DDH 123 or the ratio of HF fugacity to H 0 fugacity was lower. 2  Because there is only one equation with two unknowns it is impossible to determine which parameter is controlling the fluorine concentration at each of the locations. At relatively low temperatures (less than 500°), however, only a small difference (Figure 4.7) in the temperature of OH - F exchange is needed to explain the fluorine concentration differences observed. Lower temperature OH-F exchange caused by hydrothermal alteration seems the most feasible cause of variations in fluorine abundance. 4.5.2 Chlorine  Much less information is available on the thermodynamics of the Cl <=> OH exchange in biotite. The concentrations of Cl in the biotites analyzed in this study were small. No important variations were observed.  4.6 Hydrothermal biotite  The subtle iron enrichment shown in Figure 4.6 was originally reported as iron enrichment in secondary biotites in the core of deposit (DeLong, 1992) and correlated to gold mineralization. On further investigation (Stanley and DeLong, 1993) it appears that the subtle Fe# zonation is more restricted and is related to: (i) the style of gold mineralization in the 66 zone, and (ii) alteration that is found peripheral to the MBX zone and in the 66 zone. The variation in the value of the Fe# is not always strictly due to simple Fe-Mg substitution. The hydrothermal biotites at Mt. Milligan do not fit on the phlogopite-annite join in the  96  Mt. Milligan Biotites Fe# for 66 and MBX zones 0.7  0.62  0.54 CD  0.46  +  0.38  •  0.3  -o -  •  -• — a  a  •  • Sorted Samples  •  MBX Zone  66 Zone(Peripheral)  Figure 4.6. Sorted samples illustrating the subtlely higher Fe# in the 66 zone compared to the MBX zone of the Main Deposit. Data are from Appendix n.  Temperatures of OH-F exchange C o n s t a n t [f(H20)/f(HF)] 1000  860  co  720  Q Q 580  440  300 300  400  500  600 T (DDH  700 123)  800  900  1000  Figure 4.7. Temperatures of OH - F exchange at constant f(H 0)/f(HF) that would account for observed biotite compositions at both DDH 123 and 168. 2  97  biotite compositional plane (Figure 4.5) where Si:Al is 3:1; they are more aluminous and plot in the more central region of the diagram. Biotites may become more aluminous by a coupled substitution known as the tschermak exchange where two A l  3 +  coordinated and one tetrahedrally coordinated) are substituted for a M g  (one octahedrally 2+  and a Si . This 4+  substitution will lower the Fe# by removing Mg, without any Fe-Mg exchange.  Biotites from location 284 (Figure 4.1) and one grain from location 308 display a significant iron enrichment. Both are from felsic bedded tuffs. All the other samples are from monzonite or intermediate to mafic volcanics. The iron enrichment may be enhanced by differences in the physical characteristics of the surrounding rock or primary chemical composition of the protolith. The bedded tuffs probably had a higher primary permeability than other rock types at Mt. Milligan. 4.6.1 Thompson space analysis The substitutions and coupled substitutions that occur in biotite can be modeled (Stanley and DeLong, 1993) by mathematically converting the cation compositional data to Thompson exchange components (Thompson, 1986a, 1986b). Instead of expressing the composition of the biotites as a sum of cation totals, the compositions were recalculated to exchange components that were thought to more closely characterize some of the coupled substitutions that occur in biotite. An arbitrary ideal composition was designated [in this case phlogopite: K Mg6Al2Si602o(OH)4] as the additive component. All biotite was 2  expressed as this additive component plus the calculated amounts of the eight exchange components listed in Table 4.4. The sum of the calculated amounts of each exchange component and the additive component, phlogopite, produces the observed composition. These exchange components do not necessarily reflect the substitutions that actually occurred in the biotite structure, but were chosen because they are significant in a number of common rock-forming minerals (Stanley and DeLong, 1993).  98  FE2  SI4  •  AL3  " I . ••  MG2  K • -V--'.  . • 'i  '  *•  -:-&r  TI4-  Vv  .  1  .*  . "•*£•* •  • 1-  *•  F  :: *  Figure 4.8. Scatterplots showing major element variation among the hydrothermal biotites at the Mt. Milligan Main deposit. Elements expressed as atoms per unit formula. Data are from Appendix U.  99  Re-expression of the analyzed composition of the biotites in this manner allows an understanding of the nature of coupled element transfer because the exchange components are consistent with both charge balance and site distribution constraints. Plotting exchange components of interest identifies coupled substitutions that co-vary. This allows recognition of net exchanges that better describe the compositional variations, because ndependent single element substitutions are not realistic representations of the exchanges that occur in biotite.  The 208 analyses from the 22 locations display a wide variation in K abundance. Sixty+  eight of the analyses had less than 1.5 potassium atoms per unit formula and were not plotted on the biotite compositional plane (Figure 4.5) or on the element variation scatterplots (Figure 4.8), because they do not exhibit compositions characteristic of biotite. However, all data (above and below 1.5 K per formula unit) were analyzed using +  Thompson exchange components.  Relationships among the exchange components are presented in Figures 4.9 to 4.13. Analyses of mineral grains with less than 1.5 K cations per unit formula are plotted as triangles. Analyses that have more than 1.5 K cations are plotted as open squares. Most of the figures indicate that the exchange components behave differently where the K  +  interlayer site is more completely filled (when K approaches 2) and the stoichiometry of the +  analyses more closely approaches true biotite. Because of this biotite stoichiometry, the slope of the linear arrays formed by analyses with K cations > 1.5 reflects exchange processes controlled by biotite crystallographic constraints.  Slopes of these trends on Figures 4.9 to 4.13 empirically define a net exchange component. For the group of analyses that have a K cation value > 1.5 the net exchange is SiAlK.iMg.3. The net exchange component for analyses that have K cation value < 1.5 is KAl3Si.)Mg.3. 100  Some significant differences can be observed in the two net exchange components, particularly in the direction of movement for Al and Si. For nearly stoichiometric biotite analyses (K > 1.5) Si and Al exchanged together (sympathetically) exhibiting the same +  subscript sign. The other analytical trend (where K < 1.5) demonstrates antithetical +  exchange where Si and Al are exchanged for each other.  Table 4.4. Thompson exchange components used to derive Figures 4.9 to 4.13.  Component Type  Component  Abbreviation Formula  Additive  Phlogopite  PH  K Mg Al Si 0 o(OH)4  Exchange  ferro-magnesian  fm  FeMg.i  Exchange  tschermak  tk  AlaMg-iS'i.,  Exchange  plagioclase  Pl  NaSiCa.iALi  Exchange  edenite  ed  NaAlSLi  Exchange  soda-potassic  kn  Kna_i  Exchange Exchange  ti-tschermak mangano-magnesian  ti mm  AlzMg-iTLi MnMg.i  Exchange  magnesio-calcic  mc  CaMg.!  2  6  2  6  2  The more stoichiometric biotites behave as expected, becoming less aluminous with net exchange. The other analyses, however, exhibit compositional variation inconsistent with biotite structure. Several studies (Ferry, 1979; Veblen and Ferry, 1983; Eggleton and Banfield, 1985) found that tetrahedral sheets being replaced by "brucite" sheets was incipient to the breakdown of biotite and the formation of chlorite, a process more consistent with the net exchange observed. Thus, mineral grain analyses of K < 1.5 may be +  101  MT MILLIGAN BIOTITES Exchange Component Analysis  •  Biotites  A  Chlorites  Figure 4.9. Plot of the plagioclase component (Table 4.4) versus the tschermak component for Mt. Milligan Main deposit biotites. MT MILLIGAN BIOTITES Exchange Component Analysis  -0.2  0.2  0.4 0.6 0.8 Tschermak Component  •  Biotites  A  1  1.2  1.4  1.6  Chlorites  Figure 4.10. Plot of the ferro-magnesian component (Table 4.4) versus the tschermak component for biotites of the Mt. Milligan Main deposit.  102  MT MILLIGAN BIOTITES Exchange Component Analysis  Tschermak Component  •  Biotites  A  Chlorites  Figure 4.11. Plot of the soda-potassic component (Table 4.4) versus tschermak component for biotites of the Mt. Milligan Main deposits. MT MILLIGAN BIOTITES Exchange Component Analysis 0 -i  -0.2  0  0.2  0.4  0.6  0.8  1  1.2  1.4  1.6  Tschermak Component  •  Biotites  A  Chlorites-mixes  Figure 4.12. Plot of the edenite component (Table 4.4) versus the tschermak component for biotites of the Mt. Milligan Main deposit.  103  MT MILLIGAN BIOTITES Exchange Component Analysis 1 •  c  0.5  •  n  •  •  CP  •  C  o Q. E o O o ca o •  g  05  u c O) CO  ^ ^ ^ ^ T - - D- -  0  AiS^  -0.5 ^  -1  ^ ^ 4*^A .  A  AA  A  •  -1.5  A  A  •  -2 -0.2  •  A  0.2  0.4 0.6 0.8 Tschermak Component  •  Biotites  1  1.2  1.4  1.6  Chlorites  Figure 4.13. Plot of the magnesio-calcic component (Table 4.4) versus the tschermak component for biotites of the Mt. Milligan Main deposit. described as chloro-biotites (Stanley and DeLong, 1993). The degree of biotite breakdown can be characterized by the magnitude of the tschermak component in the biotite. Lower tschermak values reflect the breakdown of biotite to chloro-biotite. Figure 4.10 is a scatterplot comparing the tschermak component with the ferro-magnesia exchange component. Ignoring the upper trend that comes from location 284 (Figure 4.1), a single linear trend with a negative slope is observed. This indicates that the more Fe-rich compositions are associated with the chloro-biotites, and therefore, marginally higher Fe# in biotite associated with the Au-rich 66 zone may be the result of the partial breakdown of biotite to chlorite rather than the formation of more Fe-rich biotite by Fe-Mg exchange. Chlorite is indicative of lower temperature alteration. Thus, partial retrograde breakdown of secondary biotite to chlorite could produce the compositional variation observed. 104  The relative magnitudes of the tschermak component on the 1,000 metre level plan are shown in the bubble plot of Figure 4.14. Where two bubbles are present the smaller filled bubble represents chloro-biotite from the same location. More abundant chloro-biotite is observed in the 66 zone (approximately 13,000 metre east and 9200 metre north on the grid). Also present in this region are higher gold grades with less elevated copper grades (cf. Chapter 3, Figures 3.4B and 3.4C). Macroscopic evidence of retrograde alteration includes common cross-cutting or overprinting of earlier potassic alteration by epidote, pyrite and chlorite. 4.7 Conclusions  Subtly higher Fe# values of hydrothermal biotite are not the result of a spatial compositional zonation during the biotite alteration event. Rather, these variations result from the incipient breakdown of biotite to chlorite during retrograde propylitic alteration as the Mt. Milligan hydrothermal system cooled and collapsed inward over the causative stocks.  Retrograde propylitic alteration overprinting earlier potassic alteration occurs in a narrow zone at the fringe of potassic alteration zone around the entire deposit. The 66 zone exhibits more pronounced and widespread propylitic overprinting of potassic alteration. Compared to the normally narrow fringe of retrograde propylitic alteration the 66 zone is wider and contains significant pyrite. It also has higher gold grades with lower copper grades compared to the rest of the deposit. The 66 zone also exhibits spotty but elevated concentrations of lead, zinc and silver (Chapter 3; DeLong 1993), which is characteristic of the periphery of many porphyry deposits.  The higher Fe/Mg ratios in the chloro-biotites are consistent with the retrograde breakdown of biotite by the destruction or replacement of tetrahedral sheets by tschermak exchange,  105  10500  10000 ® ®  x  oE O  9500  ®  j ® ccc«  o  : ® O ®  9000  3500 12000  12500  13000  13500  14000  EAST  Figure 4.14. Plot demonstrating the average relative size of the tschermak component at each of the sample sites for hydrothermal biotites at the Mt. Milligan Main deposit. Open circles represent tschermak component for biotites and filled bubbles for chloro-biotites. Data are from Apendix UJ.  106  indicative of lower temperatures than the earlier potassic (biotite) phase. The abundance of pyrite is also consistent with higher iron magnesium ratios. The presence of lead, zinc and silver may also indicate cooler (200°C to 250°C) hydrothermal fluids (Huston and Large, 1989).  Gold observed in these retrograde areas may be a result of Au being transported as a bisulphide complex [Au(HS)2 ]. This occurred at significantly lower temperatures (less -1  than 250°C) than those that prevailed during earlier Cu and Au mineralization, where both gold and copper were likely transported as chloride complexes (Henley, 1973).  The overprinting of two periods of gold (as AuCl" and Au(HS)2 ) deposition at Mt. 1  _  Milligan, would explain the high gold concentrations (= lg/t) in the 66 zone. Other characteristics consistent with a two phase model of gold deposition are: (1) two populations of gold (Stanley, 1993; Sketchley et al., 1995) one correlating with copper (gold transport as AuCl" similar to the higher 1  temperature copper chloride complexes), and one independent of copper (transport as Au(HS)2*);  (2) two periods of alteration, potassic (consistent with gold being transported as AuCl" ), and overprinting retrograde epidote-pyrite assemblages, 1  associated with evidence of biotite to chlorite breakdown; and (3) elevated zinc, lead and silver concentrations, indicative lower temperature solutions that would favour gold transport as Au(HS)2".  The above observations explain the deposition of gold without appreciable copper in the 66 zone. The relatively high gold grades are postulated to reflect superposition of deposition from an Au(HS) " complex on earlier precipitation from an AuCl" complex. Concentrations 2  of gold > 500 ppb over large volumes of rock were achieved by this process in the 66 zone. 107  The lower temperature Au(HS)2~ event may have extended over a large volume because of special physical hydrodynamic flow conditions. The 66 zone is proximal to the Rainbow fault which is a major syn-mineral structure, and some of the units intersected by the Rainbow fault may have had high primary permeability, e.g. the strongly altered trachyte units (Figures 2.3 and 2.4). A deep seated intrusion may have provided a heat source that prolonged lower temperature fluid flow through the fault conduit and the permeable units.  108  Chapter 5  Summary of Conclusions  The Mt. Milligan gold-copper alkaline porphyry deposits were formed by hydrothermal activity related to emplacement of the MBX and Southern Star stocks into the Takla Group. They have, in a general sense, alteration and mineral zoning patterns consistent with previously described models for alkaline porphyry deposits. They are, however, large compared to most other deposits in this class. They are also younger, Middle Jurassic (about 183 Ma) than most of the alkaline deposits of the Quesnel Terrane, which are Early Jurassic (about 195 to 210 Ma).  The Mt. Milligan Main deposit attained its larger size (close to 300 million tonnes compared to deposits in other camps that although cumulative totals may be large individual deposits are in the 25 to 50 million tonne range) because of the intersection of the MBX stock and the protruding Rainbow dike with a permeable stratigraphic interval, which permitted hydrothermal fluids to move laterally. This resulted in the development of the large proximal copper-gold-rich MBX zone and the more distal gold rich 66 zone.  The Mt. Milligan intrusions are aligned along a north-northwesterly trending belt indicating that their emplacement was controlled by an undefined regional structure.- Assuming that the stocks are essentially coeval with their host rocks, a rotation of the east-dipping trachytic rocks in the MBX and 66 zones to a sub-horizontal position indicates that the stocks were probably nearly vertical when they were emplaced. The hydrothermal system that formed the deposits probably developed contemporaneously with or soon after the emplacement of the stocks and the Rainbow dike.  109  Within and around the MBX stock, the intrusions and hydrothermal system spread laterally indicating that stratigraphy was a dominant control on intrusion and mineralization. Major features are: 1. The alignment of the Rainbow dike subparallel to stratigraphy; 2. Intense potassic alteration along volcanic stratigraphy, particularly the trachytic units; 3. Large lateral extent of potassic alteration; and 4. Metal zoning patterns where copper and gold within and adjacent to the MBX stock grade laterally to gold in trachytic and latitic rocks away from the stock.  Within and around the Southern Star stock, the intrusions and hydrothermal system were laterally constrained indicating that structure was a dominant control on intrusion and mineralization. Significant features include: 1. The elongate shape of the Southern Star stock; 2. The restricted occurrence of potassic alteration in and immediately adjacent to the stock; 3. The dominance of vein and fracture filling over disseminated sulphides.  Mineralogical zoning is well developed within the deposits. A biotite rich subzone of the potassic alteration zone forms the core of the deposit. Potassic alteration is surrounded by a propylitic alteration zone. Most of the copper and gold occurs in the biotite-rich subzone.  110  The distribution of copper, gold, zinc, lead and silver at the Mt. Milligan and Southern star deposits reflect, in a general sense, patterns (Figure 3.5) that have been recognized in the porphyry environment (Emmons, 1927; Jones, 1992). More specifically gold and copper are located in the central parts of deposits whereas lead, zinc and silver are peripheral.  Retrograde propylitic alteration overprinting earlier potassic alteration occurs in a narrow zone at the fringe of potassic alteration zone around the entire deposit. The 66 zone exhibits more pronounced and widespread propylitic overprinting of potassic alteration. Compared to the normally narrow fringe of retrograde propylitic alteration the 66 zone is wider and contains significant pyrite. It also has higher gold grades with lower copper grades compared to the rest of the deposit. The 66 zone also exhibits spotty but elevated concentrations of lead, zinc and silver, which is characteristic of the periphery of many porphyry deposits.  The compositional characteristics of biotite also provide evidence of retrograde hydrothermal fluids affecting the 66 zone. Subtly higher Fe# values of hydrothermal biotite found in the fringes of the Mt. Milligan deposit and more specifically in the 66 zone are not the result of a spatial compositional zonation of iron in the biotite during the potassic biotite alteration event. Rather, these variations result from the incipient breakdown of biotite to chlorite during retrograde propylitic alteration. This occurred as the Mt. Milligan hydrothermal system cooled and collapsed inward over the causative stocks.  The higher Fe/Mg ratios in the chloro-biotites are consistent with the retrograde breakdown of biotite by the destruction or replacement of tetrahedral sheets by tschermak exchange, indicative of lower temperatures than the earlier potassic (biotite) phase. The presence of lead, zinc and silver may also indicate cooler (200°C to 250°C) hydrothermal fluids (Huston and Large, 1989). Ill  Gold occurs in two different alteration assemblages and in different subzone environments at the Mt. Milligan deposits. Thus two different modes of gold transportation and precipitation are implied. Possibly comparable dual populations of gold have been described from volcanogenic massive sulphide deposits (Huston and Large, 1989). Gold is associated with copper in the base of the massive sulphide and with pyrite, sphalerite and galena at the top and fringes of the massive sulphide.  A mechanism for gold being observed in two different types of assemblages may be derived from studying solubility data of gold (Figure 3.8) as chlorocomplexes and thio-complexes (Jones, 1992; Huston and Large, 1989). Summaries of the evolution of fluid chemistry (Meinert, 1982; Large et al., 1988; Huston and Large, 1989) in porphyry systems suggest that gold is transported as a chlorocomplex in early higher temperature hydrothermal fluids and is precipitated by the following reaction: 4AuCl "+2H 0 » 4Au°+4H + 8Cl"+0 +  2  2  2  Equation 3.1  Copper and iron are also transported in similar chlorocomplexes. Gold and copper traveling together is suggested by the correlation between copper and gold in the inner bornite and chalcopyrite-gold subzones (MBX and WBX zones and the Southern Star Deposit, Figure 3.17). High gold grades associated with copper are promoted by higher temperatures (275°C to 350°C) and relatively oxidized fluids as indicated by the presence of bomite (Huston and Large, 1989), as in the inner bornite subzone (Figure 3.2D).  If the gold budget of the system allows, as it appears to do in the 66 zone at Mt. Milligan, gold may continue to deposit beyond the chalcopyrite-gold zone into the pyrite halo from a thio-complex. This is similar to the gold associated with lead and zinc in massive sulphides (Huston and Large, 1989). The relevant reaction for gold precipitation from the thiocomplex (Au(HS) ~) is: 2  112  4Au(HS)2"+2H O+4H <^4Au +8H S+O2 +  Equation 3.2  0  2  2  Gold is precipitated by this reaction from cooler (120-250C ), near neutral pH, relatively 0  oxidized fluids. At constant pH the solubility gold as Au(HS)2" actually increases with decreasing temperature (Jones, 1992). The reduction in the activity of sulphur by the precipitation of sulphide, in this case pyrite, is an effective method of depositing gold from this type of solution. Iron precipitation evolving from the magnetite stability field to the pyrite stability field in the 66 zone is indicated by unique textures in the 66 zone where magnetite grains are surrounded concentrically by pyrite and epidote.  Gold observed in these retrograde areas may be a result of gold being transported as a bisulphide complex [Au(HS)2 ] at significantly lower temperatures (less than 250°) than -1  those that prevailed during earlier copper and gold mineralization events, where both gold and copper were likely transported as chloride complexes (Henley, 1973).  The overprinting of two periods of gold (as AuCl" and Au(HS)2~) deposition at Mt. 1  Milligan, would explain the high gold concentrations (= lg/t) in the 66 zone. Other characteristics consistent with a two phase model of gold deposition are: (1) two populations of gold (Stanley, 1993; Sketchley et al., 1995) one correlating with copper (gold transport as AuCl" similar to the higher temperature copper 1  chloride complexes), and one independent of copper (transport as Au(HS)2 ). -  (2) two periods of alteration, potassic (consistent with gold being transported as AuCl" ), and overprinting by retrograde epidote-pyrite assemblages, associated with 1  evidence of the breakdown of biotite to chlorite. (3) elevated zinc, lead and silver concentrations, indicative of lower temperature solutions that would favour gold transport as Au(HS)2 . _  113  The above observations explain the deposition of gold without appreciable copper in the 66 zone. The relatively high gold grades are postulated to reflect superposition of deposition from an Au(HS)2 complex on earlier precipitation from an AuCl" complex. Concentrations _  of gold > 500 ppb over large volumes of rock were achieved by this process in the 66 zone.  The lower temperature Au(HS)2~ event may have extended over a large volume because of special physical hydrodynamic flow conditions. The 66 zone is proximal to the Rainbow fault which is a major syn-mineral structure, and some of the units intersected by the Rainbow fault may have had high primary permeability, (e.g. the strongly altered trachyte units [Figures 2.3 and 2.4]). A deep seated intrusion may have provided a heat source that prolonged lower temperature fluid flow through the fault conduit and the permeable units.  Gold, therefore, can be transported and deposited at both higher (>300° C) and lower (150° to 275°C) temperatures if both gold-chloride complex and later gold' thio-complex bearing fluids are available. If a structure, such as the Rainbow fault, were to act as a long lived conduit for evolving, cooling fluids then overprinting zonal characteristics, such as observed in the 66 zone, can be explained. Propylitic alteration overprinting potassic alteration over a broad area, an expanded pyrite halo, and a broader occurrence of lead, zinc and silver all suggest that the 66 zone was exposed to a hydrothermal fluids that evolved from saline fluids that caused potassic alteration to cooler, near neutral pH fluids that were responsible for propylitic alteration and minor lead, zinc and silver enrichment. This overprinting is also marked by the chloritization of earlier precipitated biotite, which is described in detail in Chapter 4.  114  REFERENCES  Ague, J.J.,and Brimhall, G.H. (1988): Regional variations in bulk chemistry, mineralogy,and the compositions of magmatic and accessory minerals in the batholiths of California; Geol. Soc. Amer. Bull., v. 100, pages 891-911. 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Milligan Sampl  DDH 52 53 54 55 57 70 72 75 81 S3 54' 89 93 99 101 103 104 110 111 113 115 120 :  —  123 120 132 125 125 142 147 149 152 154 155 157 153 160 153 154 133 157 163 175 175 179 205 205 212 214 217 219 221 228 230 223 237 244 252 253 255 253 259 272 278 283  '  NORTH 94C5 91S2 9205 9175 9791 95C0 9783 9500 9175 921C 9195 921C 9372 ' 9294 9012 918 = S005 9500 922C 9017 9795 98C1 9803 9500 8992 9024 9734 9293 9801 9500 9192 9021 9031 9193 8825 9500 3821 8831 9997 9232 9380 9404 9797 9395 9500 9500 9500 9500 8203 8831 8852 8401 8199 9998 3203 9994 9988 8600 8600 8752 8847 8976 8949 8983 9600 9202 8401 8203 '  Mn psrr, Mc% S% Au ssm Pb porn I n pc Ac Dorn Ni szm Fs% Mo ppm V pom ^ A S ; Cu DDm A 4 437 1.51 0.59 100 0.54 22 3.5 25 " 0.4 1753 13212 194 0.05 A 507 3 C.54 19 0.4 S3 5.12 19 271 12900 424 1.52 3.31 0.57 2 244 133 54 0.4 17 7.25 8 13004 1.C5 527 2.77 3.55 3 131 0.492 5.29 1004 58 8 133S3 0.29 44 159 713 2.54 3.29 79 20 7.22 4 0.8 2147 13101 0.91 54 9 572 2.94 1.49 4 1 48 4.73 60 5255 13C59 0.33 4 155 1392 4.03 0.99 0.4 2 4742 95 5.51 27 13202 0.22 A 0.4 2 50 801 1.12 3.77 13 4.27 23 1425 12240 AA C.22 0.4 217 5.94 139 355 3.91 4.31 £2 1071 6 12248 7 237 0.22 0.4 I452 2.75 2 . 2 " 71 6.12 25 SO 0.44 425 2.93 2.22 0.4 231 4 113 9.42 15 45 2003 13430 C.22 7 0.4 5 72 475 1.52 1.99 23 4.19 71 2251 13535 174 307 3.47 C.25 0.5 = 4 0.4 122 4.11 2 27 1073 12901 0.74 A 7 57 425 1.53 2.22 3 2.7 0.4 - 29 13007 5409 17 529 0.75 0.47 0.22 0.4 5 2.29 37 102 32 13400 1.41 9 5.54 50 455 1.25 4.97 0.4 1457 53 12339 A 78 325 1.51 2.5 0.2 0.4 21 4.34 45 4S 125C2 A 0.25 338 2.32 2.94 112 135 27 7.09 27 3903 1.3 12325 0.14 •43 515 1.03 0.22 0.4 12 4 1.85 12334 159 5 59 14= 0.22 4 1091 2.14 2.22 0.4 2 22 5.35 151 25 13309 • 14 G.22 310 2.27 2.21 4 222 21 5.53 1252 2 0.3 13003 A 712 2.99 2.37 0.21 0.4 223 24 149 22 5.42 1073 12SS3 . 0.25 293 2.59 2.22 4 0.4 129 92 5.53 14S7 22 ' 12530 174 5 2.74 0.09 4 203 1.23 :.~3 25 1.3 320 12229 * =' • T : 977 2.21 2.1: 0.09 0.4 35 7.59 1555 25 12504 194 22" 2.29 2.25 0.12 A 0.4 40 9.23 430 13120 274 2.S2 C.43 0.33 207 A 0.4 9 73 4.33 30 4222 12507 4 145 345 1.71 1."2 0.12 9 22 4.59 0.5 23 350 12300 14 A =7 423 2.23 2 . ' 2 0.2 41 0.4 73 5.05 4295 12409 114£. 2.31 4.1" C.' A 41 5 " 7.08 0.5 455 12455 1.79 1.35 0.02 4 122 0.4 15 7 4.c2 19 215 '12201 325 1.71 0.4 95 2.1 0.51 20 4.0S 2 221 12 43 12957 / 1 0.14 3 5.41 - 751 1.09 2.33 2 0.7 4 21 459 12838 290 1.53 1.72 0.05 A 0.4 12 112 29 5 535 12392 ' 245 0.78 2.21 2 142 0.03 3 2.2 201 2.5 127 15 12205 114 294 1.74 2.54 0.04 0.4 24 7.05 72 290 29 12235 r 0.12 0.4 3 200 759 2.54 2.27 32 3.95 24 39 12970 57 ' 1 2 2.05 7.91 0.04 A 3 14 0.4 35 S.79 13074 85 2S7 1.15 1.91 A 0.13 42 3.33 6 90 1.1 20 373 12905 ~~ 0.29 4 552 2.52 4.23 0.4 4S 5.07 5 101 552 13077 0.46 1720 2.91 1.12 0.4 3 122 93 4.5 2175 105 55 12573 0.04 1221 2.71 2.23 155 33 5.93 ICS 353 30 1.8 12293 0.01 551 2.12 4.43 2 73 0.4 105 5.16 55 5 5 13305 0.17 544 2 553 2.55 2.11 42 5.55 173 4 0.5 25 12805 14 0.54 4 491 1.47 2.37 7S 65 2.22 47 0.5 15S0 12925 1.17 4 11 0.4 93 5.97 142 521 2.S9 5.22 55 4381 13125 554 0.99 0.21 C.29 A 97 0.4 2 5 2.2 55 1731 12246 . 0.8 2 53 480 1.04 2.08 4 3.62 4 1 52 4639 13047 0.23 22 2.14 2 100 532 1.47 0.74 3.7 224 124 1995 12290 0.05 2 2.97 2 113 552 0.95 0.25 1 4 13 320 12255 187 0.12 453 2.09 2.99 4 2 0.4 82 7.05 25 888 12292 0.4 229 390 3.31 2.85 5 0.4 85 7.31 27 4 790 12825 *l 34 1.2 1.23 0.43 .1 288 57 4.27 4 0.4 22 1810 12357 0.14 57 725 2.18 2.22 4 0.4 28 5.97 3 314 53 13198 0.09 44 7 449 2.31 4.51 41 0.4 9 169 4 176 13030 4 0.15 82 255 2.37 2.57 95 4.7 4 0.4 297 32 12500 0.49 4 455 1.51 1.77 2.4 3 152 27 6.19 25 3676 12338 0.25 101 488 1.57 3.35 334 30 5.92 4 19 3.1 — 12425 0.05 2 161 1005 3.14 1.59 41 90 4.82 5 0.4 502 12135 104 0.08 88 4.91 16 589 1.62 3.5 0.4 7 544 22 12489 0.34 51 6.69 119 2 513 2 1 6 4.34 0.4 4 21 3981 12521 0.11 4 9 5.12 3 158 •351 1.65 2.53 0.4 23 218 13157 504 1.25 1.02 0.78 5 3.8 2 119 0.7 4 29 8712 12552 484 0.05 0.4 67 5.29 3 135 2.9 2 2 5 4 31 325 13248 10 0.14 22 5.97 63 703 2.11 2.88 1.2 4 1687 59 12303 0.4 1.1 2 47 4 0.4 2 1.85 250 0.51 733 52 13184 54 287 0.75 0.49 0.02 3 2.13 3 254 0.6 4 25 12753 71 287 0.87 0.54 0.41 3 2.75 2 0.6 27 1155 4 12748  123  DDH 284  NORTH 9210  296 301 304 307  9020 9020 9020 8200 3975 3975 3402 3202 3983  308 312 318 320 324 329 325 340 342 243 344  349  9046 7995 3601 3750 3600  381 398 529 549 553 593 597 598 600 327 532 550 367 569 573 575 538 589  12399 12965 13318 1349.9  9199 9039 3839 34C0 9200 8825 7997  369 371 374 377  13056 12949  3975 8949  245 346 347 352 355 357 353  P b p p m Z n PP EAST Cu ppm 4 32 2097 13324 38 373 13186 4 4 31 346 13250 9 4 • 31 13042 4 31 797 12645  3600 3401 3743 3752 3843 3C02 3592 3600 9201 9600 S601 9600 3600 9600 9600 9800 9601 3800 9600 9600 9400 9800  .  733 103 996 228 62 9  12460 13511 12555 12257  231 73 560 1745  12920 12611 11941 13017  253 551 202 275  12632 12700 12982 12612 12590 12683  2105 517 233 3106 2359 720 3753 766 318 2935  125CS 12702' 12730 12230 12944 12946 13000 12899 12700 12395 12298 12000 13191 12934 12593 12595 12645 12535 12860 12S45  5 4 4  38 41 23 46 34  4  72 38  4 4  262 554 542 372 348 140 4024 1410 1710 1287 1700 1842 1042 1916 2705  2.15 7.35 4.91 5.57  2 2 3 11  22 243 92 71  533 482 342 374  3.06 2.57 0.4 0.4 7 9 4.57 2 0 6 4.47 1.1 8 4 2.37 0.4 • 40 7.33 1.1 0.4 5 3 5.52 41 5.5 0.4 22 3.93 0.4 0.4 202 3.57 25 4.22 0.4 0.4 101 5.21 0.4 19 5.71  9 4  151  342 721 416  47 25 20  0.4  23  2.3 4.6  31 22 22 25  0.5  0.4  5 '30 70 38 17 6  4 4  35 117 19 22 19 15 17 10  0.4  -  19 35 35  33 29 20 25 53 23 54 25 60 24  0.4 0.4  240  4.53 3.25 4.27  96  5.53  0.5  15 89 33 5 39 12 9 24  5.36 3.04  10 4 4  4  4  4  298  14Q  Mn c c m M g % p p m Ni p p m F e % M o p p m V p p m 1.93 2 504 111 6 7 4.36 0.4 0.94 899 35 2 0.4 2 4.17 1.9 462 157 18 37 5 . 6 9 1.1 1.52 413 144 2 3 0 5.01 0.9 1.5 1052 169 2 12 5 . 5 3 2.2 2.04 541 4 143 0.4 2 2 5.52 1.53 114 279 19 0.4 6 4.15 0.9 71 321 2 0.6 3 3.35 2.13 532 116 2 111 4 . 3 2 0.7 1.77 1473 112 2 0.4 35 5 . 2 2 1.57 499 51 8 0.4 12 5.32 2.74 632 120 5 0.4 2 9 7.96  4 4  5 5 4 12 4 4  4 4 4 4 4 4 4  14  41 16 41 23  0.4 0.4  0.4 0.5  0.4 0.4 0.5 0.4  25 226 49  5.71  3.23 3.1 5.18 2.34  0.4  2.53 3.27 8 4.45 4 2.35 24 3.53  0.4  35  1.1 0.4 0.4  7.08  2 21 n  £-  3 3 10 18  S3 109 193 74 191 216 130 192 124  "I 4.  201 216  2±  212 219 125 64  d 23 10 10 12 -) i.  2 3 2 3  405 374 1117 234 . 432 714 471 363 402 514 285 240 173  1.02 2.57  S% Au ppm 1.23 0.02 6.12 0.11 2.4 0.77 1.06  0.11 0.55 0.25 1.24  4.51 2.78 1.95 1.27 4.84  0.16 0.19 0.05 0.32  2.23 0.02 0.22 0.11 0.14  5.59 0.35  4.4  1.55 2 . 5 1 1.07 5 . 0 9 1 . 4 1.41 1.23 0 . 1 7 2.25 4.49  2.19 0.45  1.32 3.07  1.5 5.33 2.53 3.39  0.03 0.19 0.05 0.14 0.47 0.22 0.22 0.24  2.53 2.51 2 . 9 2 4.1 1 0.9 2.73 2.55 0.13 2.42 5.64 2.65 2.47 3.G3 0 . 4 8 2 , 3 3 1.29  0.17 0.05 0.22 0.03 0.22 0.23 0.04 0.07  2.23  1.23 1.66 4.47  1.5 3.34  0.44 ' 3.4  2.23 0.73 2.51 0.33 0.32  4.05 0.9  35 23  221 542 202 373  152  120 125  338 .124  2C0 240 157  953 529 512  43 1 14  1.G5 3.47  2.59 0.72 0.13  0.13 0.11 0.07 0.09 0.17 1.71 0.24  252  2.05  2.75  0.23 0.11 0.12  2 2 5  44  509  102  3.23 0.25  0.12 0.1S  108  400 241  1.55 1.01 1.16  139  511  2.3  1.39 2.68  0.32  C  3 2 4  0.3  124  Appendix II Biotite Analytical Data for the Mt. Milligan Main deposit  125  DOH 168 168 168 168 168 168 163  Grains? North 93S5 131 r 13-1 Rl 131-1 131-2 131-3 131-4 131-5  East 12SC5  Type igneous igneous igneous igneous igneous igneous igneous  Fe2+ Mn2+ Si4+ 2.39 0.018 5.55 2.27 0.028 5.57 2.15 0.009 5.51 2.05 . 0.009 5.55 2.08 0.009 5.46 2.07 0.009 5.54 2.10 0.C09 5.59  AI3+ • Mg2+ 2.49 2.70 2.46 2.31 3.03 2.38 2.47 2.96 3.18 2.50 2.37 3.08 2.70 2.53  K+ 1.82 1.84 1.77 1.78 1.65 1.77 1.55  TI4+ Cl0.037 0.52 0.046 0.50 0.037 0.55 0.037 0.48 0.046 0.43 0.046 0.52 0.046 0.50  CA2+ 0.01 • 0 0.01 0.01 0 0 0.01  NA+ 0.04 0.02 0.05 0.03 0.02 0.02 0.02  F0.14 0.22 0.43 0.58 0.55 0.52 0.43  168 168 163 168 168  132-1 132-2 1323 132-4 132-5  igneous igneous igneous igneous igneous  2.04 2.35 2.21 2.28 2.03  0.009 0.009 0.009 0.009 0.000  5.82 5.46 5.52 5.49 5.45  2.73 2.56 2.46 2.42 2.55  2.71 2.58 2.75 2.31 2.97  1.64 1.71 1.77 1.74 1.49  0.018 0.046 0.037 0.037 0.037  0.22 0.47 0.32 0.57 0.63  0.01 0.01 0.01 0 0.02  0.02 0.03 0.03 0.03 0.02  0.49 0.45 0.51 0.3 0.5  163 168 163  133-1 133-2 133-3  igneous igneous igneous  2.21 2.09 2.17  0.009 0.009 0.009  5.57 5.50 5.55  2.37 2.28 2.23  2.93 2.97 2.98  1.75 1.77 1.73  0.055 0.037 0.055  0.51 0.59 0.53  0 0.01 0.01  0.01 0.02 0.02  0.5 0.55 0.53  153 168 163 163 163 158  134-1 134-2 134-3 134-4 134-5 134-5  igneous igneous igneous igneous igneous igneous  2.27 2.10 2.15 2.14 2.24 2.29  0.009 0.018 0.018 0.009 0.018 0.018  5.53 5.51 5.47 5.48 5.65 5.59  2.45 2.36 2.41 2.43 2.48 2.51  2.73 3.04 2.97 3.02 2.70 2.52  1.77 • 0.028 0.046 1.73 0.037 1.31 1.74 0.037 0.037 1.63 1.57 0.046  0.53 0.53 0.55 0.55 0.50 0.52  0.01 0 0 0.01 0.02 0.02  0.02 0.02 0.03 0.04 0.03 0.03  0.48 c:=5 0.49 0.44 0.43 0.29  168 163 163 153 163  135-2 125-3 135-4 135-5 135-5  igneous igneous igneous igneous igneous  2.11 2.07 2.09 2.05 2.14  0.009 0.018 0.009 O.COO 0.009  5.33 5.56 5.54 5.55 5.59  2.29 2.37 2.28 2.31 2.48  2.00 2.05 3.08 3.01 2.93  1.34 1.31 1 33 1.65 1.35  0.046 0.037 0.C37 0.046 0.037  0.52 0.54 0.51 0.51 0.42  0 0 0 0.01 0  0.03 0 05  0.02  0.42 0.24 0.49 0.51 0.44  163 163 163 168  135-1 135-2 136-3 136-4  igneous igneous igneous igneous  2.23 2.17 2.18 2.19  0.009 O.COO 0.009 0.C09  5.54 5.54 5.56 5.55  2.52 2.41 2.39 2.32  2.75 2.91 2.93 2.93  1.32 1.32 1.73 1.36  0.046 0.027 0.037 0.037  0.50 0.54 0.52 0.54  0 0 0.01 0  0.04 0.05 0.03 0.03  0.42 0.4 0.44 0.43  123 123 123  71cor 71-4 71-6  igneous igneous igneous  2.15 1.92 1.74  0.018 0.018 0.018  5.55 5.57 5.01  2.42 2.73 2.73  3.04 2.96 2.57  1.84 1.55 1.93  0.046 0.009 0.009  0.50 0.17 0.17  0 0.4 0  0.04 0.2 0.04  0.13 0.25 0.25  123 123 123 123 123 123 123  72-1 72-2 72-3 72-4 72-5 72-6 72-7r  igneous igneous igneous igneous igneous igneous igneous  2.19 2.18 2.23 2.24 2.25 2.20 2.20  0.023 0.018 0.018 0.018 0.018 0.018 0.018  5.57 5.55 5.54 5.50 5.55 5.52 5.53  2.42 2.37 2.49 2.32 2.41 2.59 2.49  3.04 3.03 2.95 3.02 3.00 2.80 2.81  1.33 1.73 1.32 1.77 1.79 1.77 1.31  0.023 0.046 0.037 0.037 0.037 0.023 0.037  0.46 0.57 0.49 0.53 0.50 0.40 0.42  0 0 0 0 0 0 0.01  0.02 0.04 0.02 0.03 0.05 0.02 0.02  0.21 0.22 0.21 0.15 0.15 0.19 0.16  123 123 123 123 123  74-1 r 74-2 74-3 74-4 74-5  igneous igneous igneous igneous igneous  2.37 2.34 2.34 2.31 2.25  0.C18 0.028 0.028 0.018 0.028  5.56 5.55 5.56 5.57 5.55  2.52 2.64 2.42 2.43 2.73  2.73 2.52 2.86 2.73 2.79  1.35 1.33 1.73 1.83 1.30  0.028 0.037 0.046 0.046 0.037  0.47 0.47 0.51 0.54 0.37  0 0 0 0 0  0.02 0.03 0.03 0.04 0.02  0.21 0.18 0.18 0.14 0.19  123 123 123 123 123 123 40 40 40 40 40 40 40 52  75-1 75-2 75-3 75-5 75-5 75-7  igneous igneous igneous igneous igneous igneous Hydrother Hydrother Hydrother Hydrother Hydrother Hydrother Hydrother Hydrother  2.44 2.45 2.47 2.15 2.43 2.40 2.39 2.31 2.22 2.43 2.42 2.41 2.18 2.14  0.018 0.018 0.028 0.018 0.028 0.018 0.028 0.028 0.028 0.028 0.028 0.028 0.028 0.037  5.51 5.51 5.45 5.59 5.54 5.51 5.70 5.78 5.86 5.68 5.73 5.79 5.95 5.48  2.49 2.59 2.59 2.62 2.46 2.61 2.47 2.53 2.35 2.49 2.52 2.47 2.25 2.77  2.66 2.50 2.84 2.31 2.57 2.53 2.34 3.16 2.99 2.82 2.99 2.96 3.14 3.90  1.31 1.73 1.59 1.83 1.82 1.85 1.907 1.888 1.925 1.925 1.916 1.888 1.898 1.320  0.037 0.037 0.037 0.018 0.046 0.046 0.064 0.018 0.055 0.064 0.046 0.073 0.018 0.009  0.54 0.51 0.46 0.22 0.54 0.49 0.284 0.046 0.220 0.275 0.156 0.147 0.138 0.055  0 0 0 0 0 0 0.009 0.009 0.018 0.018 0.009 0.009 0.018 0.009  0.04 0.04 0.04 0.01 0.04 0.04 0.07 0.06 0.06 0.06 0.01 0.01 0.12 0.01  0.24 0.2 0.18 0.25 0.19 0.22 0.064 0.119 0.138 0.046 0.046 0.046 0.147 0.055  9600  9150 9150 9150 9150 9150 9150 9150 9405  12829  12995 12995 12995 12995 129S5 12995 12995 13212  0.04 0.0i  126  OOH 62 62 52 62 62 52 62 52 66 66 56 65 70 70 70 70 70 70 72 72 72 72 72 i 2 72 72 72 72 72 72 72 77.  72 72 31 31 31 31 31 31 31 31 31 99  S9 39 99 39 99 99 39 99 39 39 101 101 101 101 101 101 101 101 101 101 101 116 116 116  Grain* North 9105 9405 3405 9405 9405 9405 9405 9405 9176 9176 9176 9175 9600 S600 3600 9600 3600 9600 9788 9788 9738 9788 9783 9738 9738 9738 9738 9788 9738 ' 3738 9788 9738 9733 9783 9175 9175 3175 3175 9175 9175 9175 9175 9175 9394 9394 9394 9394 9394. 3394 9394 9394 9394 9394 9394 3012 9012 9012 9012 9012 9012 9012 9012 9012 9012 9012 9018 9018 9018  Type East 13212 Hydrother 13212 Hycrother 13212 Hydrother 13212 Hydrother 13212 Hydrother 13212 Hydrother 13212 Hydrother 13212 Hydrother 13383 Hydrother 13383 Hydrother 13383 Hydrother 13383 -Hydrother 13059 Hydrother 13059 Hydrother 12059 Hydrother 12059 Hydrother 12059 Hydrother 15059 Hydrother 13202 Hydrother 13202 Hydrother 13202 Hydrother 13202 Hydrother 13202 Hydrother 12202 Hydrother 13202 Hydrother 12202 Hydrother 13202 Hydrother 13202 Hydrother 13202 Hycrother 13202 Hydrother 12202 Hydrother 13202 Hydrother 12202 Hydrother 13202 Hydrother 13248 Hydrother 13248 Hydrother 13248 Hyarother 13248 Hydrother 13248 Hydrother 13248 Hydrother 13243 Hydrother 12248 Hydrother 13243 Hyarother 12007 Hydrcther 13007 Hydrother 13007 Hydrother 13C07 Hyarother 13007 Hydrother 13007 Hydrother 13007 Hydrother 12007 Hydrother 13C07 Hydrother 12007 Hydrother 13G07 Hydrother 13400 Hydrother 13400 Hydrother 13400 Hydrother 13400 Hydrother 12400 Hydrother 13400 Hydrother 13400 Hydrother 13400 Hydrother 13400 Hydrother 13400 Hydrother 13400 Hydrother 13205 Hydrother 13205 Hydrother 13205 Hydrother  Fe2+ 1.39 '1.98 2.15 215 2.15 1.95 2.20 1.92 1.36 1.96 1.97 1.97 2.05 2.03 2.07 2.20 2.12 2.02 2.23 2.29 2.26 2.19 2.22 2.47 2.37 2.52 2.59 • 2.57 2.17 2.10 2.04 2.15 2.16 2.15 2.20 2.26 2.24 2.22 2.36 2.29 2.26 2.13 2.20 2.27 2.24 2.20 2.26 2.45 2.45 2.26 2.44 2.16 2.16 2.17 2.26 2.29 2.20 2.21 2.35 2.33 2.38 2.36 2.16 2.18 2.19 1.88 1.39 1.83  Mn2+ 0.013 0.028 0.028 0.028 0.023 0.018 0.018 0.028 0.000 0.018 0.018 0.009 0.018 0.C09 0.018 0.028 0.009 0.009 0.009 0.009 0.009 0.009 0.009 0.C09 0.C09 0.009 0.009 0.C09 0.CC9 0.0C9 0.000 0.009 0.C09 0.009 0.000 0.000 0.000 0.009 0.000 0.000 0.009 0.000 0.009 0.028 0.018 0.028 0.028 0.028 0.028 0.023 0.028 0.018 0.028 0.018 0.028 0.037 0.028 0.028 0.023 0.018 0.028 0.028 0.028 0.018 0.028 0.028 0.028 0.028  Si4+ 6.01 5.67 5.63 5.75 5.70 5.71 5.78 5.60 5.91 5.86 5.73 5.78 5.67 5.82 5.56 5.41 5.47 5.83 5.74 5.57 5.-51 5.73 5.71 5.27 5.54 5.25 5.05 5.05 5.73 5.67 5.91 5.37 5.30 5.76 5.84 5.93 5.83 5.87 5.15 5.33 5.34 5.93 5.89 5.38 5.27 5.62 5.31 5.12 4.92 5.40 5.06 5.77 5.58 5.72 5.57 5.10 5.45 5.67 5.22 5.295.01 4.98 5.48 5.57 5.70 5.71 5.33 5.67  AI3+ 2.55 2.77 2.54 2.57 2.57 2.76 2.53 2.55 3.03 2.33 2.34 2.83 2.53 2.47 2.78 2.91 2.33 2.53 2.31 2.42 2.45 2.29 2.44  2.77 2.45 2.70 2.30 2.97 2.31 2.50 2.24 2.42 2.24 2.37 2.28 2.43 2.54 2.23 2.93 2.43 2.40 2.46 2.27 2.75 2.82 2.77 2.33 2.93 3.04 2.65 2.92 2.53 2.59 2.60 2.63 3.00 2.94 2.55 2.39 2.39 3.03 3.10 2.90 2.63 2.49 2.59 2.74 2.59  Mg2+ 2.91 3.31 3.74 3.27 3.18 3.22 3.15 4.10 2.44 2.77 2.97 2.91 3.40 3.23 3.48 3.57 3.72 3.11 3.31 3.34 3.65 3.30 3.16 4.00 3.70 4.C8 4.35 4.29 3.32 2.32 3.25 3.00 3.27 3.35 2.77 2.71 2.70 2.77 3.34 2.86 2.76 2.53 2.91 4.06 4.17 3.23 4.16 4.35 4.31 3.34 4.67 3.20 3.22 3.29 3.25 4.34 3.36 3.20 4.00 3.90 4.56 4.52 3.28 3.59 3.36 3.64 4.70 3.52  K+ 1.388. 1.307 1.284 1.797 1.398 1.325 1.379 1.329 1.861 1.916 1.852 1.383' 1.723 1.843 1.577 1.430 1.430 1.773 1.532 1.255 1.448 1.705 1.315 0.393 1.211 0.380 0.533 0.514 "\ . i Z i 1.324 1.352 1.379 1.751 1.553 1.907 1.738 1.737 1.370 0.533 1.363 1.343 1.333 1.373 1.018 0.889 1.714 0.917 0.706 0.257 1.008 0.422 1.751 1.595 1.531 1.568 0.697 1.531 1.733 0.843 1.073 0.558 0.559 1.504 1.421 1.705 1.673 0.523 1.769  c:-  TI4+ 0.C09 0.092 0.009 0.110 0.009 0.073 0.009 0.110 0.009 0.110 0.009 0.110 0.009 0.119 0.018 0.073 0.013 0.165 0.028 0.155 0.037 0.165 0.037 0.165 0.018 0.101 0.046 0.174 0.009 0.083 0.009 0.083 0.009 0.083 0.028 0.147 0.028 0.174 0.018 0.119 0.018 0.138 0.018 0.183 0.028 0.183 0.013 0.083 0.009 0.138 0.018 0.083 0.009 0.064 0.003 0.034 0.C23 0.174 0.018 0.165 0.023 0.174 0.013 0.155 0.018 0.174 •0.018 0.165 0.037 0.223 0.028 0.193 0.037 0.220 0.037 0.229 0.013 0.073 0.028 0.193 0.037 0.229 0.046 0.220 0.023 0.223 0.009 0.073 0.009 0.073 0.009 0.119 0.009 0.064 0.009 0.055 0.009 0.018 0.009 0.083 0.009 0.037 0.046 0.138 0.009 0.248 0.009 0.110 0.009 0.193 0.000 0.064 0.009 0.147 0.009 0.165 0.000 0.092 0.009 0.110 0.009 0.073 0.000 0.064 0.009 0.174 0.009 0.156 0.009 0.174 0.028 0.138 0.013 0.028 0.028 0.138  CA2+ NA+ 0.018 0.02 0.003 0.01 0.018 0.02 0.009 0.02 0.009 0.01 0.000 0.01 0.018 0.02 0.013 0.02 0.018 0.06 0.000 0.01 0.0G0 0.01 0.000 0.01 0.009 0.04 0.009 0.04 0.055 0.07 0.018 0.06 0.009 0.02 0.0C9 0.07 0.018 0.01 0.009 0.00 O.CCO 0.02 0.009 0.01 0.009 0.01 0.009 0.01 0.0C9 0.01 0.CC9 0.01 0.009 0.01 0.000 0.01 0.009 0.01 0.0C9 0.01 0.000 0.01 C.CC9 0.01 0.0C9 0.01 Q.C03 0.01 0.000 0.01 0.C09 0.01 0.009 0.01 0.C09 0.01 0.009 0.01 0.009 ' 0.01 0.003 0.01 0.009 0.01 0.000 0.01 0.046 0.03 0.064 0.04 0.009 0.01 0.027 0.02 0.037 0.02 0.018 0.01 0.155 0.03 0.046 0.02 0.04 0.073 0.193 0.02 0.055 0.17 0.037 0.02 0.C09 0.01 0.009 0.01 0.018 0.01 0.009 0.06 0.009 0.01 0.009 0.01 0.009 0.01 0.000 0.01 0.009 0.01 0.009 0.00 0.C09 0.01 0.202 0.01 0.013 0.01  127  F0.110 0.023 0.083 0.119 0.073 0.092 0.083 0.156 0.064 0.028 0.046 0.013 0.165 0.155 0.123 0.174 0.123 0.193 0.513 0.394 0.353 0.504 0.4*19  0.294 0.412 0.394 0.303 0.235 0.422 0.449 0.453 0.257 0.402 0.44Q 0.413 0.403 0.422 0.375 0.220 0.253 0.235 0.312 0.203 0.018 0.046 0.092 Q.G92 0:083 0.023 0.083 0.083 0.092 0.073 0.083 0.174 0.123 0.055 0.110 0.119 0.147 0.064 0.138 0.128 0.156 0.138 0.018 0.083 0.110  DDH Grain* 116 116 116 116 116 116 116 116 116 116 123 123 123 123 123 123 123 136 136 136 136 136 136 136 136 136 135 167 167 167 167 167 167 167 167 167 167 167 167 163 163 163 163 163 168 168 168 168 168 168 168 168 168 163 175 175 175 175 175 175 178 178 178 178 178 184 184 184  FNA+ T14+ CA2+ ClK+ Mg2+ AI3+ Mn2+ Si4+ Fe2+ Type . East North 0.01 0.119 0.018 0.018 0.083 1.503 4.01 5.69 2.49 0.028 1.83 Hydrother 13205 9018 0.02 0.155 0.018 0.018 0.092 1.238 4.12 5.50 2.70 0.037 1.90 13205 Hydrother 9018 0.03 0.055 0.037 0.248 1.513 0.018 3.65 2.60 0.028 1.96 3.So 13205 Hydrother 9018 0.02 0.101 0.009 0.037 0.678 0.018 4.76 2.95 0.028 5.14 13205 Hydrother 2.01 9018 0.01 0.138 0.009 1.283 0.018 0.064 4.20 2.68 0.028 5.50 1.91 13205 Hydrother 9018 0.01 0.028 0.211 0.037 0.623 4.69 0.009 2.75 0.037 5.32 1.91 13205 Hydrother 9018 0.01 0.101 0.229 1.623 0.138 0.018 3.69 2.60 5.60 0.028 1.80 13205 Hydrother 9018 0.01 0.101 0.202 1.577 0.238 0.028 3.57 2.61 5.59 0.028 1.79 13205 Hydrother 9018 0.01 0.083 1.595 0.009 0.092 3.88 0.028 2.53 5.72 1.82 0.028 13205 Hydrother 0.01 0.147 9018 1.833 0.018 0.220 3.35 0.023 2.58 5.74 0.028 1.81 0.02 0.193 13205 Hydrother 9018 1.888 0.000 0.101 3.00 0.009 2.92 5.70 0.009 1.94 0.02 0.220 1-2839 Hydrother 9600 1.815 0.009 0.165 2.73 0.037 2.92 5.83 1.35 0.018 0.01 0.228 12839 Hydrother 9600 1.833 0.000 2.81 0.018 0.321 2.62 5.69 0.018 0.01 0.202 12839 Hydrother 2.15 9600 1.824 0.009 0.083 3.09 0.009 2.94 5.63 0.018 0.01 0.193 12839 Hydrother 2.03 9600 1.577 0.009 0.155 3.26 0.009 2.85 5.54 0.009 0.02 0.174 12839 Hydrother 2.16 9600 1.769 0.000 0.403 2.30 0.023 2.69 5.52 0.018 0.02 0.174 12839 Hydrother 2.30 9600 1.797 O.COO 2.79 0.037 0.367 2.73 5.55 0.028 0.02 0.119 12839 Hydrother 2.26 96C0 1.742 0.C09 3.10 0.009 0.165 2.88 5.54 0.009 0.02 0.174 12600 Hydrother 2.22 9393 0.972 0.009 0.101 3.95 3.06 0.018 2.32 5.16 0.04 0.220 0.018 12600 Hydrother 9393 0:614 0.018 4.27 3.26 0.009 0.073 2.45 4.88 0.02 0.064 0.018 12500 Hydrother 9393 0.550 0.009 4.43 3.22 0.000 0.055 2.44 4.92 0.03 0.155 0.009 12600 Hydrother 0.463 9393 0.009 4.45 3.31 0.009 0.055 2.48 4.82 0.02 0.092 0.018 0.018 12500 Hydrother 9393 0.009 4.79' 3.81 0.009 0.000 4.39 0.01 0.018 0.312 12600 Hydrother 2.53 9393 0.046 4.51 0.009 3.44 0.009 0.037 4.70 0.02 0.018 1.663 12600 Hydrother 2.54 S393 0.083 0.009 0.156 3.23 0.009 2.71 0.05 5.66 0.018 1.815 12600 Hydrother 2.17 0.357 9393 0.028 0.165 2.96 2.41 0.055 0.01 5.89 0.009 1.306 12500 Hydrother 2.11 9393 0.119 0.009 3.06 0.054 0.202 2.59 0.01 5.76 0.018 0.999 12500 Hydrother 2.12 0.202 9393 0.018 0.092 4.25 2.88 0.000 0.01 5.24 2.12 0.009 0.009 13308 Hydrother 0.028 9797 0.009 5.27 0.009 3.77 0.01 4.42 0.000 0.018 1.641 0.229 13308 Hydrother 2.16 9797 0.018 3.48 0.147 2.74 0.01 5.56 0.009 0.018 0.981 0.321 13308 Hydrother 2.04 9797 4.24 0.009 2.92 0.02 5.23 0.009 0.092 2.04 0.303 0.009 0.133 13308 Hydrother 9797 0.018 3.25 4.83 0.037 0.00 4.82 0.009 2.29 0.293 0.018 0.174 13308 Hydrother 9797 0.018 3.25 0.06 4.80 0.023 4.80 0.009 0.321 0.018 0.155 13308 Hydrother 2.28 9797 3.49 0.018 0.01 4.75 4.72 0.037 0.009 1.924 0.018 13308 Hydrother 2.18 9797 0.12 0.413 O.COO 5.32 2.70 2.45 0.037 0.228 1.146 0.000 13308 Hydrother 2.29 9797 0.04 0.257 0.028 3.80 0.110 2.74 0.018 5.56 1.352 1.94 0.018 13308 Hydrother 9797 0.02 0.257 0.023 2.57 3.24 0.147 0.009 5.74 1.623 1.86 0.018 13308 Hydrother 9797 0.01 0.248 Q.0C9 2.44 3.65 0.018 0.147 5.75 1.595 1.90 0.018 13308 Hydrother 9797 0.03 0.223 0.018 3.57 0.155 2.46 0.009 5.72 1.833 1.93 0.009 13308 Hydrother 9797 0.02 0.541 3.19 0.009 0.248 2.55 1.824 0.037 1.96 5.65 0.000 12805 Hydrother 9395 0.02 0.453 2.94 0.000 0.275 1.696 1.98 2.59 0.037 5.74 0.009 12805 Hydrother' 9395 0.02 0.504 3.18 0.018 1.637 1.96 2.60 0.037 0.211 5.68 0.000 12805 Hydrother 9395 0.03 0.650 0.009 0.265 1.742 1.93 3.15 0.046 2.53 5.69 0.000 12805 Hydrother 9395 0.02 0.559 0.000 1.806 0.238 1.93 3.07 0.046 2.52 5.69 0.009 12805 Hydrother 9395 0.02 0.449 0.000 1.815 2.00 2.94 2.37 0.037 0.394 5.78 0.000 12805 Hydrother 9395 0.01 0.623 1.742 0.009 1.98 0.248 3.16 2.54 0.037 5.64 0.000 12805 Hydrother 0.03 0.431 9395 1.861 0.009 0.422 3.03 2.520.037 5.59 0.018 0.02 0.348 12805 Hydrother 2.03 9395 1.641 0.000 0.375 2.57 2.84 0.037 5.67 0.009 0.01 0.449 12805 Hydrother 2.07 9395 1.324 0.009 2.71 0.018 0.220 2.73 5.82 0.009 0.02 0.504 12805 Hydrother 2.04 9395 1.623 0.000 0.284 2.98 0.028 2.47 5.87 1.82 0.009 0.03 0.453 12805 Hydrother 9395 1.714 0.009 0.165 0.018 2.85 2.81 5.37 1.76 0.009 0.02 0.559 12805 Hydrother 9395 1.742 0.000 2.59 3.06 0.037 0.248 5.73 1.84 0.000 0.03 0.605 12805 Hydrother 9395 1.797 0.000 2.43 0.046 0.275 3.25 5.71 1.93 0.000 0.04 0.559 12805 Hydrother 9395 1.333 0.000 0.293 2.28 3.36 0.046 1.90 5.75 0.009 0.05 0.000 12805 Hydrother 9395 1.824 0.009 0.138 2.89 3.57 0.009 5.63 0.009 0.06 0.055 12926 Hydrother 1.61 9600 1.738 0.028 2.81 0.009 0.138 3.48 1.60 5.70 0.06 0.028 0.009 12926 Hydrother 9600 1.723 0.018 2.59 0.028 0.238 3.45 1.58 5.84 0.04 0.083 0.009 12926 Hydrother 9600 1.843 2.84 0.028 0.009 0.147 1.56 3.66 5.57 0.05 0.009 12926 Hydrother 1.824 9600 0.119 2.81 0.009 0.156 0.009 1.65 3.47 5.67 0.03 0.000 1.379 12926 Hydrother 9600 0.046 0.018 0.266 1.64 3.36 0.046 0.02 2.53 5.83 0.018 1.769 12926 Hydrother 9600 0.211 0.009 0.238 3.31 0.037 0.02 2.36 5.81 0.009 1.95 1.898 13246 Hydrother 9600 0.128 0.009 0.229 0.055 3.46 ,0.02 2.38 5.78 0.009 1.361 13246 Hydrother 1.96 9600 0.147 0.000 0.202 0.037 3.36 0.02 2.43 5.83 0.009 1.88 1.843 13246 Hydrother 9600 0.165 0.009 0.202 3.39 0.018 0.04 5.94 2.30 0.009 1.329 13246 Hydrother 1.83 9600 0.193 0.009 0.202 0.028 0.06 3.49 2.31 5.88 0.009 1.83 1.833 13246 Hydrother 9600 0.055 0.092 0.04 0.009 0.000 3.36 2.74 5.57 0.028 1.357 13006 Hydrother 2.41 8967 0.055 0.037 0.128 0.009 2.62 2.76 5.78 0.028 13006 Hydrother 2.37 8967 0.064 0.055 0.101 3.22 0.009 2.79 5.52 0.046 13006 Hydrother 2.54 3967  128  DDH 184 184 228 228 228 228 228 228 228 284 284 284 284 284 284 284 284 284 284 284 284 284 284 284 284 284 284 308 324 324 324 324 324 324 324 324 324 600 600 500 500 600 600 600 600 600 600 627 627 627 627 627 627 627 643 643 643 643 643 643 643 643 643  Grain*  North 8967 8967 9988 9988 9988 9988 9988 9988 9988 9210 9210 3210 9210 9210 9210 9210 9210 9210 9210 9210 9210 9210 9210 9210 9210 9210 9210 8975 8983 8983 8983 8983 8933 8983 8983 8983 8983 9600 9600 9600 9600 9600 9600 9600 3600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600 9600  East 13006 13006 12638 12638 12638 12638 12638 12638 12538 13324 T3324 13324 13324 13324 13324 13324 13324 13324 13324 13324 13324 13324 13324 13324 13324 13324 13324 13055 13318 13318 13318 13318 13318 13318 13318 13318 13318 12298 12298 12298 12298 12298 12298 12298 12298 12298 12298 13000 13000 13000 13000 13000 13000 13000 12645 12645 12645 12645 12645 12645 12645 12645 12645  Fe2+ Type. Hydrother 2.96 Hydrother 2.34 Hydrother 1.49 Hydrother 1.52 Hydrother 1.56 Hydrother 1.49 Hydrother 1.39 Hydrother 1.47 Hydrother 1.47 Hydrother 3.31 Hydrother 3.38 Hydrother 3.03 Hydrother 3.14 Hydrother 3.57 Hydrother 3.77 Hydrother 3.78 Hydrother 3.70 Hydrother 3.72 Hydrother 2.54 Hydrother 2.55 Hydrother 2.50 Hydrother 2.51 Hydrother 2.73 Hydrother 2.95 Hydrother 3.03 Hydrother 2.88 Hydrother 2.91 Hydrother 1.93 Hydrother 2.42 Hydrother 2.40 Hydrother 2.55 Hydrother 2.53 Hydrother 2.39 Hydrother 2.21 Hydrother 2.18 Hydrother 2.35 Hydrother 2.22 Hydrother 2.38 Hydrother 2.35 Hydrother 2.38 Hydrother 2.53 Hydrother 2.41 Hydrother 2.28 Hydrother 2.27 Hydrother 2.28 Hydrother 2.36 Hydrother 2.30 Hydrother 1.39 Hydrother 1.91 Hydrother 1.93 Hydrother 1.85 Hydrother 1.79 Hydrother 1.31 Hydrother 1.81 Hydrother 2.37 Hydrother 2.32 Hydrother 2.39 Hydrother 2.37 Hydrother 2.28 Hydrother 2.10 Hydrother 2.06 Hydrother 2.16 Hydrother 2.11  Mn2+ 0.055 0.055 0.018 0.009 0.018 0.018 0.009 0.009 0.009 0.000 0.009 0.009 0.000 0.018 0.009 0.009 0.009 0.009 0.009 0.000 0.009 0.000 0.000 0.000 0.009 0.009 0.000 0.009 0.028 0.023 0.037 0.028 0.037 0.018 0.023 0.028 0.028 0.018 0.028 0.018 0.028 0.018 0.018 0.018 0.028 0.018 0.018 0.009 0.009 0.009 0.000 0.000 0.009 0.009 0.009 0.009 0.009 0.009 0.009 0.009 0.009 0.009 0.009  Si4+ 4.70 4.77 5.76 5.68 5.78 5.76 6.42 5.91 5.92 5.31 5.12 5.36 5.35 4.62 4.62 4.53 4.49 4.48 5.91 5.71 5.80 5.77 5.87 5.67 5.44 5.57 5.65 5.99 5.57 5.77 5.24 5.46 5.72 5.97 5.92 5.74 5.89 5.56 5.67 5.62 5.41 5.60 5.73 5.69 5.69 5.58 5.76 5.78 5.78 5.74 5.80 5.79 5.80 5.78 5.15 5.29 5.31 5.23 5.64 5.82 5.95 5.86 5.91  AI3+ 3.22 3.22 2.94 3.01 2.77 2.97 2.45 2.53 2.81 3.05 3.33 3.08 3.19 3.54 3.47 3.47 3.55 3.64 3.01 3.12 3.07 3.31 2.77 2.93 3.10 2.37 3.01 2.37 2.70 2.64 3.08 2.73 2.59 2.48 2.50 2.64 2.53 2.95 2.83 2.35 2.34 2.92 2.76 2.79 2.86 2.92 2.78 2.57 2.27 2.41 2.53 2.60 2.55 2.60 2.84 2.71 2.66 2.70 2.33 2.27 2.05 2.24 2.16  Mg2+ 4.65 4.62 3.26 3.34 3.35 3.22 2.86 3.41 3.11 2.51 2.70 2.59 2.48 3.53 3.55 3.75 3.84 3.77 1.74 2.09 1.73 1.82 1.79 1.95 2.59 2.32 1.97 2.70 3.47 2.34 3.53 3.28 2.99 2.85 2.84 2.97 2.93 2.33 2.34 2.36 3.43 2.75 2.85 3.01 2.80 2.83 2.73 3.09 3.39 3.27 3.13 3.14 3.22 3.07 4.48 4.27 4.23 4.41 3.77 3.34 3.49 3.24 3.26  K+ 0.009 0.055 1.751 1.559 1.769 1.759 1.359 1.773 1.705 1.192 0.738 1.063 0.953 0.128 0.101 0.018 0.073 0.018 1.541 1.403 1.324 1.513 1.398 1.513 0.597 1.430 1.595 1.532 1.255 1.733 1.013 1.293 1.738 1.843 1.388 1.797 1.833 1.870 1.824 1.379 1.329 1.361 1.852 1.760 1.833 1.398 1.879 1.852 1.833 1.824 1.861 1.898 1.870 1.879 0.542 0.853 0.853 0.752 1.384 1.806 1.773 1.870 1.934  Cl0.018 0.000 0.018 0.018 0.018 0.009 0.009 0.018 0.018 0.023 0.013 0.018 0.009 0.000 0.000 0.009 0.000 0.000 0.023 0.009 0.023 0.013 0.023 0.023 0.023 0.023 0.023 0.009 0.009' 0.009 0.009 0.023 0.046 0.028 0.037 0.009 0.046 0.009 0.009 0.009 0.009 0.009 0.009 0.000 0.009 0.009 0.009 0.046 0.055 0.055 0.046 0.046 0.046 0.046 0.009 0.009' 0.018 0.018 0.028 0.028 0.018 0.028 0.028  TI4+ CA2+ 0.028 0.009 0.073 0.000 0.037 0.156 0.147 0.028 0.165 0.046 0.165 0.028 0.018 0.165 0.183 0.018 0.147 0.046 0.138 0.028 0.037 0.101 0.183 0.046 0.092 0.110 0.018 0.028 0.101 0.023 0.018 0.018 0.046 0.009 0.009 0.009 0.033 0.202 0.018 0.165 0.037 0.220 0.156 0.013 0.238 0.018 0.018 0.211 0.110 0.083 0.037 0.183 0.009 0.202 0.009 0.119 0.092 0.009 0.009 0.138 0.073 0.009 0.119 0.023 0.128 0.009 0.147 0.C09 0.000 0.156 0.009 0.128 0.009 0.101 0.110 0.009 0.023 0.101 0.009 0.110 0.018 0.083 0.018 0.110 0.028 0.101 0.037 0.083 0.037 0.083 0.009 0.110 0.037 0.101 0.000 0.229 0.248 0.000 0.248 0.000 0.009 0.220 0.000 0.193 0.000 0.193 0.000 0.229 0.083 0.000 0.101 0.101 0.009 0.009 0.092 0.174 0.009 0.009 0.211 0.009 0.193 0.009 0.220 0.000 0.220  o.coo  F0.009 0.018 0.101 0.055 0.174 0.110 0.073 0.083 0.119 0.113 0.073 0.110 0.110 0.064 0.009 0.055 0.009 0.018 0.073 0.174 0.202 0.092 0.202 0.174 0.046 0.110 0.110 0.055 0.147 0.101 0.110 0.147 0.119 0.119 0.147 0.138 0.147 0.073 0.073 0.064 0.046 0.128 0.155 0.092 0.155 0.138 0.138 0.353 0.532 0.477 0.458 0.413 0.403 0.422 0.183 0.202 0.238 0.193 0.229 0.348 0.321 0.238 0.293  NA+ 0.08 0.01 0.06 0.15 0.05 0.03 0.04 0.06 0.19 0.03 0.02 0.05 0.05 0.05 0.07 0.04 0.07 0.05 0.06 0.05 0.09 0.06 0.04 0.05 0.43 0.03 0.04 0.02 0.00 O.CO  0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.01 0.02 0.02 0.03 0.03 0.05 0.01 0.02 0.03 0.04 0.03 0.03 0.02 0.03 0.04 0.01 0.01 0.01 0.00 0.01 0.01 0.00 0.01 0.00  1  2  9  

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