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Nature and Origin of Gold-Rich Carbonate Replacement Deposits at the Rau Occurrence, Central Yukon Kingston, Scott P. 2009-04-15

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NATURE AND ORIGIN OF GOLD-RICH CARBONATE REPLACEMENT DEPOSITS AT THE RAU OCCURRENCE, CENTRAL YUKON  by SCOTT P. KINGSTON  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF BACHELOR OF SCIENCE (HONOURS) in  THE FACULTY OF SCIENCE Geological Sciences  This thesis conforms to the required standard ……………………………………… Supervisor THE UNIVERSITY OF BRITISH COLUMBIA Vancouver, BC MARCH 2009  ABSTRACT  The Rau occurrence is an unusual gold-rich carbonate replacement style deposit located in the Keno Hill District of central Yukon. It is most similar to the Ketza River deposit in southeastern Yukon, which is one of a small number of known gold-rich and base metal-poor carbonate replacement deposits in the world. The Rau occurrence itself hosts economically significant gold grades, and new information on the occurrence provides constraints on the genesis of this poorly understood type of deposit. A detailed petrographic study, utilizing both reflected light and a scanning electron microscopy, was completed using polished thin sections prepared from samples of drill core from two 2008 diamond drill holes on the Rau property. The petrographic analyses identified a simple hydrothermal mineralogy consisting of pyrite, pyrrhotite, and arsenopyrite as the dominant sulphides and a suite of accessory minerals that includes bismuthenite, sphalerite and chalcopyrite. No free gold was identified in the samples, and it remains unclear whether any of the gold in the deposit occurs as isolated grains or it occurs entirely in a more refractory form, perhaps in solid solution in arsenian pyrite. The mineralogy and paragenetic sequence observed at the Rau occurrence show a number of differences from the Ketza River deposit, including an abundance of talc and early stage marcasite in the Rau occurrence. Major and trace element geochemistry suggests that felsic dykes at the Rau occurrence appear to be of similar age and composition to the McQuesten Plutonic Suite to the south; however, the Rau dykes were likely derived from different source rocks and this may explain why their metallogenic signature differs from that of the McQuesten Suite. This suggestion is supported by Pb isotopic compositions of feldspars from the McQuesten Suite and Rau dykes. A Pb isotopic study was undertaken to constrain the source of metals in the sulphide mineralization at the Rau occurrence. The Pb isotopic signature of the sulphides is consistent with much of the Pb having been derived from the Rau intrusions themselves, with a variable contribution of Pb from the host carbonate rocks. Although the new work has shed some light on the nature and origin of gold-bearing carbonate replacement mineralization at the Rau occurrence, many questions regarding the genesis of the mineralization remain unanswered.  ii  TABLE OF CONTENTS  TITLE PAGE………………………………………………….….…………….………..…....i ABSTRACT…………………………………………………….……………………...……..ii TABLE OF CONTENTS……………………………….…...…………….……….…….......iii LIST OF FIGURES………………………………….………..…….……….…….……….....v LIST OF TABLES…………………………………………….………………...……….…...vi ACKNOWLEDGEMENTS………...…………………………..…………………….…...…vii 1  INTRODUCTION………………………………………….…………………………1  2  GEOLOGY……..………………………………………...……….…………….…….3  3  2.1  Regional geology….……………………….………….………………………3  2.2  Local geology…....……………………….……...…..………………………...3  2.3  Previous work on the Rau Property…….……………………………………..5  A REVIEW OF SEDIMENT-HOSTED GOLD DEPOSIT TYPES……………...…..9 3.1  Carlin-type gold deposits……………………………………………………...9  3.2  Distal disseminated gold deposits……………………………………………10  3.3  Skarn deposits………………………………………………………………..11  3.4 4  5  3.3.1  Gold skarns…………………………………………………………..11  3.3.2  Tungsten skarns……………………………………………………...12  Gold-bearing carbonate replacement deposits……………………………….12  METHODOLOGY…………………………………………………………………..14 4.1  Petrographic study and SEM work…….……………………………...……..16  4.2  Geochemistry…..……………………………..……………………………...16  4.3  Lead isotopic studies..……………….…………………………….…………16  PETROGRAPHIC STUDY……..………………………………………….…..……18 5.1  Overview of diamond drill hole core..……..………………………………...18  5.2  Polished thin section analysis………………………………………………..22 5.2.1  Pyrite and pyrrhotite…………………………………………………23  5.2.2 Marcasite……………………………………………………………..26 5.2.3  Arsenopyrite and arsenic-bearing pyrite……………………………..29  5.2.4  Accessory minerals…………………………………………………..32  iii  5.2.5 6  7  Host rock mineralogy and textures…………………………………..35  RESULTS OF GEOCHEMISTRY AND LEAD ISOTOPIC STUDIES....…………39 6.1  Geochemistry...……...……………………………………………………….39  6.2  Lead isotopic studies…………………………………………………………39  DISCUSSION AND IMPLICATIONS FOR MINERAL EXPLORATION..………45 7.1  Petrographic study and SEM work…………………………………………..45  7.2  Geochemistry...………………………………………………………………46  7.3  Lead isotopic studies…………………………………………………………47  7.4  Recommendations for future work…………………………………………..49  REFERENCES CITED……………………………………….……………………………...51 APPENDIX A…………………………………………………………………………...…...55  iv  LIST OF FIGURES  Figure 1. Location map of the Rau occurrence………………………………………..............2 Figure 2. Simplified geology of the western Selwyn Basin…..……………………...………..4 Figure 3. Geological interpretation of the Rau occurrence and location of the study area……6 Figure 4. Geological interpretation of the Rau occurrence, with alteration assemblages, veins and mineralization………………………….………………..….…....…………..……...…….8 Figure 5. Rau property diamond drill hole locations and cross sections….…………………15 Figure 6. Carbonate host rocks in the sedimentary sequence….……….……..…..................20 Figure 7. Volcanic rocks in the sedimentary sequence..….……….…….…...........................21 Figure 8. Marble developed in diamond drill hole 11 (235.8m)…………..............................21 Figure 9. Sulphide textures in the diamond drill hole core……………………...…………...22 Figure 10. Pyrite textures in polished thin section…………………….……………………..24 Figure 11. Pyrite textures in polished thin section…………..….…....…………..……...…...25 Figure 12. Relationship between pyrrhotite and pyrite………………………………………27 Figure 13. Sulphide relationships………….……………………..……….……….……..….28 Figure 14. Pyrrhotite alteration and relationship between sulphides….……….…….…........29 Figure 15. Relationship between banded marcasite and surrounding sulphides…………….30 Figure 16. Relationship between arsenopyrite and pyrite in presence of arsenic-rich pyrite..31 Figure 17. Bismuthenite in thin section……………………………..…………….…………33 Figure 18. Marcasite and bismuthenite occurrences……………….………………………...34 Figure 19. Sphalerite and chalcopyrite occurrences…………………………………………35 Figure 20. Goethite occurrences……………………..……….……….……………………..36 Figure 21. Matrix mineralogy……………………………………………………………......37 Figure 22. Matrix mineralogy and alteration products…….…………………………….......38 Figure 23. Total alkali vs. silica diagram……………………………..…………………...…40 Figure 24. Shand-type plot for discrimination of magma source for the Rau dykes and McQuesten suite plutons……………………………………….....………………………….40 Figure 25. Tectonic discrimination plot for the Rau dykes and McQuesten suite plutons…..41 Figure 26. Plot of 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/206Pb vs. 207Pb/206Pb for Pb isotopic analysis …………………………………………………….………………………………...44  v  LIST OF TABLES  Table 1. Location of polished thin sections…………….……………………………............23 Table 2. Pb isotopic compositions………………………………………………....………...42 Table 3. Description of mineralogy and texture of samples for Pb isotope analysis.....……..43  vi  ACKNOWLEDGMENTS  Throughout the course of this project I was fortunate enough to have been surrounded by many people who provided me with both guidance and support. Without them this project truly would not have been possible. I must first thank my supervisor Dr. Jim Mortensen for always finding the time to meet with me to discuss the project. His incredible knowledge of geology and of scientific writing has helped me every step of the way. Thank you to Matt Dumala and everyone else at ATAC Resources Ltd. for allowing me to work on this project. It is exciting to know that my preliminary research may contribute to something amazing in the future. To Mary Lou Bevier, thank you so much for everything you have done in organizing this course. To my friends and family, I thank you not only for your support during this project but for your continued support in everything I do. I am grateful for all of the amazing people that I have met during my time in the geology program at UBC. The memories I have made during these last few years will last a lifetime.  vii  CHAPTER 1 INTRODUCTION Gold-bearing carbonate replacement deposits are a rare and poorly understood class of mineral deposits. They may be related to typical base-metal rich and gold-poor carbonate replacement deposits, and/or to sedimentary-hosted gold deposits such as Carlin-type gold deposits and gold skarns. The Rau gold-bearing carbonate replacement occurrence in central Yukon (Figure 1) was initially staked and targeted as a lead-zinc prospect, and was later identified for its tungsten potential. ATAC Resources Ltd. was attracted to the property in 2006 due to a Geological Survey of Canada (GSC) reconnaissance silt sample that showed 99th percentile gold (150 ppb) values. Grid soil sampling in 2006 outlined anomalous gold and arsenic values, and in 2008 a drilling program outlined significant gold grades (Appendix A). The nature and origin of the gold mineralization at the Rau occurrence is poorly understood, and it does not conform exactly to any known deposit type. Gold-bearing carbonate replacement style deposits such as the Ketza River deposit in Yukon (Fonseca, 1998) and Mosquito Creek mine in British Columbia (Alldrick, 1983; Rhys et al., 2009) are perhaps the closest analogues. As such, the Rau occurrence represents both an interesting deposit with substantial economic potential in its own right, as well as an opportunity to carry out investigations that will shed light on the genesis of this unusual deposit type and exploration strategies for this style of mineralization elsewhere in the world. Better constraining the nature of and origin of gold mineralization in the study area will provide a framework for understanding how the Rau deposit formed and determining its economic potential. Characterizing the unique mineralogy and texture of the rocks within the study area is of particular importance in achieving this goal. The purpose of this thesis is to develop a descriptive and genetic model for gold mineralization at the Rau property which can be developed into an exploration model for similar deposits worldwide. Laboratory based studies were carried out on sample suites selected from two 2008 diamond drill holes on the Rau property. The scope of the project included the following research thrusts:  1  1) Providing detailed lithological and mineralogical descriptions of certain segments of the drill core based on petrographic and scanning electron microscope (SEM) analysis to define the nature and origin of mineralization. 2) Determining the trace Pb isotopic signatures of the sulphide mineralization and those of the carbonate host rocks and associated intrusions to attempt to determine the main metal source(s) for the mineralization. 3) Comparing the geochemistry of felsic intrusive dykes at the Rau occurrence that may be genetically related to the sulphide mineralization with that of the McQuesten and Tombstone plutonic suites to the south to establish the origin and regional affiliation of the Rau dykes.  Figure 1. Location map of the Rau occurrence.  2  CHAPTER 2 GEOLOGY 2.1 REGIONAL GEOLOGY The Rau occurrence is located within the Keno Hill District of central Yukon, 100 km northeast of the town of Mayo (Figure 1). The deposit is hosted in carbonate rocks of the Mackenzie Platform, which occur in the footwall of the Dawson Thrust on the northern edge of the Selwyn Basin (Figure 2). The study area is near the northern edge of the Tintina Gold Province (Hart et al., 2002), which extends across Yukon and Alaska and hosts many intrusion related gold deposits, including Dublin Gulch, Scheelite Dome and Clear Creek. The east-trending Dawson Thrust is localized along an abrupt linear facies change between strata of the Selwyn Basin and shallow water carbonates of the Mackenzie Platform to the northeast (Pyle et al., 2000). The Selwyn Basin is a 700 x 200 km, fault controlled epicratonic basin that developed on the North American cratonic margin (Abbott et al., 1987), and is composed of Neoproterozoic to Mississipian metasedimentary and sedimentary rocks (Marsh et al., 2003). The Dawson Thrust is the structurally lowest of three low angle, north-verging thrust faults; the Tombstone and Robert Service thrust faults are located structurally above the Dawson Thrust (Figure 2). The Dawson Thrust is thought to have had a profound influence on geology of this region (Mair et al., 2006). Prior to JurassicCretaceous contractional deformation in the area, the Dawson thrust fault greatly influenced sedimentation and magmatism as a major-basin bounding structure (Mair et al., 2006). 2.2 LOCAL GEOLOGY The geology of the study area was first mapped at a 1:250,000 scale in 1961 by L.H. Green and J.A. Roddick for the Geological Survey of Canada (Green, 1973). Green mapped a large part of the study area as a Cambrian gritty quartzite, sandstone and quartz cobble conglomerate unit. A second unit mapped in the vicinity of the study area is an OrdovicianSilurian, medium-to thick-bedded dolomite and limestone. Green also identified and mapped the Keno Hill quartzite, which he described as massive, commonly graphitic and argillaceous  3  4  Figure 2. Simplified geology of the western Selwyn Basin (modified from Mair et al., 2006).  quartzite of inferred Cretaceous age. Abbott (1990) completed 1:50,000 scale mapping of the area. He subdivided the Ordovician-Silurian thick bedded dolostone and limestone unit of Green (1973) into two different units, and in addition mapped a Cambrian finely laminated grey-black dolostone in the area. The Keno Hill quartzite was assigned a Mississippian age based on newly discovered fossil occurrences (Abbott, 1990). Abbott’s mapping showed the Dawson Thrust to pass immediately to the south of the study area. Abbott noted two intrusions to the south east of the main mineralized area at the Rau occurrence. A smaller (~ 700 m diameter), medium-grained pegmatic granite that was mapped north of the Dawson Thrust is described by Abbott (1990) as leucocratic, fluorite rich, and containing biotite and muscovite. This pluton is located within the Rau occurrence and is locally known as the Rackla pluton. Dating of zircons from the pluton by U-Pb methods yielded a minimum age of 61.4 ± 0.2 Ma, which is considered a minimum age given the likelihood of some postcrystallization Pb-loss (J.K. Mortensen, pers. comm., 2009). This is approximately the same age as the McQuesten Suite intrusions (Figure 2), which were assigned an age of 65 ± 2 Ma (Murphy, 1997). Numerous dykes and sills in the central part of the Rau property were mapped by Panton (2008), who speculated that they are associated with the same intrusive suite as the Rackla pluton (Figure 3), and therefore are most likely of Late Cretaceous age. The dykes and sills typically range from 30cm to 7m in thickness and are commonly more fractionated than the Rackla Pluton (Panton, 2008). They include garnet bearing aplite and coarse pegmatite bodies composed primarily of orthoclase and quartz but also commonly exhibit abundant lithium- and vanadium-rich micas on their margins (Panton, 2008). A larger (~2500m diameter) intrusion, comprising dark grey fine grained biotite hornblende granidiorite of inferred mid-Cretaceous age, was mapped by Abbott (1990) just south of the Dawson Thrust. This pluton is locally referred to as the Mount Westman pluton and is located to the southeast of the Rau property. The most recent compilation of the geology of the study area can be viewed online at the Yukon Geological Survey webpage.  2.3 PREVIOUS WORK ON THE RAU PROPERTY The Rau property is wholly owned by ATAC Resources Ltd. Collectively the property covers an area of 290 sq km, and consists of 1437 contiguous mineral claims. It was 5  Figure 3. Geological interpretation of the Rau occurrence showing the general geology of the study area (modified from Panton, 2008). Red outline indicates the zone of main gold mineralization at the Rau occurrence and study area of this project.  6  the focus of sporadic lead-zinc exploration between 1967 and 1980, and tungsten potential was subsequently recognized in the area. Six main types of mineralization have been identified on the Rau property, as discussed by Dumala (2009): (1) carbonate-hosted replacement-style gold; (2) zinc±silver±lead±gold±bismuth in limonite-rich veins and replacement bodies; (3) scheelite in tremolite skarns; (4) pyrrhotite ±scheelite±chalcopyrite in actinolite-diopside±garnet skarns; (5) wolframite±tantalite in granite; and (6) iron±zinc in hydromorphically transported gossans (Figure 4). Panton (2008) described Au- and W-Snbearing skarn deposits located on the Rau property, and proposed a deposit model for this mineralization. Skarns at the Rau occurrence hosts the bulk of the tungsten, which occurs as scheelite in disseminated form as well as in wolframite inclusions. Panton (2008) used major element chemistry to classify the Rackla pluton as an I-type, magnesian, calc-alkalic, weakly metaluminous intrusion, falling within the granodiorite field of the IUGS classification. Panton (2008) suggested all or most of the mineralization at the Rau occurrence is contained within a complex intrusion related gold system, and speculated that the Rackla pluton appears to be the source of spatially associated tungsten, gold, lead, zinc and silver mineralization. ATAC Resources Ltd. was attracted to the property in 2006 on the basis of a GSC reconnaissance silt sample that showed 99th percentile gold (150 ppb) values and was supported by a 99th percentile tungsten value (25 ppm). Grid soil sampling by the company in 2006 identified anomalous gold and arsenic values. In 2007, geological mapping, prospecting, geochemical sampling, and geophysical surveys were completed on the Rau property by Archer, Cathro & Associates (1981) Ltd., the exploration consultant for ATAC Resources Ltd. In 2008, a total of 3423 m of diamond drill core were obtained from 18 drillholes on the property, and included significant gold grades over large intervals (Appendix A). This gold mineralization is the main focus for the present study. ATAC Resources Ltd. and Yankee Hat Minerals Ltd. signed an option agreement on April 9, 2008, concerning 40 claims that cover the Rackla Pluton and the tungsten-bearing skarns. Yankee Hat refers to this area as the Wau property and the company continued surface exploration during the 2008 field season. A detailed outline of the exploration history on the Rau property can be found in the assessment report prepared by M. Dumala for ATAC Resources (Dumala, 2009).  7  Figure 4. Geological interpretation of the Rau occurrence with alteration assemblages, veins and mineralization (Panton, 2008). Red outline is the same as in Figure 3.  CHAPTER 3 A REVIEW OF SEDIMENT-HOSTED GOLD DEPOSIT TYPES A wide variety of sediment-hosted gold deposits have been recognized throughout the world. Each deposit type has unique characteristics related to their formation, mineralization and tectonic setting. A subset of these deposit types are described below as potential analogues to mineralization at the Rau occurrence.  8  3.1 CARLIN-TYPE GOLD DEPOSITS The largest Carlin-type gold deposits in the world occur in the Basin and Range Province in the northern part of Nevada in the western United States. The unique geologic history in northern Nevada may explain why this deposit type is almost entirely confined to this single region (e.g., Ilchik and Barton, 1997). Although they are much smaller, Carlintype gold deposits have also been discovered in southern China; these deposits share many similarities with those from Nevada (Rui Zhong et al., 2002). Ilchik and Barton (1997) noted three features that were central to the development of the Carlin-type deposits in Nevada. First, the Cordilleran miogeocline represents the host and possible source rocks for mineralization. Second, there was widespread late Mesozoic and mid-Tertiary magmatism, which is related to hydrothermal systems. Finally, mid-Tertiary extension, with or without accompanying magmatism, promoted hydrothermal circulation. Carlin type ore occurs as replacement bodies in carbonate host rocks, and is characterized by submicron gold which occurs in disseminated and trace element rich pyrite and marcasite (Cline et al., 2005). The localization of ore zones is largely structurally controlled by faults and folds. Ore zone geometry typically consists of a feeder zone controlled by normal (or possibly reverse) fault zones, and an upper stratiform zone (Arehart, 1996). Fold control is also important to ore deposition, as fold crests may trap or inhibit movement of ore fluids, or act as a release point for over-pressurized fluid (Arehart, 1996). Individual ore bodies represent zones of porosity and permeability that result from faults and favorable lithologic features, and are typically capped by less permeable horizons (Cline et al., 2005). The highest ore grades tend to concentrate beneath domes or anticlines where, like petroleum reservoirs, high angle structures act as feeders (Cline et al., 2005). Alteration proceeds both temporally and spatially from distal decarbonatization through silicification (jasperoid formation) to argillization (Arehart, 1996). Within the gold bearing carbonate leached zone, altered host rocks are typically enriched in gold between 100 to 1000 times over unaltered host rocks, whereas the accompanying accessory suite of elements including As, Sb, Hg, Tl and Ag are enriched 100 times over background (Ilchik and Barton, 1997). Characteristic of many of the deposits is the lack of accessory minerals of hydrothermal origin, particularly base metal sulphides, and a lack of mineral zoning (Arehart, 1996). The fine grained nature of the  9  volumetrically minor ore and gangue minerals, as well as extensive post-mineral oxidation and weathering, have hampered development of a model for formation of Carlin-type deposits (Cline et al., 2005). The three proposed models include, (1) metal leaching and transport by convecting meteoric water (Ilchik and Barton, 1997), (2) hydrothermal systems related to epizonal intrusions (Chakurian et al., 2003), and (3) circulation of deep metamorphic and/or magmatic fluids (Heinrich, 2005). Unlike skarns, the large regional extent of Carlin-type deposits suggests that they are unlikely to be associated with a single pluton (Meinert et al., 2005). The Carlin trend accounts for 1.3% of all the gold ever mined in the world, with total gold production from the Carlin Trend having reached in excess of 2,000 tonnes of gold by the end of 2007 (Nevada Bureau of Mines and Geology, 2007). 3.2 DISTAL DISSEMINATED GOLD DEPOSITS Distal disseminated gold deposits and “classic” Carlin-type deposits (as described above) were previously classified together due to the many characteristics that they have in common. In some cases these two deposit types cannot be readily distinguished. The genesis of Carlin-type deposits, however, is still very much in question, and an association of the mineralization with igneous activity is debatable. Distal disseminated deposits, on the other hand, appear to be closely associated with the intrusion of a proximal porphyry system or with volcanic/subvolcanic activity (Presnell et al., 1996). Sillitoe and Bonham (1990) proposed that gold mineralization in classic Carlin-type deposits was not entirely related to igneous activity, but rather that gold was scavenged from host sedimentary sequences by circulating meteoric hydrothermal fluids and that the deposits were distal products of porphyry-related hydrothermal magmatic systems. Barneys Canyon is a well studied distal disseminated gold deposit that occurs within 7 km of the large, gold-rich Bingham porphyry Cu deposit (Presnell et al., 1996). As with classic Carlin-type gold deposits, gold at Barneys Canyon occurs as micron to submicron grains and is only detectable by assay (Presnell et al., 1996). Pyrite, occurring both as individual grains and veins, and marcasite, which typically occurs as bladed clusters and aggregates that are commonly intergrown with pyrite, are the primary sulphides in the deposit (Presnell et al., 1996).  10  3.3 SKARN DEPOSITS Skarn deposits are typically hosted in minerallogically simple fine grained clastic and carbonate sedimentary rocks. The deposits are dominated by silicate minerals such as garnet and pyroxene (Meinert et al., 2005). Skarn mineralogy and metal content is largely dependent on the crystallization history and genesis of associated plutons (Meinert et al., 2005). As they represent intrusion related disseminated gold deposits hosted in carbonate rocks, gold skarns are an important analogue to the deposit style seen at the Rau occurrence. Tungsten skarns are also described below because the Rau occurrence was first targeted as a W-skarn (Panton, 2008). 3.3.1 GOLD SKARNS Skarns carrying significant gold grades (5-15 g/t Au) are more reduced than other skarn deposit types, lack economic concentrations of other metals, and have a distinctive AuBi-Te-As ± Co geochemical association (Meinert et al., 2005). They are typically associated with dike/sill complexes and diorite-granodiorite plutons that are ilmenite bearing with reduced compositions (Fe2O3/(Fe2O3+FeO) << 0.75) (Meinert et al., 2005). The low oxidation and sulfidation state of Au-skarns suggest that the primary mechanism of Au precipitation is by reduction of oxidized hydrothermal fluids (Meinert, 1998). Gold skarns have a characteristic alteration style that includes biotite ± K-feldspar (potassic) alteration within a distal alteration zone (Meinert, 1998). Gold mineralization forms late in magmatichydrothermal systems and is commonly intimately associated with Bi minerals (Meinert et al., 2005). The Canadian Cordillera is host to many important skarn deposits, some of which were world class producers. Cumulative production at the Nickel Plate mine in the Hedley District of British Columbia between 1904 and its closing in 1995 was 13.4 Mt, averaging 5.3 g/t Au, 1.3 g/t Ag, and 0.02% Cu (Ray et al., 1996), making it the largest and highest grade Au skarn in Canada (Meinert, 1998). The dioritic Hedley intrusions, which exhibit strong endoskarn alteration with abundant pyroxene, biotite, garnet, amphibole and Kfeldspar, are genetically and temporally linked to the skarn deposit at the Nickel Plate mine (Meinert, 1998).  11  3.3.2 TUNGSTEN SKARNS Tungsten skarns are found throughout the world in major orogenic belts, typically in association with calc-alkaline plutons (Meinert et al., 2005). The northern Canadian Cordillera hosts two major tungsten skarn deposits, Mactung and Cantung. Abundant calc silicate hornfels and skarnoid are observed in the high temperature metamorphic aureoles common to W-skarn deposits (Meinert et al., 2005). Tungsten-skarns show evidence of a progressive sequence of hydrothermal events and significant remobilization of metals during late, low temperature fluid flow (Newberry, 1998). Oxidized W-skarns are dominated by pyrite and intermediate-Fe pyroxene, whereas reduced W-skarns contain abundant pyrrhotite and high-Fe pyroxene (Newberry, 1998). Tungsten skarns have a unique metal concentration pattern due to the fact that they are interpreted to form at significantly greater pressure (depths) than other skarn types. The high pressure keeps the magma water-undersaturated until late in the crystallization history; thus the deposits are a result of fluids exsolved from a melt that has undergone significant crystallization prior to fluid loss (Newberry, 1998). 3.4 GOLD-BEARING CARBONATE REPLACEMENT DEPOSITS The following summary of the main characteristics of carbonate replacement deposits is taken from Nelson (2005). Carbonate replacement deposits are intrusion related, sulphide rich replacement deposits that are typically small, irregular and discontinuous. They are hosted by dolostone and/or limestone which has commonly been silicified and/or dolomitized, and typically form distal to skarns and small, high level felsic intrusions. The host carbonates typically occur within a thick sedimentary package that includes siliclatic rocks, cross cutting igneous porphyrytic intrusions and in some cases a volcanic horizon near the top of the sequence. The majority of carbonate replacement deposits within the Canadian Cordillera occur within the miogeocline due to the common association of felsic intrusions and abundant carbonate rocks (Nelson, 2005). Carbonate replacement deposits are typically polymetallic, base metal rich and gold poor. Within British Columbia the geochemical signature is Ag+Pb+Zn+Sn, whereas deposits in the United States show an outward zonation from a Cu-rich core through Ag-Pb zone to a fringe zone of Zn-Mn (Nelson, 2005).  12  Carbonate replacement deposits that contain a significant amount of gold are relatively rare; however, there are examples in the Canadian Cordillera such as the Ketza River deposit, which is located in the Pelly Mountains of central Yukon (Fonseca, 1998). The Ketza River deposit is classified by Fonseca (1998) as a gold-rich and base metal poor, carbonate hosted manto-style replacement deposit. It is hosted by a thick Lower Cambrian reef complex, which occurs within a shallow marine, carbonate platform sequence known as the Pelly-Cassiar Platform (SRK Consulting, 2008). The carbonate sequence was deformed and intruded by granitic plutons during the Mesozoic (Fonseca, 1998). Two main folding events followed by three episodes of faulting affected the Ketza River area from Jurassic to mid-Cretaceous time (Fonseca, 1998). Mississippian stocks and dykes are the only exposed intrusive rocks on the property and show no spatial correlation with the mineralization (Fonseca, 1998). The presence of submicroscopic bismuth within all ore types on the property suggests that the hydrothermal activity was triggered by the emplacement of a pluton. Although the source of metals, sulfur, heat, and mineralizing fluids is uncertain, Fonseca (1998) speculates that an intrusion of Early Cretaceous age may exist beneath the deposit. Massive sulphide replacement bodies at Ketza River occur within the Lower Cambrian limestones as sub-horizontal tabular bodies (mantos) and sub-vertical bodies (chimneys). Fonseca (1998) identified three styles of mineralization in the Ketza River area, all of which have similar sulphide mineralogy and contain rare, typically submicroscopic, native bismuth. 1) Massive sulphide and oxidized mantos 2) Au-rich Fe-silicate alteration (skarn) 3) Quartz-sulphide veins/stockworks A fourth style of mineralization, Pb-Zn veins, occurs outside of the main mineralized area. The following descriptions of mineralization at the Ketza River deposit are taken from Fonseca (1998). Sulphide mineralogy in the quartz-sulphide vein/stockwork is dominated by pyrrhotite, pyrite and arsenopyrite with minor marcasite and chalcopyrite and rare galena and sphalerite. Marcasite is identified as a late stage mineral that occurs as open space fillings in pyrrhotite and replaces early pyrite. The paragenetic sequence proposed by  13  Fonseca (1998) also classifies chalcopyrite as a late stage mineral, together with arsenopyrite and gold. Gold-rich manto style massive sulphide orebodies and their oxidized equivalents are preferentially hosted by three discrete limestone facies within the Cambrian reef complex, implying strong lithological control on the localization of mineralization. The exact position of the mantos is controlled by high-angle planar and listric normal faults, fold hinges, and by the location of the three favorable facies. Dolomitization and decalcification of the carbonate host rocks are the only visible alteration products associated with the mineralization, and the alteration was determined to precede metal deposition. Based on trace Pb isotopic analyses, Fonseca (1998) suggests that sulphides from all four mineralization styles were derived from the same metal source during a single mineralizing event. Trace Pb isotopic analyses from the host carbonates themselves are scattered, which may be due to unobserved metamorphic or hydrothermal effects, or due to variations in the Pb isotopic compositions in different limestone facies (Fonseca, 1998). The Mosquito Creek Mine is one of a group of gold-bearing pyritic carbonate replacement deposit that occurs within the Cariboo Gold District in east-central British Columbia (Rhys et al., 2009). Gold ore occurs as massive, stratabound auriferous pyrite lenses or mantos, within metamorphosed Paleozoic limestones (Alldrick, 1983). Gold in the highest grade zones occurs as grains along crystal boundaries and fractures within fine grained pyrite (Rhys et al., 2009). The pyritic lenses are structurally controlled, and commonly occur within the crests or noses of minor folds and along the steeply dipping limbs of the main fold structure (Alldrick, 1983). They contain coarse-grained dolomite, pyrite and arsenopyrite along their margins and are commonly encased by sericite±ironcarbonate/dolomite±fushcsite alteration or silicification replacing the host limestone (Rhys et al., 2009). Mineralization is commonly banded, alternating between pyrite and carbonatedominated bands (Rhys et al., 2009). CHAPTER 4 METHODOLOGY Samples for analysis were taken from two 2008 diamond drill holes on the Rau property. The diamond drill holes are 08-18 and 08-11 and referred to from here on as diamond drill holes number 11 and 18 (Figure 5). 14  Figure 5. Rau property diamond drill hole locations and cross sections (ATAC Resources Ltd., 2009). Red boxes highlight the holes sampled for this study.  15  4.1 PETROGRAPHIC ANALYSES AND SEM WORK Detailed lithological and mineralogical descriptions of diamond drill core recovered from drill holes 11 and 18 were completed using petrographic techniques and scanning electron microscope (SEM) analysis. Fifteen samples were selected and cut into 2-3cm billets by the author and submitted to Vancouver Petrographic Ltd. for polished thin section preparation. Samples were selected to be representative of a variety of mineralization styles, ranging from disseminated to massive throughout the entire sulphide interval in both diamond drill holes. Polished thin sections were analyzed using reflected light and transmitted light techniques on a research grade binocular microscope. Digital photographs were taken using a Nikon 995 digital camera mounted to the binocular microscope. Further analysis of the lithological and mineralogical characteristics of the polished thin sections was completed using the SEM in both back-scattered electron (BSE) and energy dispersive spectrometry (EDS) mode. Individual grain mounts were created for a variety of minerals within the diamond drill hole core and identified using SEM/EDS methods. 4.2 GEOCHEMISTRY The purpose of this part of the study was to define the origin of felsic dykes at the Rau occurrence. Major and trace element geochemical compositions were investigated to test potential relationships between the Rau dykes and the Late Cretaceous McQuesten Suite intrusions to the south of the study area. 4.3 LEAD ISOTOPIC STUDIES Lead isotopic studies were undertaken in order to compare and relate the Pb isotopic signature of the sulphide mineralization to that of host carbonates, as well as volcanic rocks in the sedimentary sequence, and to known intrusive suites in the area. Lead isotopes are useful as a geochemical tracer for metal sources in ore deposits, as discussed in Tosdal et al. (1999). Lead derived from different source reservoirs is typically isotopically distinct. The  16  degree of interaction between hydrothermal fluids and wall rocks, as well as the influence of basement rocks and tectonic setting, can be constrained using Pb isotopes (Tosdal et al., 1999). The Pb isotopic signature of minerals can also provide some information about the age of mineralization if the source of the metals is known or can be inferred, and/or if the age of the Pb source is known or can be dated by independent geochemical methods (e.g., U-Pb, 40  Ar/39Ar). If the isotopic signature of a closely associated pluton matches that of the  mineralization then it can be inferred that the mineralization has the same age as the pluton and may be genetically related to it. The radioactive decay of one isotope of thorium (232Th Æ 208Pb) and two isotopes of uranium (238U Æ 206Pb and 235U Æ 207Pb), create the framework of Pb isotope analysis. Lead isotopic data are typically shown as plots of 207  Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb, which are referred to as uranogenic  and thorogenic plots, respectively. An alternative method of presenting the data is to plot 208  Pb/206Pb vs. 207Pb/206Pb; this method avoids the higher analytical error typically associated  with measurement of the weak 204Pb peak. Lead isotopes record the ratio of U/Pb and Th/Pb from the source at the time that the metals were extracted and deposited in sulphide minerals. Samples of 16 sulphides, 2 unaltered carbonate whole rocks and 1 volcanic whole rock from diamond drill holes 11 and 18 on the Rau property, as well as 4 feldspar samples from the Rau dykes were analyzed for their Pb isotopic compositions. Sample selection was done by chipping out fragments of the desired mineral(s) from hand samples and separating the cleanest pieces with clean tweezers under a binocular microscope. Sulphide samples (pyrite and pyrrhotite) were separated from a variety of mineralization styles, ranging from disseminated to massive, from throughout the entire sulphide interval in each of the two diamond drill holes. Carbonate and volcanic rock samples were chosen for Pb isotopic analysis from an unmineralized zone at the top section of diamond drill hole 18. Sample preparation, geochemical separations, and isotopic measurements were done at the PCIGR facility at the University of British Columbia. For trace lead sulfide samples, approximately 10-50 milligrams of hand picked sulfides were first leached in dilute hydrochloric acid to remove surface contamination and then dissolved in dilute nitric acid. Samples of galena required no leaching and were directly dissolved in dilute hydrochloric acid. Following ion exchange chemistry, approximately 100-250 nanograms of Pb in chloride form was loaded on rhenium filaments using a phosphoric acid-silica gel emitter. Isotopic  17  ratios were determined with a modified VG54R thermal ionization mass spectrometer in peak-switching mode on a Faraday detector. Measured ratios were corrected for instrumental mass fractionation of 0.12%/amu based repeated measurements of the NBS 981 standard and the values recommended by Thirwall (2000). Errors were numerically propagated throughout all calculations and are reported at the 2σ level. CHAPTER 5 PETROGRAPHIC STUDY 5.1 OVERVIEW OF DIAMOND DRILL HOLE CORE The Rau occurrence is a stratabound deposit which hosts sulphide and gold mineralization within a sequence of dolomitized limestone and volcanic and/or volcaniclastic rocks. Strata in the study area are folded into a series of relatively open synclines and anticlines (Dumala, 2009). Gold occurs with sulphides that wholly or partially replace the host brecciated dolomite at the crest of an anticline (Dumala, 2009). The unaltered carbonate host rock is a fine grained, dark grey dolostone (Figure 6a), that is commonly brecciated (Figure 6b). Calcite veining and zebra-striped dolomite textures are also locally observed within the unaltered host dolostones (Figure 6c,d). The zebra stripped dolomite texture is characterized by alternating light and dark bands of sparry dolomite. This distinctive texture has been described from elsewhere in the world in carbonate platform settings that have experienced elevated heat flow and extensional faulting (Diehl et al., 2007). Sulphide mineralization at the Rau occurrence tends to be best developed immediately beneath a volcanic unit. The main mineralized zone of each of drill holes 11 and 18 is in the footwall of the volcanic unit, which may have acted as an aquitard that controlled hydrothermal fluid flow. Unaltered volcanic rocks at the Rau occurrence are typically dark green in color and contain calcite amygdules, whereas altered volcanics display a tan color and exhibit strong foliation and biotite and sericite alteration (Figure 7). Bleached white sucrosic marble develops near the bottom of diamond drill hole 11 (Figure 8a). It is bordered by a white-grey banded marble with disseminated magnetite (Figure 8b). The presence of this marble may be important as a tool to model deposit model 18  vectoring, as it may represent a “marble front” associated with skarn formation. The marble front, or “marble line”, marks the outer limit of skarn formation in the vicinity of many skarn deposits. At the Nickel Plate mine in the Hedley District, for example, the majority of the economic orebodies occur within 80 m of the marble line (Ettlinger et al., 1992) Drill hole 18 was terminated in massive pyrite, which occurs as large euhedral grains, with quartz, calcite and sphalerite (Figure 9a,b). Within the main mineralized zone of each of the diamond drill holes, sulphides occur in a wide range of forms from disseminated to massive. Sulphides commonly form blebby disseminations (Figure 9c) and banding defined by variation in sulphide concentration is observed in many sections (Figure 9d).  19  Figure 6. Carbonate host rocks in the sedimentary sequence. (a) Unaltered light-dark grey dolostone; (b) Zebra-stripped texture developed in dolostone; (c) Strong brecciation and veining; (d) Multiple generations of calcite vein growth. Scale: Image height is equal to full diameter of the diamond drill hole core (4.1cm).  20  Figure 7. Volcanic rocks in the sedimentary sequence. (a) Typical banded volcanic texture; (b) Blotchy volcanic texture. Scale: Image height is equal to full diameter of the diamond drill hole core (4.1cm).  Figure 8: Marble developed in Rau 08-11 235.8m. (a) Grey-white banded marble with disseminated magnetite; (b) Bleached white sucrosic marble. Scale: Image height is equal to full diameter of the diamond drill hole core (4.1cm).  21  Figure 9. Sulphide textures in the diamond drill hole core. (a) Large euhedral pyrite with small quartz inclusions. Sphalerite within the surrounding quartz/calcite matrix. Bottom of diamond drill hole 18; (b) Massive pyrite. Bottom of diamond drill hole 18; (c) Blebby disseminated pyrite; (d) Banded sulphides. Scale: Image height is equal to full diameter of the diamond drill hole core (4.1cm). 5.2 POLISHED THIN SECTION ANALYSIS A total of 15 samples were examined in polished thin section; the locations of these samples along with a brief description of the immediately surrounding rock are given in  22  Table 1. The nature and occurrence of the various mineral phases observed is described below. Table 1. Location of polished thin sections. py=pyrite, po=pyrrhotite, apy=arsenopyrite. Sample # 18-108.2 18-116.4 18-72.7 18-121.12  Hole Number Rau 08-18 Rau 08-18 Rau 08-18 Rau 08-18  Meterage 108.2 116.4 72.7 121.12  18-129.6 18-131.06 18-162 18-206.7 11-203.1 11-235.3  Rau 08-18 Rau 08-18 Rau 08-18 Rau 08-18 Rau 08-11 Rau 08-11  129.6 131.06 162 206.7 203.1 235.3  11-219.1  Rau 08-11  219.1  11-219.6  Rau 08-11  219.6  11-216.3  Rau 08-11  216.3  11-235.8 11-187.7  Rau 08-11 Rau 08-11  235.8 187.7  Hand Sample Description Brecciated limestone, talc on fractures, minor py Brecciated limestone, talc on fractures, minor py transition zone dolomite into volcanic, minor py, oxidation Main mineralized zone, dolomite, sulphides along foliation Main mineralized zone, dolomite, sulphides along foliations Main mineralized zone, dolomite, sulphides along foliation massive fine grained py, near calcite veins Brecciated dolomite, trace py fine grained euhedral pyrite along foliation planes disseminated py and minor apy, 1.62 g/t Au euhedral py, apy, trace po, disseminated, talc on fractures euhedral py, apy, trace po, disseminated, talc on fractures Main mineralized zone, disseminated py form parallel bands diss. py and minor apy, 1.62 g/t Au, borders banded marble diss. py and apy, minor po, oxidized, talc on fractures  5.2.1 PYRITE AND PYRRHOTITE Pyrite is the dominant sulphide present in the polished thin sections, occurring in a variety of styles, ranging from disseminated to massive, and in association with a large number of other mineral phases. There is evidence of at least two generations of pyrite in many of the samples, but a consistent paragenetic sequence is difficult to establish based on observation thus far. Many interesting textures were observed in pyrite grains as shown in Figure 10 and Figure 11. Pyrite occurs as thin wispy strands, and in sample 11-219.6 pyrite develops this way in close proximity to a separate generation of pyrite that develops as euhedral crystals (Figure 10a,b). It is suggested that the thin wispy strands develop as a later stage and surround the earlier formed euhedral pyrite. In sample 18-206.7 the thin wispy strands of pyrite border the euhedral pyrite (Figure11c,d). The contact between the two generations of pyrite here is typically sharp, but in some places the euhedral pyrite seems to  23  Figure 10. Pyrite textures in thin section. (a) and (b) Thin wispy strands of pyrite developed in proximity of euhedral pyrite grains; (c) Pyrite which appears to be dissolving away (occurs along the edge of the thin section); (d) Euhedral pyrite grain in contact with a mass of anhedral pyrite. Po=pyrrhotite, Py=pyrite, Qtz=quartz. be separating into strands along its edges. This “bladed” texture along the grain boundaries of euhedral pyrite is clearer in sample 11-219.1 where a single generation of pyrite is present (Figure 11b). Samples 11-219.6 and 11-216.3 display two generations of pyrite. A later generation of fine grained euhedral pyrite borders an early generation that has a significant concentration of inclusions which makes it appear more grey than the typical off-white color that pyrite displays in thin section (Figure 11a). Along the edge of the thin section for sample 12-129.6 a fine grained mass of pyrite appears to be dissolving away (Figure 10c). This 24  Figure 11. Pyrite textures in thin section. (a) Late stage euhedral pyrite generation bordering an earlier generation of pyrite with more inclusions; (b) Bladed texture on the edge of a euhedral pyrite grain; (c) and (d) Euhedral pyrite grains surrounded by a separate fibrous generation of pyrite (photos from two different slides). unique texture is only observed in this one section and it is unclear if this is simply a product of slide preparation or if it is an alteration product. Euhedral pyrite grains commonly occur close to fine grained masses of pyrite, and, in sample 11-203.1, for example, the two pyrite generations are in contact (Figure 10d). The euhedral pyrite grain encloses small euhedral grains of pyrrhotite and shows a sharp contact with the surrounding fine grained pyrite. Pyrrhotite is the second most common sulphide observed in the polished thin sections. Pyrite and pyrrhotite are commonly in close association with one another and 25  intimately intergrown. Pyrrhotite typically occurs as fine grained irregular masses and as anhedral grains. In a number of sections, including sample 11-187.7, pyrrhotite is intimately associated with talc. Pyrrhotite is either cut by talc that develops in the center of the pyrrhotite masses, as shown in Figure 12a,b, or small anhedral pyrite grains sit within fans of fine grained talc. The most common configuration involves pyrite completely surrounding the outside edges of pyrrhotite grains, or occurring along interior grain boundaries and fractures within pyrrhotite. This configuration is seen in many samples and shown in Figure 12c,d and Figure 13c. The fine grained masses of pyrrhotite commonly develop small cores that appear to eaten away along the edges and rimmed by marcasite and then outward through pyrite (Figure 13a,b). The pyrrhotite develops wavy grain boundaries. The marcasite here appears to be recrystallized by pyrite, suggesting that it is an early stage mineral. Marcasite shares the same chemical formula as pyrite (FeS2), but because of its unstable crystal structure is more brittle and more prone to breaking than pyrite. It cannot be confidently determined how pyrrhotite fits into the paragenetic sequence. In the sections shown in Figure 13a,b a second euhedral generation of pyrite is present. Figure 13d shows the above relationship between pyrrhotite, marcasite, and pyrite in an SEM image. Marcasite and pyrite cannot be distinguished in this image, but if the brightness and contrast levels are adjusted variations in the greyscale can be observed. In sample 235.3, there is evidence of some alteration effect on the pyrrhotite (Figure 14a,b). It appears that the euhedral arsenopyrite and surrounding pyrrhotite were present together initially. The pyrrhotite was then altered giving it a lobate texture and changing it to a more grey color with a brown patch in the middle. Pyrite replaces pyrrhotite along the grain boundaries and along discontinuous fractures within pyrrhotite grains. The replacement is irregular and does not occur along specific grain fractures. 5.2.3 MARCASITE Marcasite is a minor sulphide relative to pyrite and pyrrhotite. It occurs either as large fine grained masses that are well banded, contain a large number of inclusions and are cut by carbonates, or as a fine grained rims being recrystallized by pyrite around pyrrhotite. The fine grained masses of marcasite are primarily seen in samples 18-108.2 and 18-121.12. In Figure  26  Figure 12. Relationship between pyrrhotite and pyrite. (a) Euhedral pyrite in contact with pyrrhotite which is intimately linked with talc; (b) Transmitted light photo showing fine grained talc; (c) Pyrite occurring as a fine rim around pyrrhotite grains and along grain boundaries in association with carbonate; (d) Large pyrite grain sandwiched between pyrrhotite grains and pyrite occurring as small fragments along the edges of the pyrrhotite. Po=pyrite, Py=pyrite. 15a,b a pyrite grain is shown that developed in the central part of the marcasite mass in sample 18-108.2. Pyrite is also present along a fracture in the marcasite that extends away from the pyrite grain and crosscuts the well developed lobate banding in the marcasite. Figure 15c shows a chalcopyrite grain that also crosscuts the marcasite banding. These photomicrographs are taken in different locations in the same thin section. Figure 15d is a  27  Figure 13. Sulphide relationships. (a) and (b) Pyrrhotite surrounded by marcasite which is recrystallized by pyrite. A euhedral generation of pyrite is also present, and chalcopyrite is seen in (b); (c) Euhedral pyrite grain in contact with pyrrhotite that is rimmed b pyrite and has fractures in filled by pyrite; (d) SEM photo showing the rimmed pyrrhotite. Po=pyrrhotite, Py=pyrite, Do=dolomite, Sid=siderite, Mrc=marcasite, Musc=muscovite. photomicrograph taken from sample 18-121.12 that shows the relationship between pyrrhotite, pyrite and marcasite. Pyrite grains are again shown crosscutting the banding developed in marcasite. The banding developed in the marcasite is less developed and less lobate than that seen in sample 18-108.2. Pyrrhotite is the dominant sulphide in this section and encloses the marcasite. Based on the relationships discussed above, marcasite is an  28  Figure 14. Pyrrhotite alteration and relationship between sulphides. (a) Alteration of pyrrhotite with replacement by pyrite along the edges of grains as discontinuous fractures, and euhedral arsenopyrite grains; (b) SEM image of the same section; (c) and (d) Arsenopyrite with variations in arsenic content in contact with pyrite, with bismuthenite occurring at grain boundaries. Py=pyrite, Po=pyrrhotite, Apy=arsenopyrite, Bis=bismuthenite, Do=dolomite, Sid=siderite. earlier mineral phase than both pyrite and chalcopyrite, but pyrrhotite is difficult to fit into the paragenetic sequence. 5.2.2 ARSENOPYRITE AND ARSENIC-BEARING PYRITE  29  Figure 15. Relationship between banded marcasite and surrounding sulphides. (a) and (b) Late stage pyrite overprints the banding in early stage marcasite; (c) Contact between marcasite and pyrrhotite, with chalcopyrite overprinting banding in the early stage marcasite and arsenopyrite occurring along the grain boundary; (d) Well-banded marcasite grain in contact with pyrrhotite. Po=pyrrhotite, Py=pyrite, Cpy=chalcopyrite, Mrc=Marcasite, Apy=arsenopyrite. In the majority of the samples, arsenopyrite accounts for less than 5% of opaques and occurs as small euhedral grains (Figure 16c). Samples 11-235.3 and 11-235.8 were taken from the bottom of the main mineralized zone of diamond drill hole 11. They both contain significantly more arsenopyrite, up to 30 % of opaques, than the other samples. Moreover, there is an interesting textural relationship in these samples between arsenopyrite grains,  30  Figure 16. Relationship between arsenopyrite and pyrite in the presence of arsenic-rich pyrite. (a) and (b) distinct euhedral arsenopyrite grains in contact with arsenic-rich pyrite, (c) SEM image of arsenopyrite and pyrite, (d) SEM image of arsenic element map for the same area shown in (c). Apy=arsenopyrite, Py=pyrite, As-py= arsenic-bearing pyrite. pyrite, and an arsenic-bearing pyrite (Figure 16). Arsenopyrite occurs as euhedral grains, which is a characteristic common of arsenopyrite, and displays a sharp contact with the surrounding pyrite. Within the pyrite there are zones that show the typical pale brassy yellow color of pyrite and zones that are the same white color characteristic of arsenopyrite. The transition between these zones appears to be gradational. Pyrite and arsenopyrite have a similar hardness (6-6.8 and 5.5-6, respectively) and therefore would display similar relief. From examination of this relationship using BSE/EDS methods, including an arsenic element 31  map, it appears that sections of the pyrite are arsenic-bearing wheras others are not (Figure 16c,d). Arsenopyrite grains show variability in arsenic content. Figure 14c,d shows SEM images of sample 235.3 where arsenopyrite grains appear the same but whose spectra show distinct variations in arsenic content. 5.2.3 ACCESSORY MINERALS Bismuthenite is a common accessory mineral in these samples, and occurs primarily as submicroscopic grains along grain boundaries and fractures within the primary sulphides throughout all of the samples. In samples 18-108.2 and 18-206.7, bismuthenite occurs as grains that are visible with an optical microscope and have a distinctive pink-beige color. Bismuthenite occurs as two fracture fillings in sample 108.2, and is associated with chalcopyrite (Figure17a,b). A section of the bismuthenite in this sample displays the pink color typical of bismuthenite, but the majority of it is full of inclusions and appears to be altered. Along the edges of the fracture fillings, the pink color typical of bismuthenite is visible and the alteration appears to cap the bismuthenite that develops as fracture fillings. A small section of chalcopyrite develops at the end of one of the strands and is bordered by bismuthenite on either side. Sphalerite occurs right next to the bismuthenite fracture fillings (Figure 17b), and also appears to be included within the bismuthenite fracture fillings. Bismuthenite was also identified in the thin section for sample 18-206.7 (Figure 18c,d). The anhedral grains occur along grain boundaries of pyrite and display the typical pink-beige color. The softness of bismuthenite (hardness=2) is evidenced here as it develops in a concave cavity within the surrounding pyrite (hardness 6-6.8). SEM images in Figure 17c,d show submicroscopic bismuthenite occurring along fractures and grain boundaries as well as individual grains associated with sulphides. Bismuthenite is most commonly observed as fracture fillings, but where it occurs as individual grains it is not preferentially associated with any one of the three main sulphides. Sphalerite, as described above, is observed rarely in the sections and is commonly closely associated with bismuthenite. It occurs consistently across the samples as individual isolated grains, but is a minor phase. Commonly it forms along fractures and grain boundaries within pyrite as seen in samples 11-219.1 and 11-235.3 (Figure 19a,b)  32  Figure 17. Bismuthenite in thin section. (a) Long individual bismuthenite fracture fillings associated with chalcopyrite surrounded by pyrrhotite; (b) Alteration of bismuthenite and association with sphalerite; (c) SEM image showing bismuthenite occurring along fractures; (d) SEM image showing bismuthenite occurring along fractures and as individual grains, some with an association to arsenopyrite. Py=pyrite, Po=pyrrhotite, Cpy=chalcopyrite, Apy=arsenopyrite, Bis=bismuthenite, Sp=sphalerite, Qtz=quartz. Chalcopyrite is a minor sulphide phase that does not show consistent textural relationships or mineral associations. It is consistently seen within the samples but accounts for a very small proportion of the opaques in a given thin section. It always occurs as individual isolated grains, typically small and euhedral (Figure 19c,d)  33  Figure 18. Marcasite and bismuthenite occurrences. (a) SEM image showing contact between marcasite and pyrite; (b) Well banded marcasite in cut by carbonate matrix; (c) and (d) Association of bismuthenite with sphalerite along grain boundaries. Py=pyrite, Mrc=marcasite, Sp=sphalerite, Bis=bismuthenite, Qtz=quartz. Goethite is present in most of the samples in minor amounts. In samples 11-162 and 11-219.1 goethite accounts for up to 20% of the non-matrix minerals. The fine grained goethite in these samples forms masses and is disseminated in bands that overprint the surrounding sulphides (Figure 20c). In sample 18-121.12 a euhedral goethite mass occurs within the carbonate matrix next to pyrrhotite and is partially rimmed by chalcopyrite (Figure 20a,b). In reflected light, the grain is characteristic of goethite because it has low reflectance,  34  Figure 19. Sphalerite and chalcopyrite occurrences. (a) and (b) Sphalerite grains developed on grain boundaries and fractures of arsenopyrite and pyrite; (c) Chalcopyrite occurring within a mass of pyrrhotite which is rimmed by pyrite; (d) Chalcopyrite in association with pyrrhotite. Sp=sphalerite, Po=pyrrhotite, Cpy=chalcopyrite, Py=pyrite, As-py= arsenicbearing pyrite. is a grey-blue color and takes a good polish (Figure 20a). The reddish brown internal reflections shown in Figure 20b are also characteristic of goethite. 5.2.4 HOST ROCK MINERALOGY AND ALTERATION  35  Figure 20. Goethite occurrences. (a) and (b) Euhedral goethite grain, partially rimmed by chalcopyrite, developed between pyrrhotite grains; (c) Fine grained euhedral goethite, that forms a band and overprints pyrite; (d) Fine grained euhedral goethite in contact with pyrrhotite and pyrite. Gt=goethite, Po=pyrrhotite, Cpy=chalcopyrite, Py=pyrite. Analysis using BSE/EDS methods shows that the matrix mineralogy is dominated by calcite with varying amounts of Fe and Mg, and a host of minor elements (Figure 21). Dolomite and siderite are common as small euhedral grains. Quartz is minor, typically occurring as small grains and less commonly as late veins. Minor elements include Al, Ti, and V, but it is unclear in which minerals they occur. Talc is common as fine grained fan shaped masses (Figure 22a), and is commonly observed cutting into the edges of sulphide grains (Figure 12a,b). Muscovite is a minor mineral which occurs as lathes and is intimately 36  Figure 21. Matrix mineralogy. (a) Magnesium rich minerals surrounding calcite and intruding into a euhedral pyrite grain; (b) Calcite matrix with titanium rich silicate and calcite minerals in the vicinity; (c) Calcite matrix in the presence of siderite and dolomite; (d) Muscovite laths, mg-silicate minerals and quartz with minor carbon dominate the matrix. Py=pyrite, Qtz=quartz, Bis=bismuthenite, Musc=muscovite. associated with pyrite and pyrrhotite (Figure 22b). As discussed above, there are some alteration effects on the sulphide mineralogy. Alteration effects on the host rock were not commonly noted in the polished thin sections. In sample 235.3, a brown alteration effect borders pyrite and pyrrhotite (Figure 22c,d). The alteration occurs within a host rock entirely composed of carbonates, and it is iridescent in reflected light. In goethite alteration it is common to see the edges of sulphide grains being “eaten away” along the grain boundaries,  37  Figure 22. Matrix mineralogy and alteration products. (a) Fine-grained, fan shaped talc dominates the matrix of sample 11-187.7; (b) Muscovite lathes in a carbonate matrix; (c) and (d) Reflected and transmitted light images showing an alteration feature rimming sulphide grains. Po=pyrrhotite, Py=pyrite, Musc=muscovite. that effect is not seen here. The contact between the sulphide grains and the alteration is smooth, with the alteration simply overprinting the carbonate matrix and the outer sulphide edge.  38  CHAPTER 6 RESULTS OF GEOCHEMISTRY AND LEAD ISOTOPIC STUDIES 6.1 GEOCHEMISTRY Whole rock geochemical analyses from two of the dykes in the central part of Rau property (from L. Groat, unpublished data, 2009) were compared with five whole rock geochemical analyses of unfoliated Late Cretaceous (67-69 Ma) intrusions in the McQuesten River region (from Murphy, 1997), including the Two Sisters intrusion, the Oliver Ridge intrusion and two miscellaneous dykes. The data indicate that in terms of major element composition the Rau dykes fall within the compositional fields defined for main McQuesten Suite plutons, which range from the diorite to granite fields. Within the McQuesten Suite, the Two Sisters pluton samples have more felsic compositions than the more mafic Oliver Ridge intrusion and miscellaneous dyke samples. Major element data are plotted on a total alkalis vs. SiO2 plot (Lebas et al., 1986) in Figure 23. A Shand-type diagram showing degree of aluminum saturation in granitoids (Maniar and Piccoli, 1989) is presented in Figure 24. McQuesten Suite plutons plot at the metaluminous/peraluminous boundary or as weakly peraluminous in this figure, whereas the Rau dykes plot as weakly to moderately metaluminous. Trace element geochemical signatures show a clear distinction between the Rau dykes and the McQuesten Suite plutons. On a plot of Rb vs. Yb+Ta (Pearce et al., 1984), the Rau dykes fall well inside the within-plate granite (WPG) field, whereas the McQuesten Suite plutons plot either in the volcanic arc granite (VAG) or syn-collisonal granite (synCOLG) fields (Figure 25). 6.2 LEAD ISOTOPIC STUDIES Table 2 shows the Pb isotopic data for the samples that were analyzed in this study, and Table 3 provides a brief description of the mineralogy and texture of the rock from which the samples were taken. The data are plotted together with Pb isotopic analyses from feldspars of typical Tombstone and McQuesten Suite intrusions (from J.K. Mortensen,  39  Figure 23. Total alkali vs. silica diagram (Lebas et al., 1986) showing major element chemistry of the Rau dykes (pink diamonds) and the main McQuesten Suite plutons (blue squares).  Figure 24. Shand-type plot (Maniar and Piccoli, 1989) for discrimination of magma source for the Rau dykes (pink diamonds) and the main McQuesten Suite plutons (blue squares).  40  Figure 25. Tectonic discrimination plot (Pearce et al., 1984) for the Rau dykes (pink diamonds) and the McQuesten Suite intrusions (blue squares). unpublished data, 2009) on 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/206Pb vs. 207Pb/206Pb plots in Figure 26. Lead isotope data from sulphide veins associated with the Tombstone Suite intrusions are also plotted. The “shale curve” of Godwin and Sinclair (1982) is a very well calibrated model growth curve for the upper crust and is specific to the North American miogeocline in western Canada. Godwin and Sinclair (1982) developed the shale curve based on a compilation of Pb isotopic data from a large collection of shale hosted syngenetic massive sulphide (SEDEX)-type deposits throughout the North American Cordilleran miogeocline. The shale curve, because of its excellent calibration, has been widely employed both to date SEDEX-style mineralization in the area of the North American miogeocline, as well as to distinguish between syngenetic and epigenetic mineralization in the area (e.g. Mortensen et al., 2006).  41  Table 2. Pb isotopic compositions. Sample Number Rau 5 Rau 7 Rau 11 Rau 19 Rau 6 Rau 9 Rau 10 Rau 11 Rau 12 Rau 13 Rau 14 Rau 15 Rau 16 Rau 17 Rau 18 Rau 23 Rau 21 Rau 22 Rau 1 Rau 2 Rau 3 volc Rau 20 Rau 45 Rau 49 Rau 53  Mineral py py py py py py py py py py py py py py py py py py carb carb wr fs fs fs fs  206Pb/ 204Pb Error 19.8679 0.10 20.0135 0.04 20.2330 0.03 20.0370 0.05 20.0305 0.28 19.6653 0.09 20.1438 0.04 19.9695 0.11 19.8541 0.04 17.4038 0.51 19.9620 0.08 20.0201 0.03 21.0458 0.87 19.5878 0.08 19.8705 0.13 19.1050 0.03 19.9971 0.06 20.3292 0.52 20.6270 0.10 20.3158 0.44 19.9577 0.16 19.9455 0.04 19.8208 0.03 19.7966 0.03 19.7683 0.17  207Pb/ 204Pb Error 15.6925 0.10 15.7285 0.05 15.7567 0.04 15.7364 0.05 15.6472 0.17 15.5958 0.10 15.7676 0.05 15.7394 0.08 15.6994 0.05 14.9707 0.50 15.7691 0.09 15.7145 0.05 15.9404 0.83 15.7100 0.09 15.8446 0.12 15.6345 0.05 15.7292 0.05 15.8078 0.49 15.7912 0.10 15.6955 0.43 15.9108 0.16 15.7671 0.05 15.7110 0.04 15.6972 0.05 15.6761 0.15  208Pb/ 204Pb Error 40.1013 0.12 40.4905 0.07 40.5737 0.06 40.4213 0.08 40.0179 0.36 39.7841 0.11 40.3922 0.07 40.3329 0.14 40.1731 0.07 36.4137 0.51 40.2087 0.10 40.4340 0.07 38.5095 0.93 39.8264 0.10 40.2480 0.14 38.9762 0.06 40.3061 0.09 40.9148 0.55 40.5067 0.12 40.2585 0.44 39.7509 0.18 40.1895 0.08 40.0372 0.06 40.0283 0.07 40.1358 0.18  207Pb/ 206Pb 0.7898 0.7859 0.7788 0.7854 0.7812 0.7931 0.7828 0.7882 0.7907 0.8602 0.7900 0.7849 0.7574 0.8020 0.7974 0.8183 0.7866 0.7776 0.7656 0.7726 0.7972 0.7905 0.7927 0.7929 0.7930  Error 0.051 0.039 0.038 0.045 0.225 0.040 0.039 0.089 0.038 0.092 0.039 0.039 0.249 0.039 0.055 0.038 0.059 0.188 0.060 0.059 0.050 0.039 0.038 0.039 0.093  208Pb/ 206Pb 2.0184 2.0232 2.0053 2.0173 1.9978 2.0231 2.0052 2.0197 2.0234 2.0923 2.0143 2.0197 1.8298 2.0332 2.0255 2.0401 2.0156 2.0126 1.9638 1.9816 1.9918 2.0150 2.0200 2.0220 2.0303  Error 0.052 0.032 0.032 0.045 0.217 0.042 0.033 0.077 0.033 0.057 0.033 0.034 0.326 0.032 0.038 0.032 0.044 0.153 0.044 0.042 0.053 0.045 0.032 0.034 0.054  Analyses by Janet Gabites, Geochronology Laboratory, Department of Earth and Ocean Sciences, The University of British Columbia. Results have been normalized using a fractionation factor of 0.12% based on multiple analyses of NBS981 standard lead, and the values in Thirlwall., 2000. Errors are reported at the 2 sigma level. fs = feldspar, py = pyrite, carb = carbonate whole rock, volc wr = volcanic whole rock.  The Tombstone Suite and McQuesten Suite feldspars show complete overlap and plot slightly below the shale curve. They fall within the field that outlines Pb isotopic data defining sulphide veins associated with the Tombstone Suite plutons. The volcanic sample and two samples of unaltered hanging wall carbonates from the Rau gives strongly radiogenic compositions, with the volcanic sample yielding a higher 207Pb/204Pb ratio but lower 206Pb/204Pb ratio than the two carbonate samples (Figure 26). The Rau feldspars are distinctly more radiogenic than, and plot well away from, feldspars from the Tombstone and McQuesten Suite intrusions. Both of the plots show the bulk of the sulphide samples plotting  42  Table 3. Description of mineralogy and texture of the rocks surrounding the location from which the samples were taken for Pb isotopic analyses. Sample #  Mineral  Hole-meterage Hand Sample Description massive pyrite (euhedral) oxidized, arsenopyrite along foliations, qtz/calc clasts 18-156.7 upper edge of main mineralized zone, dolomite, m.gr blebby/disseminated py/po, sericite in voids 18-129.6 massive pyrite (euhedral), quartz/calcite host rock, sphalerite (orange/red, glassy) with qtz along infills 18-225.8 18-231.65 massive pyrite, small quartz and calcite inclusions  Rau 5  py  Rau 7  py  Rau 11 Rau 19  py py  Rau 6  py  18-89.05  Rau 9  py  18-140.23  Rau 10  py  18-102.71  Rau 11  py  18-225.8  Rau 12  py  18-149.35  Rau 13  py  18-107.9  Rau 14  py  18-73.65  Rau 15  py  18-224.03  Rau 16  py  18-167.1  Rau 17  py  11-216.3  Rau 18 Rau 23  py py  11-199.35 11-235.5  Rau 21  py  18-60.6  grey-brown talc-sulphide, strongly foliated, pyrite along foliations main mineralized zone, dolomite, 1.96 g/t Au, massive pyrite grading into blobby m.gr pyrite dark-grey to white, not as well foliated as surrounding rocks, chlorite, minor disseminated pyrite euhedral massive pyrite set in quartz/calcite host, sphalerite (orange/red, glassy) with quartz massive pyrite (~70% of sample), quartz/calcite host, small quartz clasts within massive pyrite large (~3cm) euhedral pyrite, near qtz vein with syntaxial vein growth, not as well foliated as surrounding small (~2-5mm) euhedral pyrite, quartz/carbonate host rock, near transition into volcanic rock massive pyrite, small quartz inclusions (3-5mm), very fine grained soft black mineral main mineralized zone, dolomite, massive pyrite surrounded by disseminated pyrite, qtz clasts in pyrite main mineralized zone, dolomite, disseminated pyrite forming parallel bands main mineralized zone, dolomite, disseminated arsenopyrite, talc on fractures disseminate pyrite and minor arsenopyrite, 1.62 g/t Au brecciated dolostone, fine grained pyrite with a very fine grained black mineral, vuggy quartz  py carb carb wr  18-89.2 18-41.76 18-23.6 18-91.0  grey-brown talc-sulphide, strongly foliated, pyrite along foliations carbonate whole rock, dark grey/black, unmineralized carbonate whole rock, light grey, quartz+biotite, unmineralized volcanic whole rock  Rau 22 Rau 1 Rau 2 Rau 3 volc  along a mixing trend between the Rau feldspars and the Rau carbonates, with most clustering closer to the feldspar compositions. There is a substantial range in Pb isotopic signature of the sulphides, however, with a small proportion of the samples yielding compositions that are significantly less radiogenic than the Rau feldspars.  43  Figure 26: Plot of 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/206Pb vs. 207Pb/206Pb for Pb isotopic analysis. The dashed line outlines the field of composition for sulphide veins associated with Tombstone Suite plutons. Solid line represents the “shale curve” of Godwin and Sinclair (1982).  44  CHAPTER 7 DISCUSSION AND CONCLUSIONS 7.1 PETROGRAPHIC STUDY The detailed petrographic study that was carried out indicates that the Rau occurrence is characterized by a simple mineralogy and a textural complexity. The mineralogy and textures of the Rau mineralization are summarized below, and some aspects are compared to those described by Fonseca (1998) for the Ketza River deposit. Principal sulphide minerals at the Rau occurrence include of pyrite, pyrrhotite, marcasite, arsenopyrite, minor chalcopyrite and sphalerite. Bismuthenite is common and typically occurs as submicroscopic grains. Talc is a common accessory phase, and the matrix is dominated by calcite together with a variety of Mg- and Fe-bearing carbonate minerals. Quartz is minor. Gold was not observed in any of the samples studied. Goethite occurs as a late alteration phase. Pyrite and pyrrhotite are the dominant sulphides present and commonly occur together within the samples. A wide range of textural variations in pyrite suggest multiple generations. Marcasite is an early stage mineral; it is both crosscut by late pyrite and recrystallized to pyrite in a number of samples, and cut also by late-stage chalcopyrite in one sample. In contrast, Fonseca (1998) found that at the Ketza River deposit marcasite was the latest stage mineral, commonly replacing early pyrite. Based on the limited number of samples analyzed in this study it is not possible to show conclusively how pyrrhotite fits into the paragenetic sequence, although in general it occurs in close association with pyrite and in the center of masses that are bordered by marcasite that has been partially recrystallized to pyrite. Sphalerite and chalcopyrite are minor phases that occur consistently within the samples analyzed. Fonseca (1998) noted that arsenopyrite was a dominant sulphide in all three of the mineralization styles seen at the Ketza River deposit, with arsenopyrite in the metasedimentary rock hosted quartz-sulphide veins accounting for between 30-100% of opaques present (Fonseca, 1998). In contrast, the samples analyzed for the Rau occurrence show arsenopyrite as a typically minor phase, commonly euhedral and associated with pyrite and pyrrhotite. Analysis of pyrite shows that some sections are arsenic-bearing.  45  The matrix mineralogy is dominated by carbonates, ranging from calcite to siderite and dolomite. The presence of talc within a large number of the samples is likely attributable to the substantial Mg content of the host dolostones. The availability of Mg in the element budget of the system allows for recrystalization to talc. The host rocks at the Ketza River deposit, in contrast, are mainly limestone which may explain the lack of talc associated with sulphide mineralization there (Fonseca, 1998). Goethite is a Fe-bearing secondary hydroxide, and appears to have formed during late oxidation of sulfides such as pyrite or pyrrhotite; however, based on the euhedral outline of some of the goethite masses it likely also replaces siderite. A detailed examination, utilizing both reflected light microscopy and SEM methods, was carried out of all of the fifteen polished thin sections of a variety of sulphide styles, including some from within intersections that yielded significant gold values. No visible gold was identified in any of the samples, and, although the sample suite that was examined is not exhaustive, it appears likely that gold does not occur as free gold in the system. Gold may occur in solid solution, possibly in the arsenic-bearing pyrite. An association between gold and the arsenic-bearing pyrite was considered but a thorough investigation of samples 11-235.3 and 11-235.8, where the arsenic-bearing pyrite is dominant, did not reveal visible gold. 7.2 GEOCHEMISTY Major element geochemistry shows that the Rau felsic dykes are compositionally somewhat similar to the more felsic portions of the McQuesten Plutonic Suite (Figure 23). Most of the McQuesten Suite plutons are slightly to moderately peraluminous (Figure 24), suggesting that they are derived mainly from partial melting of a sedimentary source and can be classified as S-type in origin. On the basis of trace element compositions, the McQuesten Suite intrusions fall either in the volcanic arc granite or syn-collisional granite fields (Figure 25). The Rau dykes differ from the McQuesten Suite intrusions, however, in that they yield weakly to moderately metaluminous compositions (Figure 24) , suggesting that the intrusions at the Rau were not entirely derived from melting of sediments. They may include a minor mantle-derived component, or alternatively could represent partial melts of either sediment  46  eroded from an arc (rather than simple pelitic compositions) or of underlying crystalline basement rocks. The trace element geochemistry of the Rau dykes and McQuesten Suite intrusions also shows that there is a clear distinction between the McQuesten Suite intrusions and the Rau intrusions, with the Rau dykes falling well within the within-plate granite field, rather than the volcanic arc or syn-collisional granite fields. Geochemical data suggests that although the Rau dykes appear to have approximately the same crystallization age as the McQuesten Suite intrusions, they are likely derived from different source rocks. It is possible that these different sources for Late Cretaceous magmatism in the area might correspond to significantly different associated metallogenic signatures. There are currently mineral occurrences presently known to be associated with McQuesten Suite intrusions (Murphy, 1997). The distinction between the Late Cretaceous McQuesten Suite intrusions and the coeval but distinct magmatic event represented in the vicinity of the Rau occurrence by the Rackla pluton and associated Rau dykes therefore may provide an important factor for targeting on-going exploration in the Keno Hill District area and possibly elsewhere in the northern Cordillera. 7.3 LEAD ISOTOPIC STUDIES The fact that the range of Pb isotopic compositions for Tombstone and McQuesten suite feldspars falls immediately below the “shale curve” (Figure 26) is consistent with derivation of these magmas mainly by partial melting of supracrustal rocks of the Selwyn Basin. The Tombstone and McQuesten suites are relatively close in age, at 92 ± 2 Ma and 65±2 Ma, respectively (Murphy, 1997). Based on this it would be expected that they would give similar 206Pb/204Pb values along the “shale curve”, as is seen here on the uranogenic Pb plot (Figure 26). However, their range of 207Pb/204Pb compositions with respect to the “shale curve” is interesting. Tombstone Suite intrusions are typically metaluminous and more mafic, with average normalized SiO2 compositions of 64.6%, whereas McQuesten Suite intrusions tend to be peraluminous in composition and somewhat more felsic, with average normalized SiO2 compositions of 66.9% (Murphy, 1997). Thus, although compositionally the McQuesten Suite plutons could represent entirely crustal derived magmas, the compositions of the Tombstone plutonic suite would have been expected to give somewhat less radiogenic  47  Pb compositions, indicating a significant mantle input. The Pb isotopic compositions of Tombstone Plutonic Suite feldspars appear to indicate that the magmas have been strongly contaminated by Selwyn Basin type sedimentary components. The feldspars of the Rau dykes are much more radiogenic (higher 207Pb/204Pb and 206  Pb/204Pb ratios) than the feldspars from the McQuesten Suite intrusions. Since lead derived  from different source reservoirs tends to be isotopically distinct, this suggests that the Rau dykes must have formed by melting a somewhat different crustal source than that which produced the main McQuesten Suite intrusions. The range of isotopic signatures for the Rau feldspars relative to the “shale curve” suggests that the source was likely upper crustal but may have included underlying crystalline basement rocks as opposed to typical Selwyn Basin supracrustal rocks. The major and trace element geochemistry discussed above also suggests that there must have been different source rocks for the Rau dykes and McQuesten Suite plutons. The clustering of sulphide Pb isotopic signatures around those of the Rau feldspars is consistent with the metals being derived mainly from the intrusions themselves. The majority of the sulphide Pb signatures plot along a theoretical mixing line between the signature of the Rau feldspars and that of the Rau carbonates. This suggests that Pb in the sulphides reflects variable mixtures of Pb produced when hydrothermal fluids derived from the crystallizing Rau intrusions moved laterally through the host carbonate rocks and altered and replaced them. The four samples that show the most radiogenic signatures and plot closest to the Rau carbonates would have derived more Pb from the carbonates, presumably due to greater wall rock interaction. These four samples do not show any characteristics in terms of mineralization style or texture that distinguish them from the other sulphide samples. Three sulphide samples are less radiogenic and plot to the left of and below the Rau feldspars signature. All three of these samples are from drill hole 11, and all three were taken from zones of fine grained disseminated pyrite. The significance of the less radiogenic components is uncertain. It is possible that the fine grained pyrite that was sampled includes a component of pre-existing, syngenetic sulphides, which would have given a significantly less radiogenic signature than the Rau feldspars (Mortensen et al., 2006). Alternatively, there may be a significant variability in the Pb isotopic composition of the various carbonate units that were replaced in different parts of the Rau occurrence and those in drill hole 11 may have been less radiogenic than the values determined for carbonates in the upper part of drill  48  hole 18. The isotopic signature of the volcanic sample plots well away from the other isotopic signatures. Unfortunately, this was the only volcanic sample submitted for analysis that ran. Based on limited data available, however, it appears that the volcanic rocks have not contributed any significant amount of Pb into the system, and it is therefore inferred that the volcanic rocks did not play a role in the formation of the mineralization, except perhaps in helping channel fluid flow 7.4 RECOMMENDATIONS FOR FUTURE WORK The most critical aspect of any future research on the Rau occurrence will be to establish the mode of occurrence of gold, since this has major implications for the economics for the deposit. If the gold does in fact occur in solid solution, possibly within arsenian pyrite, rather than as free grains, this will make the metallurgical extraction of the gold considerably more challenging. Fonseca (1998) undertook preliminary cathodoluminescence studies and identified a hydrothermal signature in cathodoluminescence at the Ketza River deposit. Cathodoluminescence is the emission of visible light by a sample excited by a beam of electrons. Manganese is generally a cathodoluminescence activator, whereas iron tends to be an inhibitor (Pierson, 1981). The high concentration of Fe in the host rocks observed at the Rau occurrence suggests that cathodoluminescence would not be particularly diagnostic. However, it may be worthwhile to carry out reconnaissance cathodoluminescence studies in less Fe-rich rocks from the Rau occurrence to try to better constrain the extent of hydrothermal alteration. Preliminary ultra-violent (UV) fluorescence studies were conducted during the petrographic analysis section of this project to attempt to identify variations in carbonate compositions in the host rocks. The sections that were studied unfortunately did not show strong or variable fluorescence. This can likely be attributed to the elevated contents of Mg and Fe in the host rocks. Strong fluorescence colors are associated with minor trace element contents of Mn, Pb, and Zn, whereas elevated contents of Mg and Fe tend to suppress fluorescence under UV light (Gies, 1975). As with cathodoluminescence, it may be  49  worthwhile to investigate sections of the Rau occurrence with lower Fe and Mg contents to analyze carbonate variations. Further work should be done to relate the dykes and sills in the central part of the Rau property to the Rackla pluton itself. A small number of zircons were separated from a Rau dykes sample; however, these could not be dated due to instrumental problems. Samples of muscovite were also separated from two of the Rau pegmatitic dyke samples for 40Ar/39Ar dating; however, these could not be dated either due to delays in the PCIGR noble gas laboratory. 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