Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
DSpace
UBCV
Parry, Robert Keagan
The Klondike Gold District (KGD) is situated in west-central Yukon, located on the pericratonic Yukon-Tanana Terrane. The region is a major producer of placer gold, famous for the Gold Rush of 1896, and has produced an estimated 10 Moz. of gold (Chapman at al., 2010) derived from gold-bearing quartz veins and greenschist Klondike Schist host rocks. This research investigates the structural controls on mineralization of the Lone Star Ridge area, providing new field observations and interpretations of the sequence of deformation events related to vein formation and gold mineralization. Overprinting relationships of ductile deformation and brittle, mineralized veins in field observations have provided evidence for deformation outlasting hydrothermal vein emplacement, contrary to the post-deformation vein emplacement hypothesis outlined in recently published literature. Detailed petrographic investigations and cathodoluminescence imaging of quartz veins and altered wall rock indicate that post-hydrothermal vein deformation has occurred and that strain partitioning may have played a role in enhancing gold grades. The evidence for post-hydrothermal vein emplacement has broad implications for the deformational history of the KGD, concluding that ductile deformation related to regional thrusting was ongoing though hydrothermal vein emplacement. Historical exploration has focused on discovering lode gold associated with primary hydrothermal veins, whereas the overprinting deformation affecting these veins may have potentially remobilized gold to later precipitate into structures controlled by the overprinting deformation.
i Mechanisms of Post-Hydrothermal Deformation in Orogenic Gold Deposits: A Case Study of the Klondike Gold District, Yukon By Robert Keagan Parry 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) APRIL 2018 © R. Keagan Parry, 2018 ii Table of Contents Title Page .................................................................................................................................................. i Table of Contents ..................................................................................................................................... ii List of Figures ......................................................................................................................................... iii List of Tables .......................................................................................................................................... iv Acknowledgements .................................................................................................................................. v Abstract .................................................................................................................................................... 1 Chapter 1: Introduction ............................................................................................................................ 2 Chapter 2: Regional Geology ................................................................................................................... 3 2.1 Tectonic History.................................................................................................................... 3 2.2 Regional Geology ................................................................................................................. 6 2.3 Deformational History .......................................................................................................... 9 2.4 Overview of Lone Star Ridge Study Area .......................................................................... 13 Chapter 3: Methods ................................................................................................................................ 15 3.1 Fieldwork ............................................................................................................................ 15 3.2 Petrography ......................................................................................................................... 15 3.3 Cathodoluminescence ......................................................................................................... 15 Chapter 4: Core Logging Observations ................................................................................................. 18 Chapter 5: Petrographic Results ............................................................................................................. 22 5.1 LS16-69_88.1m .................................................................................................................. 23 5.2 LS16-69_88.8m .................................................................................................................. 26 5.3 LS16-69_89.0m_A .............................................................................................................. 28 5.4 LS16-69_89.0m_B .............................................................................................................. 31 5.5 LS16-69_89.4m .................................................................................................................. 34 5.6 LS16-69_89.6m .................................................................................................................. 36 5.7 LS16-69_90.4m .................................................................................................................. 38 Chapter 6: Cathodoluminescence Observations..................................................................................... 40 Chapter 7: Discussion ............................................................................................................................ 44 Chapter 8: Conclusion ............................................................................................................................ 47 Chapter 9: References Cited .................................................................................................................. 49 Appendices ............................................................................................................................................. 53 iii List of Figures 2.1.1 Map of Northern Cordillera ............................................................................................................ 3 2.1.2 Schematic diagram of YTT accretion ............................................................................................. 5 2.2.1 Map of Klondike District ................................................................................................................ 7 2.3.1 Examples of deformation fabrics in hand sample ......................................................................... 10 2.3.2 Deformation overprint of discordant veins ................................................................................... 12 2.4.1 Map of Lone Star Ridge ................................................................................................................ 13 4.1 Schematic log of LS16-69, depths 88.0m to 90.5m ......................................................................... 18 4.2 Mineralized quartz vein and visible gold in silicified halo .............................................................. 19 5.0 Stitched photograph of core showing thin section locations ............................................................ 22 5.1.1 Thin section scans of LS16-69_88.1m .......................................................................................... 23 5.1.2 Photomicrograph of a carbonate porphyroblast in LS16-69_88.1m ............................................. 24 5.1.3 Photomicrograph of quartz-carbonate segregation in LS16-69_88.1m ........................................ 25 5.2.1 Thin section scans of LS16-69_88.8m .......................................................................................... 26 5.3.1 Thin section scans of LS16-69_89.0m_A ..................................................................................... 28 5.3.2 Photomicrograph of contact of vein and country rock in LS16-69_89.0m_A .............................. 29 5.3.3 Annotated photomicrograph showing principle stress axis and associated fractures ................... 30 5.4.1 Thin section scans of LS16-69_89.0m_B ..................................................................................... 31 5.4.2 Photomicrograph of microcrystalline quartz and pyrite in LS16-69_89.0m_B ........................... 32 5.4.2 Photomicrograph of carbonate filled fractures in LS16-69_89.0m_B ......................................... 32 5.5.1 Thin section scans of LS16-69_89.4m .......................................................................................... 34 5.6.1 Thin section scans of LS16-69_89.6m .......................................................................................... 36 5.7.1 Thin section scans of LS16-69_90.4m .......................................................................................... 38 6.1 CL image of LS16-69_89.0m_A quartz vein and country rock margin .......................................... 40 6.2 CL image of LS16-69_89.0m_A sulphides ..................................................................................... 41 6.3 Triptych of Microcrystalline quartz in LS16-69_89.0_B (CL, cross-polarized, plane-polarized) ... 42 7.1 Faulting and drag-folding at 88.7m .................................................................................................. 44 7.2 Annotated photomicrograph showing principle stress axis and associated fractures ...................... 45 7.3 Schematic diagram of interpreted deformation and mineralization ................................................. 46 8.1 Late quartz associated gold ............................................................................................................. 48 iv List of Tables 2.3.1 Summary of deformational history ............................................................................................... 11 3.3.1 Emission bands of crystal lattice defects in quartz ....................................................................... 16 4.1 Summary of core samples ................................................................................................................ 21 8.1 Re-interpreted deformational history diagram ................................................................................. 47 v Acknowledgments First, and foremost, I would like to thank my supervisor, Dr. Murray Allan, for taking me on as a field assistant for the 2017 field season, helping me find my thesis project topic and providing boundless insight and guidance throughout my research. Time spent in the field with Murray was an invaluable learning experience. He patiently answered my seemingly never-ending list of questions and provided insight into the rock-reading abilities of an experienced geologist. I would also like to thank Brodie Stroh for having me as his field assistant in the summer of 2017. Without this opportunity, I would have never been able to visit the plethora of localities in Yukon and observe rocks which permitted my research. I would like to thank the McDougall family for boarding us during our stay in the Sixtymile valley. They were captivating people, providing insight into the workings of a modern-day family-run placer gold operation, and provided a feeling of home even in the remote locality of their property. I am grateful to the folks at MDRU who have allowed me to join the Yukon-Alaska Metallogeny Project, as well as those who assisted me with lab equipment, specifically the SEM technicians and Dr. Farhad Bouzari who trained me on the CL microscope. 1 Abstract The Klondike Gold District (KGD) is situated in west-central Yukon, located on the pericratonic Yukon-Tanana Terrane. The region is a major producer of placer gold, famous for the Gold Rush of 1896, and has produced an estimated 10 Moz. of gold (Chapman at al., 2010) derived from gold-bearing quartz veins and greenschist Klondike Schist host rocks. This research investigates the structural controls on mineralization of the Lone Star Ridge area, providing new field observations and interpretations of the sequence of deformation events related to vein formation and gold mineralization. Overprinting relationships of ductile deformation and brittle, mineralized veins in field observations have provided evidence for deformation outlasting hydrothermal vein emplacement, contrary to the post-deformation vein emplacement hypothesis outlined in recently published literature. Detailed petrographic investigations and cathodoluminescence imaging of quartz veins and altered wall rock indicate that post-hydrothermal vein deformation has occurred and that strain partitioning may have played a role in enhancing gold grades. The evidence for post-hydrothermal vein emplacement has broad implications for the deformational history of the KGD, concluding that ductile deformation related to regional thrusting was ongoing though hydrothermal vein emplacement. Historical exploration has focused on discovering lode gold associated with primary hydrothermal veins, whereas the overprinting deformation affecting these veins may have potentially remobilized gold to later precipitate into structures controlled by the overprinting deformation. 2 1.0 Introduction The Klondike Gold District (KGD) has by far been Yukon’s largest producer of placer gold since the Bonanza Creek discovery in 1896, which marked the beginning of the Klondike Gold Rush. Since then, extraction of placer gold has been unabating and exploration for lode gold sources continues throughout the region. Overall, the Yukon Territory has an estimated minimum total placer gold production of 20 Moz., at least half of which has been reportedly mined out of the Klondike District (Chapman, 2010). Between 2000 and 2006, the territory had an annual production of 100,000oz. (LeBarge, 2007), and to this day it remains a major gold producer. The KGD is considered to host orogenic style gold mineralization, with the majority of gold-bearing quartz veins hosted in the Permian greenschist facies Klondike Schist. Phanerozoic orogenic deposits are typically found in similar greenschist-facies host-rocks within accretionary orogenic belts, several of which have experienced similar mineralization styles and erosional processes, which result in their rich placer deposits (Bierlein & Crowe, 2000). As the Klondike has remained unglaciated since the Pleistocene (Duk-Rodkin, 1999), stream incision has remained largely uninterrupted, contributing to the rich placer deposits, and thoroughly developed soil horizons (MacKenzie et al., 2008). Poor rock exposure in the district due to soil and colluvium cover has restricted exploration for lode sources of gold, therefore creating a research demand to delineate the region’s geology and lode gold sources. This study is a contribution to the Yukon-Alaska Metallogeny (YAM) project, an NSERC-CRD supported collaboration between MDRU – University of British Columbia, members of the exploration industry, and the Yukon Geological Survey. The project was launched in response to the demands of gold and copper exploration companies for geoscientific data in areas where it is either absent, of poor or outdated quality, or requiring synthesis and interpretation. Some key objectives are to establish the regional, geological, and structural controls on gold mineralization, and develop a robust geochronologic framework for mineralization with respect to important geologic events such as magmatism, deformation, and exhumation/cooling. This thesis aims to answer a subset of these fundamental questions by addressing structural controls on mineralization in the Klondike, with a focus on the relative timing of deformation, alteration and mineralization. Thorough a combination of field observations and petrographic analysis of microstructures, this study challenges existing structural models for the KGD that purport vein formation and mineralization to be entirely post-ductile, post-collisional features. This study has broad implications 3 for how ductile and brittle deformation are inter-related, and how this might impact conceptual models for orogenic gold formation in collisional or accretionary orogens. 2.0 Regional Geology 2.1 Tectonic History The KGD is situated within the Yukon-Tanana Terrane, the largest pericratonic terrane in the Canadian Cordillera, which extends from eastern Alaska, through southern Yukon, and into northern British Columbia (Fig. 2.1.1) (Colpron et al., 2006). The terrane is separated from the Laurentian craton by the Slide Mountain Terrane and continental margin sediments to the east, and is bound by far-travelled terranes of the Insular and Intermontane belts to the southwest (Beranek et al., 2011). Current interpretations of the parautochthonous YTT indicate a complex origin that includes the superimposed continental margin, continental arc, back arc and island arc tectonic environments (Colpron et al., 2006). Figure 2.1.1: Terrane map of the Northern Cordillera (Modified from Colpron et al., 2006). 4 The oldest and stratigraphically lowest sub-unit of the YTT is the meta-siliciclastic Snowcap Assemblage, which consists of quartzite, psammitic, pelitic, and calc-silicate schists, marble, and local amphibolite and ultramafic rocks (Colpron et al., 2006). This assemblage is pre-Late Devonian in age, constrained by the intrusion of Early Mississippian and Late Devonian plutons as well as Late Devonian to Early Mississippian zircons in unconformably overlying sediments (Colpron et al., 2006). The intercalation of amphibolite with MORB to OIB geochemical signatures and pelitic metasedimentary rocks resembling the Laurentian Margin in the Snowcap Assemblage signifies its formation occurred along the rifted margin of Ancestral North America (Colpron et al., 2006, Nelson et al., 2006), and is interpreted to have been deposited from the Proterozoic breakup of Rodinia until the Late Devonian (Piercey and Colpron, 2009). Unconformably overlying the Snowcap Assemblage are Upper Devonian to Lower Mississippian metavolcanic and metasedimentary rocks of the Finlayson Assemblage. This assemblage consists of mafic to felsic metavolcanics, basinal sediments including anoxic basin derived carbonaceous pelites, as well as intrusive rocks of the Grass Lakes and Simpson Range Plutonic Suites (Colpron et al., 2006). Southwest of the Tintina Fault, metavolcanic and metaplutonic rocks of the Snowcap Assemblage display calc-alkaline geochemical signatures consistent with a continental arc affinity (Creaser et al., 1997; Fallas et al., 1999, Colpron, 2006), while northeast of the fault, arc facies rocks are juxtaposed the with back-arc derived alkalic to tholeiitic volcanics (Piercey et al., 2001, 2002, 2006) by the Money Creek thrust (Colpron et al., 2006, Murphy et al., 2006). The variety of continental and back-arc volcanics in the Finlayson Assemblage is interpreted to record the evolution of a Late Devonian to Early Mississippian continental arc to back-arc system (Colpron et al., 2006). This continental arc sequence is understood to have formed by westward migration of Laurentia during the Middle to Late Devonian, resulted in east dipping subduction at its margin (Nelson and Colpron, 2007, Nelson et al., 2013). Formation of the upper sedimentary and back-arc volcanic units of the Finlayson Assemblage is interpreted to have formed by slab roll-back processes initiating near the termination of the Devonian which caused extension and rifting of the YTT from the Laurentian margin as well as the initial opening of the Slide Mountain Ocean (Beranek and Mortensen, 2011, Nelson et al., 2013). The Klinkit Assemblage is the next unit to unconformably overlie the Snowcap and Finlayson group. It is composed of mafic to intermediate calc-alkaline volcanic rocks, minor alkali basalt as well as limestone and marble, dated from the Middle Mississippian to Early Permian, which lay on top of a basal conglomerate composed of recycled Finlayson assemblage rocks (Roots et al., 2006, Colpron et al., 2006). The metavolcanic and metasedimentary sequences of this unit are interpreted to record the further 5 opening of the Slide Mountain Ocean, where deposition of shallow oceanic facies occurred, and continued back-arc volcanism (Beranek and Mortensen, 2011). The Klondike Assemblage is the youngest and stratigraphically highest tectonostratigraphic unit of the Yukon-Tanana Terrane (Fig. 2.1.2). Rocks of this assemblage include Middle to late Permian calc-alkaline felsic and minor mafic metavolcanics and co-magmatic intrusions of the Sulphur Creek suite (Colpron et al., 2006). Geochemically, the Klondike assemblage is characteristic of a continental arc tectonic setting (Piercey et al., 2006), and is interpreted to have formed by the subduction of Slide Figure 2.1.2: Schematic diagrams showing the formation, accretion and uplift of the YTT (Grimshaw, 2018). Mountain oceanic lithosphere beneath the Yukon-Tanana terrane (Mortensen, 1992, Nelson et al., 2006). The initiation of this west-dipping subduction zone beneath the led to closure of the Slide Mountain Ocean (Nelson and Colpron, 2007). 6 Final closure of the Slide Mountain Ocean during the Late Permian to Early Triassic marked the beginning of the Klondike Orogeny, which is characterized by initial obduction of the YTT and Slide Mountain terrane onto the Laurentian Margin (Nelson et al., 2013, Beranek and Mortensen, 2011). Subduction then stepped east to the Cache Creek Ocean, continuing in the east dipping direction until the accretion of the Insular Terrane (Nelson et al., 1993). Nelson and Mihalynuk (1993) proposed that the Insular Terrane rotated counter-clockwise and enclosed the YTT in the Early Triassic in an oroclinal fashion, which culminated in collisional deformation and exhumation of the YTT in the early Jurassic (Nelson et al., 2013) Further insight into the Jurassic exhumation of the YTT has been provided by geochronological data recorded by Berman et al. (2007). At 187 ± 5Ma, retrograde resorption of garnet was recorded by the crystallization of high-Y, matrix monazite while 40Ar/39Ar metamorphic cooling ages for hornblende, muscovite, and biotite were dated to 185-175 Ma, implicating significant exhumation during the Jurassic. As changing crustal levels are poorly linked to the ~10-20˚C Myr-1 cooling rate and P-T-t paths are not sufficiently constrained to distinguish exhumation through thrusting, extensional faulting or erosion, Berman et al. (2007) suggests a sinistral transtension model during slab rollback after the onset of east-dipping subduction during the Early Jurassic. 2.2 Bedrock Geology The KGD is a region in roughly bound by the Tintina Fault to the northeast, the Indian River to the south and the Yukon River to the west. The Klondike Gold District is underlain by thrust panels of meta-sedimentary and meta-igneous rocks creating an imbricated structural stack. Three distinct rock groups make up these thrust panels, the meta-volcanic and meta-plutonic Klondike assemblage of the Middle to Late Permian, the carbonaceous Nasina Schist of the Late Devonian to Early Mississippian, and mafic igneous rocks of the Slide Mountain Terrane (Chapman et al., 2010, MacKenzie et al., 2008). 7 Figure 2.2.1: Simplified geological map of Klondike District showing Lone Star Ridge study area (Modified from Chapman et al., 2010). The Klondike Schist, which underlies the majority of the KGD, is the structurally uppermost unit and the principal host rock for gold bearing veins that have eroded to form the KGD’s placer deposits. This unit is Middle to Late Permian in age and comprises felsic to mafic metavolcanic rocks and their intrusive equivalents. The rock package includes mafic quartz-feldspar-actinolite-chlorite schist and metagabbro, as well as felsic quartz-muscovite schist, quartz-feldspar-augen schist, and quartz monzonitic orthogneiss (Mortensen, 1990). Protoliths of the Klondike Schist are interpreted to be clastic sedimentary rocks, mafic and felsic volcanics, felsic intrusives and minor exhalative sediments (Mortensen, 1990). The unit is polydeformed, and is characterized by penetrative foliation and recrystallization fabrics developed with simultaneous metamorphism to middle-greenschist facies (MacKenzie et al., 2008). Peak metamorphic conditions in the KGD are interpreted to have been reached during the Late Permian Klondike Orogeny (Beranek and Mortensen, 2011), as supported by monazite U-Pb dating studies by Berman et al. (2007) Carbonaceous metasedimentary rocks of the Nasina Assemblage structurally underlie thrust panels of Klondike Schist in the northern and western parts of the Klondike. The most abundant rock 8 types within this unit are medium to dark grey carbonaceous phyllite, schist and quartzite. Some chloritic schist and fine to medium grained marble have been mapped by Mortensen (1990) which occur in narrow bands (up to 150 m) in the western area of the district. Regional scale thrust faults in the KGC are defined by horizontal to shallowly dipping shear zones that contain discontinuous fault-bounded imbricate slices of greenstone and ultramafic rocks which occur along specific contacts between lithologic assemblages (Mortensen, 1990). Greenstone rocks are composed of altered pyroxene-phyric basalt, fine-grained mafic tuff, diabase and minor gabbro, whereas the ultramafic rocks are composed of partially serpentinized harzburgite, massive to sheared serpentinite, listwanite (carbonate-fuchsite-silica altered serpentinite) and talc-carbonate schist (Mortensen, 1990). These thrust bound lenticular bodies typically define the thrust fault separating Klondike and Nasina assemblages, and are interpreted as potential ophiolitic slices of the Slide Mountain Terrane (Chapman et al., 2010, Mortensen, 1990). Units postdating thrust faulting intrude major fault surfaces. Narrow dikes and small plugs (up to 500m diameter) of quartz-feldspar +/- biotite porphyry exist throughout the region, with a larger intrusion outcropping along a ridge between Hunker Creek and the Klondike River (Mortensen 1990). A major phase of uplift and erosion of metamorphic rocks of the KGD in the Jurassic to Early Cretaceous culminated in deposition of conglomerate and shallow marine sediments of the mid to Late Cretaceous Indian River Formation and mafic volcanic rocks of the Late Cretaceous Carmacks Group. During the early Eocene, strike-slip movement on the northwest-trending Tintina fault commenced, and broadly coeval mafic to felsic dikes and small stocks were emplaced in the KGD. Today this fault is still active and is responsible for at least 450km of dextral displacement of the Klondike District from the faulted segment of the Yukon-Tanana Terrane in the Finlayson District of southern Yukon (Fig. 2.1.1.; Mortensen, 1990). Additional exhumation of the KGD during the Late Tertiary contributed to further erosion of the Klondike schist, and deposition of the Pliocene White Channel Gravels, which are a major source of paleoplacer gold in the KGD. Erosion and downcutting from the Pleistocene and Holocene led to the incision of gravel terraces and establishment of modern streams, which now lie some 70m below the base of the White Channel Gravels (Chapman et al., 2010). These stream beds host rich placer deposits, with gold derived both from re-concentrated from the White Channel Gravels and from ongoing erosion of mineralized Klondike Schist (Chapman et al., 2010). 9 2.3 Deformational History Rocks of the Klondike District have experienced a complex polyphase deformational history. Four deformational events have been documented for the metamorphic bedrock by MacKenzie et al. (2008), including initial metamorphic foliation development (D1 to D2), thrust slice emplacement and fold development (D3), reverse faulting and buckle folding (D4), and late stage normal faulting (D5). Soon after the Permian deposition of the volcanic protolith of the Klondike Schist, compression related to the Klondike Orogeny led for formation of a principle foliation and metamorphic segregations. Folding, simultaneous dynamic recrystallization and metamorphism occurred during this event resulting in a middle greenschist facies schist with sub-horizontal, layer parallel foliation, observable throughout most metamorphic rocks in the region (D1 of MacKenzie et al., 2008). Further compression during the Permian followed D1, producing tight to isoclinal folds and additional development of the pervasive transposition fabric characteristic of the Klondike Schist (Fig. 2.3.1 (a)). Rootless hinges of these folds are preserved in more competent quartzofeldspathic rock types as well as syn-metamorphic lensoidal quartz segregation veins ranging from 1mm to 20cm in width (D2 event of MacKenzie et al., 2008). These two initial compressional events and associated fabrics have been bracketed by the protolith age of 260 Ma and an unfoliated post-tectonic pluton dated to 252 Ma (Beranek and Mortensen 2011). These events coincided with peak greenschist facies metamorphism and formation of a quartz-albite-muscovite-chlorite +/- biotite mineral assemblage (MacKenzie et al., 2008). Thrust stacking of the Klondike, Nasina assemblages and Slide Mountain Terrane commenced during the Early Jurassic (Mortensen, 1990). This event is responsible for northwest-trending recumbent folds with rounded hinges, crenulation and spaced cleavage (Fig. 2.3.1 (b)) affecting foliation as well as dispersed phacoidal cleavage. Spaced cleavage is most common near hinges of 10 m-scale recumbent folds in rocks of the Klondike and Nasina assemblages, and is parallel to southwest-dipping axial surfaces of these folds. In hinge zones, the spaced cleavage interrupts foliation on the cm-scale whereas on limbs, spaced cleavage is subparallel to the pervasive foliation and enhances the fabric. Phacoidal cleavage resulting from the reactivation of shearing in the spaced cleavage occurs within 20 m of thrust slice boundaries, anastomosing around more competent rock pods on the 10 to 100cm scale. This reactivation of shear surfaces is characterized by slickensided surfaces, and is not observed in the mafic volcanics (D3 event of MacKenzie et al., 2008). 10 Figure 2.3.1. (a) Klondike schist samples showing metamorphic foliation (S2) and a structurally controlled vein infilling an F4 kink fold. (b) Klondike schist showing poorly defined foliation folded by F3 as well as S3 spaced cleavage defined by recrystallized micas. (MacKenzie et al., 2008). Angular kink folds found throughout the region have axial surfaces characterized by fractures, shearing and gouge zones. These features are interpreted by MacKenzie to having formed during a compressional event in the Middle to Late Jurassic. The kink folds are 1 to 50 cm-wide with cm-scale gouge zones and occur in reverse fault zones which are 10 to 50 m wide. Two sets have been observed with N-NE and W-NW orientations and 30 cm to 10 m spacing. They are best developed within 150 m of fault zones, being most prominent in the Klondike Schist, less so in the Nasina assemblage and non-existent in the ophiolitic rocks of the Slide Mountain Terrane (D4 event of MacKenzie et al., 2008). Localized post-D3 extension during the Late Jurassic is thought to be responsible for the formation of massive, up to 3m wide, discordant mesothermal quartz veins occurring throughout the region, with twenty-two vein systems with a north-northwest strike documented more than a century ago (MacLean 1914). Independent of their size, these veins typically coincide with the axial planes of kink folds (Fig. 2.3.1 (a)), although several occur parallel to the penetrative foliation (MacKenzie et al., 2008). Along with quartz, the mesothermal veins typically contain minor ferroan carbonate and pyrite, along with varying amounts of argenite, arsenopyrite, barite, chalcopyrite, freibergite, galena, gold, polybasite, pyrostilbnite, scheelite, sphalerite, and sulphosalts (Hoymann and Friedrich, 1992). Gold occurs as free grains within these mesothermal veins as well as adjacent to or as inclusions in pyrite along vein selvages. Hydrothermal alteration associated with the mesothermal veins is most prominent in mafic host rocks (Mortensen et al., 1992), in which strong carbonate alteration occurs up to 5 meters from vein margins and sericitization occurs within 1 centimeter of vein margins. Pyrite commonly replaces magnetite porphyroblasts in mafic lithologies, and occurs within 2 meters of veins (MacKenzie et al., 2008). This 11 sulphidation of the wall rock is considered largely responsible for gold deposition through destabilization of gold-bisulphide complexes in mineralizing fluids (MacKenzie et al., 2008, Chapman et al., 2010). Extension during the Late Cretaceous led to normal faulting and well-developed fault gouge zones. These normal faults are generally oriented parallel to the D4 kink folds and have been interpreted to be reactivated planes of weakness established during previous deformation events. Hydrothermal alteration accompanies the normal faulting with pyritization and sericitization (D5 event of MacKenzie et al., 2008). Deformation Stage Klondike Schist Event Main Feature Orientation Metals Deformation Age Deposition S0 Bedding NA Sulphides in some rocks NA Permian First Foliation S1 Foliation, segregations Variable NA Compression Permian Pervasive foliation S2 Foliation, isoclinal folds Variable NA Compression Permian Thrust stacking D3 Phacoidal cleavage, recumbent folds, spaced cleavage (S3), serpentine emplacement Shallow dip, variable, mainly shallow dip, SE trend NA Compression Jurassic? Fault-folding D4 Angular kink folds (F4), fold and axial surface fractures (S4), faults, shears, gouge zones Two orthogonal, N to NE, W to NW; steeply dipping ? Compression Jurassic? Mesothermal veins Au veins Massive discordant quartz veins Variable, often N to NW Au, pyrite, other sulphides Local extension late Jurassic? Normal faulting D5 Gouge zones N to NE, W to NW strike, steeply dipping Pyrite in silicified schist Regional extension Cretaceous Table 2.3.1: Summary of deformational history (adapted from MacKenzie et al., 2008). For this study, evidence for S2 isoclinal folding in outcrop was limited and therefore difficult to distinguish from the S1 fabric. For this reason, these deformational structures will be combined and referred to as the pervasive transposition fabric (ST). Structural elements related to the D3 and D4 event of MacKenzie et al (2008) were widely recognized in the current study, but observations made in the 2017 field season indicate that these are part of a progressive deformation sequence that also involve the formation and deformation of discordant quartz veins. These observations include sheared and boudined 12 discordant veins (Fig. 2.3.2) indicating post-emplacement ductile deformation as well as discordant quartz vein boudinage with extensional fractures oriented perpendicular to spaced cleavage developed in the surrounding schist, demonstrating a transition from a ductile to brittle deformation regime. Figure 2.3.2: (A) Discordant vein boudinage with extensional fractures oriented perpendicular to spaced cleavage developed in surrounding schist, (B, C) boundined discordant veins, (D) isolated vein boudin (Allan, 2017). A B C D 13 2.4 Overview of Lone Star Ridge Study Area This current study focusses on the Lone Star Ridge area, located in the western part of the KGD. The area is flanked by two of the most productive placer gold-bearing streams, Bonanza Creek to the north and Eldorado Creek to the southwest (Fig. 3.4.1) which have produced more than half of all the gold found in the Klondike District (Chapman et al., 2010). The geology of the Lone Star Ridge has been Mapped in detail by Dr. J. K. Mortensen and is thoroughly described in M. Grimshaw’s 2018 research on the area as follows: Figure 2.4.1: Map of Lone Star Ridge, Modified from Mortensen (2013). In general, the Lone Star Ridge area is underlain by rocks of the Klondike Assemblage, including the Sulphur Creek Orthogneiss and the Klondike Schist member. Mapping by J. Mortensen in 2013 delineated four separate members of the previously undifferentiated Klondike Schist Member (chlorite schist, carbonaceous schist, quartz-chlorite mica schist, meta-rhyolite and a quartz mica schist), and a quartz augen schist member which was differentiated from the Sulphur Creek Orthogniess. The quartz augen schist member underlies the southwest portion of Lone Star Ridge and is a strongly foliated quartz mica schist containing deformed quartz and orthoclase augen up to 8mm in 14 diameter (Mortensen et al., 2007, as cited in Grimshaw, 2018). Mortensen (2013) has separated this member from the Sulphur Creek Orthogneiss, and previously defined its protolith as a felsic porphyry intrusion formed during arc volcanism during the Late Permian (Mortensen et al., 2007) The chlorite schist member is a strongly-foliated green chlorite-schist with centimeter-scale spaced cleavage fabric (Knight et al., 1999, as cited in Grimshaw, 2018), and its protolith has been interpreted as Late Permian mafic seafloor volcanic rocks by Mortensen (2013). Carbonaceous schist outcrops along Eldorado Creek where is has been described as varying from 2 to 20m thick and lacking in continuity. It is a fine-grained, dark-grey quartz-muscovite schist rich in graphite (Mortensen et al., 2007), and contains large aggregates of coarse (1cm) euhedral pyrite (Grimshaw, 2018). The depositional environment has been interpreted as basins proximal to volcanic vents which accumulated organic rich clastic material (Mortensen et al., 2007). The quartz-chlorite schist member appears blocky in outcrop and is pale green to tan in colour (Knight et al., 1999b) and makes up a major component of the previously undifferentiated Klondike Schist (Mortensen, 2011). Mineralogically, it is composed of quartz, muscovite, and minor chlorite with cm-scale layers of quartz (Knight et al., 1999b). Mortensen (2007) has interpreted the origin of this member to be volcanic and clastic material deposited on the Late Permian seafloor. The meta-rhyolite member has been interpreted to be a product of Late Permian volcanism. It is pale green to yellow with a foliated sugary texture when weathered in outcrop, it appears flaggy but is difficult to differentiate in drill core. Mortensen (2011) completed chemical analysis on this unit to determine its rhyolitic composition. Like previous units, the meta-rhyolite unit has been interpreted as being produced by Late Permian volcanism. The current study focuses on a zone of veining and mineralization in the quartz mica schist member. It has been described by Mortensen (2011) to contain mm to cm scale laminations of quartz, feldspar, and mica. Alteration occurs as spotty chlorite and carbonate with fine-grained euhedral pyrite. 15 3.0 Methods 3.1 Fieldwork Sample collection was carried out during the summer of 2017. Quartz vein showings across the KGD were studied to investigate relationships between veins and host rock fabrics. Drill core and assay data were provided by various exploration companies including Klondike Gold Corporation to assist with logging and sample selection. Samples were collected that showed unambiguous relative timing relationships between structural and hydrothermal elements, or that required additional petrographic and microstructural observations. 3.2 Petrography Seven samples were prepared into polished thin sections for petrographic investigation (Vancouver Petrographic Ltd.). Thin sections of the host rock were chosen based on their development of microstructures and extent of silicification for characterization of deformation among variously altered rock. Thin sections were also cut from the mineralized quartz vein and the highly silicified host rock along its margin for interpretation and verification of the vein’s origin. Billets were cut perpendicular to foliation and parallel to any apparent lineations to exhibit the pervasive deformation fabrics. Thin sections were scanned under plane and cross polarized light for future reference. A Nikon Eclipse E600 POL polarizing microscope was used for petrographic analysis and photomicrographs were taken using a Canon Rebel EOS T21 camera mounted on the microscope. 3.3 Cathodoluminescence Cathodoluminescence (CL) imaging was performed on the main quartz vein and immediately surrounding host rock using a Philips XL-30 scanning electron microscope equipped with a Robinson CL detector at the Earth, Ocean and Atmospheric Science Department’s Electron Microbeam and X-Ray Diffraction Facility at UBC. Studies of CL textures found in quartz from various geologic environments by J. Götze et al. (2012), Rusk (2012), and Allan et al. (2007) have concluded that CL textures represent the dynamics of the hydrothermal systems under which the quartz precipitated, which may include fluctuations in pressure, temperature, fluid composition, and growth kinetics. Various cathodoluminescent characteristics of quartz have been related to different defects in its crystal structure by J. Götze et al. (2012). Types of defects include extrinsic (incorporation of inclusions into the crystal lattice or interstitial positions) and intrinsic (lattice vacancies). The most common 16 extrinsic defect is aluminum substitution for silicon in the tetrahedral site, requiring addition of a charge-balancing cation (e.g., H+, Li+ or Na+), whereas other tetrahedral substitutions are less common (Ti4+, Ge2+/4+, and Fe2+/3+). Oxygen and H2O excess can lead to defects known as peroxy linkages and radicals (extra oxygens) and silantol groups (-OH-). Intrinsic defects of vacancies occur when silicon or oxygen are missing from the lattice. CL colours and emission bands have been correlated to defects as follows: Defects Emission band Neutral oxygen vacancy 290nm (4.3 eV) Al3+ substitution 385nm (3.3 eV) Ti4+ impurity 440-480nm (2.7 eV) Non-bridging oxygen hole centres 635 and 650nm (1.95 eV and 1.9 eV respectively) Fe3+ substitution 750nm (1.65 eV) Interstitial molecular oxygen 1280nm (0.968 eV) Table 3.3.1: Emission bands of common crystal lattice defects in quartz (J. Götze et al., 2012). Quartz forming in various environments has been studied by B. Rusk (2012) to correlate the variety of textures created under different formation conditions. Euhedral, zoned quartz is more common primary feature in low temperature quartz which becomes less common with increasing temperatures (~100 - ~>700˚C). Non-luminescent bands that crosscut primary quartz growth features are common, and may relate to the annealing of fractures. Quartz precipitation grading from dark bands into euhedral quartz indicates a consistent source of hydrothermal fluids migrating into a void space with increasing dilation rates. Metamorphism is also found to influence the CL response of quartz; with increasing metamorphic grade, the CL response of quartz becomes increasingly mottled to homogenous, relating to the annealing and eradication of crystallographic defects and trace elements that quartz may inherit from hydrothermal fluids and the protolith (Schilles et al., 2001). A study of the CL response of hydrothermal quartz in conjunction with trace element, oxygen isotope, and fluid inclusion analysis by Allan and Yardley (2007) also linked the concentration of Al3+ substitution into quartz with growth kinetics. It has also been noted experimentally that there is a positive correlation between quartz growth rate and the concentration of H-bearing impurities in the lattice (Ihringer and Zink, 2000), the distribution of which is expected to influence the CL response. In the study of Allan and Yardley (2007), it was shown that quartz highest in Al3+ concentration also correlated with isotopically light quartz (δ18Oquartz=0‰, vs. V-SMOW) that was interpreted to represent an influx of 17 meteoric water proceeding a drop in pressure and temperature following release of overpressured hydrothermal fluid. Although there is no direct correlation between CL intensity and the concentration of any particular lattice defect, textural observations of quartz’s CL response can be a useful tool in delineating the dynamics of vein emplacement. 18 4.0 Core Logging Observations Figure 4.1: Schematic log of drillhole LS16-69, depths 88.0m to 90.5m. The specific section of core chosen for this study is from hole LS16-69, drilled by Klondike Gold Corporation in 2016 at the collar location: 586763m E 7086291m N (UTM zone 7). The drillhole was 19 oriented plunging 51o towards an azimuth of 208o, and drilled to a total depth of 108.2m. The core intersects quartz-muscovite schist of the Klondike Schist, and at depths 88.1m to 90.6m occurred a 4-cm thick mineralized quartz vein (51.49 g/t, Klondike Gold Corporation, 2016) containing euhedral pyrite, galena, and visible gold within the silicified halo of this vein. Schist in the surrounding silicified halo is texturally and mineralogically overprinted, lacking the high-contrasting Q and abundant M-domains of the typical transposed fabric of the region. There is also an absence of crenulation folding and spaced cleavage which are generally well developed. Along with the elevated gold concentrations, assay results from this section of core (88.71m to 89.12m) returned anomalously high concentrations of lead, zinc, silver, and tellurium (925.4 ppm, 264 ppm, 28.7 ppm, and 13.4 ppm respectively) (Klondike Gold Corporation, 2016). The vein located 89.0m downhole is cross-cut by barren microfractures and the quartz continues into 5 mm-wide fractures emanating 2-3 cm into the host rock. Galena porphyroblasts rimmed with pyrite up to 1cm in width are visible in the vein, and display a crystal habit varying from euhedral to subhedral on opposite sides (Fig. 4.2). In the silicified zone directly above the vein, at a depth of 88.8m, there is a Figure 4.2: Mineralized quartz vein (89.0m depth) with galena and pyrite porphyroblasts, visible gold fragments (surrounded by red circle) are visible in hanging wall. lack of any obvious crenulation development. From the vein margin into the host rock are fine (less than 1mm thick) veinlets filled with carbonate. Euhedral pyrite porphyroblasts up to 2mm in diameter are situated in line with these carbonate veinlets and appear to be related. Disseminated visible gold is also 20 present in the silicified halo, restricted to a few centimeters from the margin of the vein (Fig. 4.2). Carbonate porphyroblasts are present in this section as sporadic spots and appear to be replacing chlorite. Towards the margins of the silicified zone are brittle faults with associated drag folding of the transposition fabric. Due to the orientation of the core axis the sense of movement along the fault is not resolvable, although it denotes brittle accommodation of strain along the margins of the silicified halo. Outside of the silicified zone, the Klondike schist is deformed by a crenulation fabric and spaced cleavage. Along the upper edge of the silicified halo of the mineralized vein is a 3cm wide foliaform quartz-carbonate segregation which has a well-developed chlorite-carbonate selvage along its hanging wall contact. In the surrounding schist occurs carbonate porphyroblasts appearing to be replacing chlorite, as well as very fine grained (<0.5mm) pyrite disseminated along foliation planes. Below the silicified zone, at a depth of 89.4m, is a foliaform 15cm white quartz-carbonate vein lacking associated alteration or silicification. The surrounding host rock is mica rich and has well developed crenulation fabrics. On the lower margin, a boudin neck appears to have passively flowed into the quartz, warping the foliation fabric with it into the vein. This quartz vein can be interpreted either as a metamorphic segregation, an early-formed vein that was sheared into parallelism with the transposition fabric (ST), or as a late vein that was emplaced within a cleavage-parallel parting in the host schist. At a depth of 90.4m, the host rock surrounding the foliaform quartz segregation grades from a foliation composed of predominantly M-domains downwards to an increasing abundance of Q-domains. Crenulation folding of the transposition fabric is well developed in this zone, and a consistent spaced cleavage cross-cuts the foliation at a near-perpendicular orientation. Carbonate alteration is not as prevalent in this section, although a weak reaction with HCl(aq) is observable along the foliation. Disseminated pyrite, with euhedral cubes less than 0.5mm in width is widespread along the foliation. Field observations of this core section prompted further inquiry into its deformational and mineralization history. Initial observations of overprinting relationships indicate that the sequence of events involved in the emplacement, alteration, mineralization, and structural modification of this quartz vein do not necessarily abide by the deformation framework laid out by MacKenzie (2008). The first variation noted is the foliaform orientation of this mineralized vein which in contrast to the more typically discordant quartz veins that cross-cut transposition fabric common throughout the KGD, as outlined by MacKenzie (2008). An absence of crenulation and spaced cleavage in the silicified halo of the mineralized vein is evidence that silicification predates the deformational event responsible for formation 21 of crenulation fabrics in the distal, less altered portions of schist. Strain partitioning between contrasting rheology is believed responsible for this lack of ductile fabric development in the silicified halo. This interpretation is further validated by the presence of reverse fractures at the margins of the silicified halo, representing brittle accommodation of strain in more competent zones of silicic alteration. These observations require that at least some veins formed prior to ductile deformation, and that brittle and ductile features may be linked, even on very local scales, to the same deformation event. Samples were chosen based on structural and mineralogical observations and are as follows: Sample ID Depth (m) Description LS16-69_88.1m 88.1m Non-silicified Klondike Schist (quartz mica schist member) with 2cm quartz segregation, chlorite and carbonate selvage present as well as carbonate porphyroblasts (replacing chlorite?) LS16-69_88.8m 88.8m Silicified Klondike Schist lacking crenulation folding and cleavage, reverse faulting with associated drag folding, carbonate-spot porphyroblasts LS16-69_89.0m 89.0m Mineralized foliaform white bull-quartz vein with pyrite-galena inclusions, barren microfractures cross cut vein, surrounded by intensely silicified Klondike Schist containing disseminated visible gold and euhedral pyrite porphyroblasts LS16-69_89.4m 89.4m Contact between 15cm foliaform quartz-carbonate segregation and M-domain rich Klondike Schist, schist ductilely deformed by crenulation and spaced cleavage LS16-69_90.4m 90.4m Quartz rich Klondike Schist with well developed crenulation and spaced cleavage fabrics, carbonate porphyroblasts less prevalent, weak reaction with HCl(aq) Table 4.1: Summary of selected core samples from Klondike Gold Corporation’s 2016 LS16-69 hole. 22 5.0 Petrographic Observations Thin sections were made from samples which include the margins of two foliaform quartz-carbonate segregations, relatively unaltered Klondike Schist with well-developed crenulation and spaced cleavage, the silicified halo surrounding the mineralized vein, and the mineralized vein containing pyrite-galena porphyroblasts. Billets were cut based on the presence visible structures and the extent of Figure 5.0: Stitched photograph of sampled core and thin section locations silicification and alteration to characterize and correlate the extent of deformation and alteration proximal to the silicified vein. The following are petrographic descriptions, arranged in a downhole fashion. 23 5.1 LS16-69_88.1m Figure 5.1.1: Thin section scans of LS16-69_88.1m, plane polarized light (left) with foliation traced by black line, cross-polarized light (right) Thin section LS16_69_88.1m intersects the margin of a boudined 3 cm-wide foliaform quartz-carbonate metamorphic segregation. The host consists of the Klondike Schist quartz mica member and is predominantly composed of very fine-grained (<0.25mm) quartz (65%), muscovite (20%), carbonate (7%), feldspar (5%), pyrite (2%) and rutile (<1%). The quartz and feldspar have characteristic polygonal boundaries commonly terminating at triple-junctions. Quartz grains appear to orient parallel to foliation forming ribbon structures. Foliation is defined by muscovite M-domains, making up around 25% of the schist, and are folded by a gentle crenulation fabric. Carbonate porphyroblasts (up to 2.5mm wide) making up around 7% of the schist appear to be replacing chlorite. They are overprinting crenulation and 24 contain inclusions of rutile and pyrite which have preserved orientation of the original transposition fabrics (see Fig. 5.1.2). Figure 5.1.2: Plane-polarized transmitted light photomicrograph showing carbonate porphyroblast with rutile inclusions defining transposition fabric foliation (ST) traced by dotted blue line Very fine-grained (<0.1mm), euhedral rutile grains, a potential product of retrograde reactions of biotite to chlorite upon exhumation (Parry & Downey, 1982), make up less than 1% of the schist and are positioned along foliation. Pyrite occurs as euhedral to subhedral cubes up to 0.25mm in width, making up around 2% of the schist. The pyrite porphyroblasts are disseminated throughout the schist, linearly arranged and commonly elongated parallel to foliation. 25 The foliaform quartz-carbonate segregation making up 40% of the thin sections is coarser-grained than the surrounding schist. The quartz, making up 80% of the segregation, is polycrystalline with inequigranular grains measuring up to 0.75mm in width. Grain boundaries are irregular and bulge into one-another and their grain boundaries commonly intersect at triple-junction intersections. Carbonate makes up around 20% of the segregation’s volume and occurs in two morphologies. One morphology, found proximal to the segregations margin, is similar to carbonate spotting found in the surrounding schist, having a mottled textures and partial alteration to chlorite. The other morphology of carbonate is interstitial to the quartz, sometimes surrounding individual quartz grains. It is more crystalline than the mottled carbonate with well-developed grain boundaries. Few subhedral and euhedral pyrite porphyroblasts up to 0.9mm in diameter are also present in the segregation. Figure 5.1.3: Cross-polarized transmitted light photomicrograph showing a quartz-carbonate segregation with mica-inclusions aligned with host-rock’s transposition fabric. Quartz displays irregular boundaries and triple junction intersections. Interstitial carbonate shows well developed crystal boundaries and can be seen wholly surrounding fine-grained quartz grains. Overall, the host rock in this section of core has a relatively high abundance of Q-domains. Crenulation fabrics are not very well developed, which implies increased competence correlating with the lower abundance of M-domains. 26 5.2 LS16-69_88.8m Figure 5.2.1: Thin section scans of LS16-69_88.8m, plane polarized light (left) with foliation traced by black line, cross-polarized light (right). Thin section LS16-69_88.8m is situated in the silicified hanging wall of the mineralized quartz vein. It is in close proximity to the brittle reverse fault-fractures along which the transposition fabric appears to have been drag folded into parallelism with displacement. Unlike the more micaceous lithologies along this section of core, a crenulation fabric is only weakly developed. The transposition fabric in this sample is well defined by fine-grained (<0.8mm) muscovite. These M-domains occur in lower quantities than in the less-silicified areas with a total abundance of around 10%. Q-domains, comprising 90% of this thin section, are composed of 90% quartz and 10% feldspar. Quartz grains are inequigranular with grains measuring from 0.05 mm to 0.2 mm in diameter. Larger quartz grains have visible undulatory extinction and variable development of sub-grain boundaries. Feldspars are comparatively coarse, with diameters ranging from 0.5 to 1mm. Cores of the 27 feldspar grains appear undeformed and are partially sericite altered. Pyrite abundances are low (<1%) and occur as subhedral, elongate grains which are oriented parallel to the transposition fabric ST. Carbonate occurs strictly as porphyroblasts up to 2mm in width along with a minor chlorite which it is replacing. Similar to LS16-69_88.1m, the carbonate porphyroblasts contains inclusions aligned with the transposition fabric which are composed of mica and skeletal pyrite. In one carbonate porphyroblast, this skeletal pyrite occupies around 10% of its area. Rutile needles are also disseminated throughout the sample (<0.5%), most commonly ~0.1mm in width and rarely up to 0.25mm. The lack of M-domains in this sample correlates with its poorly developed crenulation and spaced cleavage. Carbonate porphyroblasts are not deformed by the transposition fabric, implying the minerals which it is replacing grew after the development of the pervasive transposition fabrics, and were unaffected by the ductile crenulation folding. Overall this sample provides evidence of increased competency due to the absence of strongly overprinting ductile fabrics. The increase in competency may be attributed to the overprinting silicification halo in which this thin section was prepared from. 28 5.3 LS16-69_89.0m_A Figure 5.3.1: Thin section scans of LS16-69_89.0m_A, plane polarized light (left) with country rock boudin neck traced with dashed red line and foliation traced by black line, cross-polarized light (right). Thin section LS16-69_89.0m_A transects the quartz vein which is central to an intense silicification halo and anomalous mineralization along the section of core under study. 29 Figure 5.3.2: Cross-polarized transmitted light photomicrograph showing contact of vein and intruding country rock showing boudin necks in vein. A small portion of wall rock is included in the top of this thin section which has boudin necks measuring 1.5mm across along its margin (Fig. 5.3.2). Relict transposition fabric is visible in the host rock, defined by M-domains (10%) composed of very fine-grained mica lathes measuring 0.2mm in length. Q-domains make up 90% of the host rock and are composed of polygonal quartz (83%) and feldspar (7%) with grain boundaries intersecting at triple-junctions. Trails of finer grained, irregularly shaped quartz dissect the host rock, interrupting the transposition fabric, and are discontinuous at the host-vein boundary. The quartz vein is composed of inequigranular quartz grains (lacking the carbonate found in foliaform segregations) which show weak undulatory extinction and a subtle crystallographically preferred orientation (See Fig. 4.4.1, thin section scan). Larger grains, up to 1.2cm in width, are elongate with irregular, serrated edges and are surrounded by finer-grained (<0.3mm) quartz. Grain boundaries show a bulging texture, and it is evident that recrystallization has occurred by dislocation of these bulges from grain boundaries (BLG recrystallization). Two sets of crosscutting fractures are present which intersect at an angle around 30˚. The first set are larger healed fractures dissecting the entire vein and are defined by fluid inclusion trails. The second set are smaller fractures dispersed throughout the vein and are discontinuous, with lengths around 1mm (Fig. 5.3.2). Together, these fractures are interpreted as 30 Figure 5.3.3: Mode I fractures oriented parallel to the principle stress axis (σ1) and cross-cutting mode II shear fractures oriented at an acute angle to σ1 (traced with dashed blue line). Stylolitic grain boundary development visible on left. brittle features which formed to accommodate compressive stress postdating the veins emplacement. The smaller, discontinuous fractures have been interpreted as mode I extensional fractures opening perpendicular to the principle stress axis, while larger fractures are interpreted as shear, mode II fractures forming at an acute angle to σ1. Comparing the quartz grains in this vein to the foliaform segregations, there is a major distinction in grain size. The finer-grained foliaform quartz-carbonate segregation in LS16-69_88.0m has a much finer grain size, attributed to intense recrystallization, while this mineralized vein shows little strain and recrystallization, but is highly fractured. These differences in deformation mechanisms acting on this vein and the foliaform segregation can be explained by strain partitioning between their varied rheologies. Since this vein is surrounded by a more competent silicified alteration halo, it is expected to behave in a brittle manner, forming tensional cracks and shear fractures to accommodate strain. In contrast, the metamorphic segregations of LS16-69_88.1m and LS16-69_89.6m, which have quartz-carbonate and quartz dominated mineralogy, respectively, are surrounded by micaceous schist which has undergone intense ductile deformation, and display intense recrystallization within the segregations. 31 5.4 LS16-69_89.0m_B Figure 5.4.1: Thin section scans of LS16-69_89.0m_B, plane polarized light (left) with foliation traced by black lines, cross-polarized light (right). LS16-69_89.0m_B is from the highly silicified hanging wall of the mineralized quartz vein (Fig. 5.0). It is composed of very fine-grained quartz (85%) up to 0.3mm in diameter, feldspar (7%) ranging from 0.5mm to 1.5mm in diameter, and sparse (3%) muscovite which subtly defines the relict transposition fabric. Quartz and felspar grains are generally polygonal with triple junction intersection, and the feldspar grains, which like in previous samples, are partially sericitized. Crosscutting the sample are planar veinlets of microcrystalline quartz, along which euhedral pyrite porphyroblasts up to 1.5mm are aligned. In macrospecimen, visible gold is also situated within this zone of intense silicification, and may be related to fluid infiltration responsible for the precipitation of microcrystalline quartz and euhedral pyrite porphyroblasts. 32 Figure 5.4.2: Cross-polarized transmitted light photomicrograph showing microcrystalline quartz trails aligned with pyrite porphyroblasts. Fracture sets, oriented sub-perpendicular to each other, cross-cut the entire sample and contain veinlets of carbonate. The carbonate veinlets typically occupy the centrelines of the microcrystalline quartz veinlets, and are interpreted to be late brittle features preferentially propagating through competent silicified zones. Figure 5.4.3: Plane-polarized transmitted light photomicrograph showing fractures infilled with carbonate. The microcrystalline quartz associated with pyrite porphyroblasts and visible gold, is interpreted to be a product of fluid infiltration due to vein emplacement, preserving the pathways of fluid responsible 33 for the silicification halo. Like the quartz vein which is central to this silicified zone, brittle fractures cross cut the Klondike Schist and are infilled with fine carbonate veinlets. These brittle fractures are interpreted to be late features accommodating compressional stress. Evidence for strain partitioning is apparent in this sample from the lack of crenulation fold development and associated spaced cleavage relative to the non-silicified Klondike Schist outside of the alteration halo. 34 5.5 LS16-69_89.4m Figure 5.5.1: Thin section scans of LS16-69_89.4m, plane polarized light (left) with foliation traced by black line, spaced cleavage traced by solid blue line and F3 crenulation fold traced by dashed blue line, cross-polarized light (right). Thin section LS16-69_89.4m was cut from a mica rich section of core in the footwall of a foliaform quartz-carbonate segregation. It is situated about 15cm below the base of the silicified halo surrounding the mineralized quartz vein, and is highly deformed by crenulation folding of the transposition fabric. Carbonate occurs as both spotted (some of which appear in tear-drop shapes extending into the foliation fabric) and foliaform morphologies parallel to foliation. The undeformed carbonate porphyroblasts range in size from 1.5 to 3mm in diameter and are associated with more quartz-feldspar rich areas, while the elongated, foliaform carbonate is generally found encompassed by micas. Disseminated pyrite porphyroblasts occur parallel to foliation, ranging in size from less than 0.05mm up to 0.25mm. Q-domains of the transposition fabric (ST) are composed of mostly quartz (95%) and minor 35 feldspar (<5%). Quartz occurs as ribbons where it is situated between muscovite segregations and individual grains have straight, parallel boundaries forming squares. Feldspar is coarser than surrounding quartz, ranging from 0.5mm to rarely 1mm in width, and is variably sericitized with the replacing mica laths up to 50µm in length. This thin section is dominated by M-domains, which can be associated with a more ductile rheology and an increased development of crenulation folding. Carbonate porphyroblasts occur with a spot morphology in the quartz-feldspar Q-domains, but when they are enclosed by M-domains, they are elongated and sheared into parallelism with the ductile fabrics. The difference in carbonate morphologies provides evidence for strain partitioning between the Q and M-domains as well as layer parallel shear occurring along the M-domains postdating the introduction of the carbonate. Overall, this thin section has a greater abundance of mica compared to the mineralized quartz vein and associated silicified halo at a depth of 89.0m, and their contrasting crenulation development provides evidence that the deformation event responsible for the formation of these fabrics postdated the emplacement of the vein and its silicified halo. 36 5.6 LS16-69_89.6m Figure 5.6.1: Thin section scans of LS16-69_89.6m, plane polarized light (left) with foliation traced by black line and spaced cleavage traced with solid blue line, cross-polarized light (right). Thin section LS16-69_89.6m was cut from a boudin neck intruding into a foliaform quartz segregation. The host rock is predominantly composed of muscovite (65%) which has undergone ductile deformation, forming the boudin neck intruding into the quartz segregation, and defines the development of a spaced cleavage. The host rock also contains quartz (20%) and feldspar (10%) which are fine-grained (less than 0.5mm) and have irregular boundaries terminating at triple junctions. Carbonate occur as elongate, 1mm by up to 5mm, foliation-parallel porphyroblasts which appear to be replacing chlorite. Pyrite porphyroblasts occur up to 0.8mm but most commonly up to 0.2mm in width and are situated along foliation with a subhedral habit, commonly elongated parallel to foliation. Rutile is more abundant in this sample than those previously described, with a modal percentage of around 1%. It occurs as fine, elongate, radiating crystals up to 50µm in length situated along foliation. 37 The foliaform quartz segregation is 90% quartz with minor euhedral feldspar grains (5%) and carbonate (5%) with the feldspar and quartz being substantially strained, displaying intense undulatory extinction. Larger quartz grains (up to 5mm) are defined by irregular, bulging grain boundaries, and are mantled with finer grained (0.2mm) quartz which appears to be a product of bulging recrystallization. Fractures infilled with carbonate which are 0.1mm wide crosscut this segregation. Like LS16_69_89.4m, the elongate carbonate porphyroblasts provide evidence for layer parallel shear after its emplacement. As with previously observed samples with an elevated abundance of M-domains, ductile fabrics are well formed evident by the flow of schist into boudin necks as well as the development of a spaced cleavage. These well-developed ductile fabrics in addition to being in a zone lacking intense silicification further supports the hypothesis of active deformation postdating the vein and silicification halo present in the mineralized zone of this section of core. 38 5.7 LS16-69_90.4m Figure 5.7.1: Thin section scans of LS16-69_90.4m, plane polarized light (left) with foliation traced by black line and spaced cleavage traced by solid blue line, cross-polarized light (right). LS16-69_90.4m was cut from quartz-mica schist located 1.5m below the mineralized quartz vein. Muscovite-rich M-domains comprise 20% of the thin section, while Q-domains composed of quartz, feldspar, pyrite, carbonate and minor rutile comprise the other 80%. Quartz is fine-grained (up to 0.4mm) and has polygonal grain boundaries which intersect at triple junctions. There are instances of ribbon quartz, where rows of square, fine grains of quartz are situated between muscovite laminae. Feldspar, making up around 7% of the thin section, occurs as coarse, blocky crystals up to 1.5mm across. It is partially sericitized and weakly strained, showing slight undulatory extinction. Carbonate, comprising 5% of the thin section, occurs in two morphologies: mottled spots measuring up to 1mm in width, and a foliaform variety measuring up to 0.7mm in width. The transposition fabric is present as a well-developed foliation defined by thin M-domains, and has undergone ductile deformation visible in the microscopic 39 crenulation and associated spaced cleavage. Subhedral pyrite (2-3%) is aligned with the foliation, and a portion of the grains (measuring up to 0.5mm) are elongate parallel to foliation. As in other samples, rutile (<1%) occurs as very fine laths, measuring <50µm in length, and is situated along foliation. Although this sample has a lower abundance of M-domains, it still exhibits ductile deformation affecting the transposition fabric. As this sample is outside of the silicified zone surrounding the mineralized quartz vein, the development of these ductile fabrics is expected under the hypothesis that silicification of the mineralized zone contributed to elevated competency and shielding of deformation postdating its emplacement. 40 6.0 Cathodoluminescence Observations SEM-based cathodoluminescence (CL) imaging was performed to examine possible growth and deformation features in quartz. Hydrothermal quartz deposited into free space typically displays clear growth zones characterized by variations in luminescence (Allan & Yardley, 2007, Götze et al., 2012), which define well-preserved primary growth textures (Rusk, et al., 2012). The presence of primary textures can therefore be used to determine if quartz preserves original growth features, or perhaps whether post-hydrothermal deformation has taken place. In the current study, if the vein and silicified zone were subjected to deformation, as hypothesized by macroscopic observations, a homogenous CL response would be expected due to dynamic recrystallization processes eradicating lattice substitutions and defects within the quartz (Schilles et al., 2001). If the vein emplacement post-dated ductile deformation, as outlined in MacKenzie’s interpretation of structural controls on mineralization in the Klondike, relict hydrothermal quartz textures would be expected. Two thin sections were chosen for analysis, the mineralized quartz vein in LS16-69_89.0m_A and the silicified halo of the same quartz vein in thin section LS16-69_89.0m_B. Within the quartz vein in LS16-69_89.0m_A and along its margin, CL textural variation was minimal showing a mottled but uniformly dark-grey response (Fig. 6.1). As determined during Figure 6.1: CL imaging of LS16-69_89.0m_A showing dark response with fine microfractures (centre) and carbonate filled veinlet (top). petrographic investigation, two sets of microfractures which cross-cut the vein were apparent with a bright grey CL response. Late cross-cutting veinlets with a bright white CL response were observed 41 which are late, carbonate filled fractures. Overall, the uniform dark-grey response is interpreted to be a result of recovery and dynamic recrystallization by overprinting deformation. The coarse sulphides contained within the vein (euhedral pyrite with a galena core) were observed to determine if their deposition was related to any specific quartz generation based on the presence of a discernable CL response. Comparable to the previously observed quartz, there was a lack of definite overprinting generations, but similar carbonate veinlets cross-cut both the pyrite and surrounding quartz. The carbonate fractures intersect the galena core and textures indicate that galena is replacing pyrite, therefore the carbonate veinlets and galena have been interpreted as potentially late, cogenetic features. Figure 6.2: CL imaging of LS16-69_89.0m_A (left) showing a dark response of quartz with fine microfractures infilled with carbonate cross-cutting the quartz and sulphides. The bright response of inclusions in galena are epoxy filled pits. Reflected light photomicrograph (right). The CL response of sample LS16-69_89.0m_B was also investigated due to its highly silicified appearance in microscopic and macroscopic sample observations. Petrographic analysis found linear arrays of very-fine quartz which emanate from the quartz vein and cross-cut relict transposition fabrics appearing to correlate with the positions of euhedral pyrite porphyroblasts and visible gold observed in macrospecimen. These veinlets of finer-grained quartz were investigated for signs of growth textures and evidence for post-emplacement recrystallization processes. In general, the schist showed a mottled appearance related to the bi-modal abundance of feldspar and quartz. Along the lengths of the micro- 42 Figure 6.3: Dark CL response of microcrystalline quartz with bright carbonate infill of fractures (top), transmitted light photomicrograph under cross-polarized light showing microcrystalline quartz and pyrite porphyroblasts along bottom left (centre), transmitted light photomicrograph under plane-polarized light showing of the same area (bottom). 43 crystalline quartz, a consistent medium-grey CL texture was observed and very fine veinlets of carbonate appeared to run along the length of the fine-grained quartz. The medium-grey response of the quartz veinlets, relative to the dark-grey response of the main vein, indicate the veinlets formed as later generations of quartz. Overall, the lack of variable CL response within the quartz vein and associated silicified halo signifies that recrystallization processes did occur after the mineralized vein’s emplacement, whereas the bright quartz veinlets crosscutting the silicified zone indicate a later generation of quartz precipitation. Due to the more competent rheology of the silicified halo, the overprinting deformation may have promoted brittle deformation into which this new generation of hydrothermal quartz, along with associated sulphide minerals and gold were introduced. 44 7.0 Discussion Macroscopic, microscopic and CL response observations of the Lone Star Ridge core support the hypothesis that existing structural models for the KGD which purport vein formation and mineralization to be entirely post-ductile, post-collisional features (MacKenzie et al., 2008), may in fact be overly generalized Upon observing the Lone Star core at the macroscale, it is apparent that ductile deformation took place after emplacement of the mineralized vein and associated silicified halo. Development of a crenulation and spaced cleavage is poor within the silicified zone but becomes progressively more developed with the transition towards decreasing silicification away from the mineralized quartz vein. Strain partitioning between the more competent silicified halo and surrounding micaceous schist has resulted in brittle failure along the margins of the halo (Fig. 7.1) and intense ductile deformation fabrics localized in the unaffected surrounding rocks. Figure 7.1: Brittle fracture at 88.7m (arrows showing sense of movement), transposition foliation (blue line) drag folded into fault (dotted line). Similar observations were made at the microscopic scale which support the hypothesis. In mica rich zones, where M-domains make up more than 50% of the rock, crenulation and spaced cleavages are well developed and carbonate porphyroblasts are sheared parallel to the transposition fabric, whereas in 45 the silicified halo, the remnant transposition fabric appears unaffected by crenulation folding and spaced cleavage shearing and carbonate porphyroblasts occur as undeformed “spots”. Within the mineralized quartz vein central to the silicified halo, mode I extensional and mode II shear fractures are observed which represent a brittle response to compressive stresses post-dating the vein’s emplacement (Fig. 7.2). With the rheological contrast between the quartz vein with its associated silicification halo and the micaceous Klondike Schist, the difference in deformation mechanisms observed can be explained by strain partitioning. In the less competent Klondike Schist outside of the silicified halo, strain was focused into ductile deformation resulted in crenulation and spaced cleavage development, while within the vein and associated halo, strain was accommodated through the dominant brittle fracturing. Figure 7.2: Mode I fractures oriented parallel to the principle stress axis (σ1) and cross-cutting mode II shear fractures oriented at an acute angle to σ1 (traced with dashed blue line). Stylolitic grain boundary development visible on left. The observed cathodoluminescence response of the quartz vein and quartz within the silicified zone, which showed a consistent dark grey mottled texture is also consistent with deformation outlasting the mineralized vein’s emplacement. This homogenous CL response indicates that dynamic recrystallization has occurred, removing atomic substitutions and lattice defects in response to heat and strain (Schilles et al., 2001). 46 Figure 7.3: Schematic diagram of silicified halo (grey) around quartz vein (red) with ductile deformation focused around its margins and a later generation of Au-bearing (yellow) quartz veinlets (modified from Allan, 2018) Overall, strain partitioning occurring around the silicification halo associated with hydrothermal vein emplacement is apparent in the 88.0m – 90.5m section of drillhole LS16-69 and provides evidence for ductile deformation outlasting its genesis. The emplacement of the hydrothermal vein and its associated alteration halo created a more competent marginal zone with enhanced competency that shielded it from formation of the ductile deformation fabrics (crenulation and spaced cleavage) that are pervasive in surrounding country rock, but that promoted late brittle deformation into which new generations of hydrothermal quartz, sulphide minerals, and gold were introduced. 47 8.0 Conclusion Previous structural model for mineralization in the KGD classifies mineralized orogenic veins to be entirely post-ductile, post-collisional features (MacKenzie et al., 2008). Through interpretations of petrographic and CL observations made in this study, it has been concluded that post vein-ductile deformation has affected this specific mineralized vein in the Lone Star Ridge region, and therefore current interpretations may not be applied to all mineralized veins in the region. MacKenzie et al. (2008) has interpreted mesothermal vein emplacement to postdate the F3 spaced cleavage whereas this study has concluded that deformation responsible for the formation of widespread crenulation and spaced cleavage outlasted mesothermal vein emplacement and mineralization. A new deformational history with overlapping Jurassic deformation events is therefore proposed as follows: Table 8.1: Deformational history diagram re-evaluating the MacKenzie et al. (2008) deformational sequence. This research may have implications for further exploration in the Klondike District and other orogenic gold districts. For example, studies considering the polyphase deformation of the Obuasi Gold Deposit of the Ashanti region in Ghana by Fougerouse, et al. (2017) have concluded that at this locality, two episodes of mineralization occurred creating the outstanding gold deposit: (1) synchronous precipitation of gold-bearing arsenopyrite during isoclinal folding of the metasedimentary protolith, and (2) the infilling of fractures with native gold controlled by fracturing and folding of quartz veins during a later deformation event. Although it is still open to interpretation as to if gold introduced during the first mineralization event was remobilized and precipitated along fractures, or if there were multiple mineralization events, this link between high-grade gold mineralization relating to post-hydrothermal 48 deformation of quartz veins in Obuasi is potentially applicable to the Klondike Gold District. Research published by Grimshaw (2018) has concluded that gold mineralization in orogenic quartz veins of the Figure 8.1: CL image of gold in limonite that is fractured by Q3 (MG28_BZ10). (Grimshaw, 2018) Lone Star Ridge is associated with a later “Q3” generation of quartz (Fig. 8.1) therefore potentially being precipitated through a deformational overprint like that observed along this Lone Star Ridge core. Although the deformation framework proposed in this study is supported by other field observations in the Klondike that are beyond the scope of this thesis, further research is recommended to validate the interpretation of ductile deformation post-dating hydrothermal veins. Instances of ductile deformation affecting orogenic veins within the Lone Star Ridge as well as all other localities in the KGD, whether in drill core or at the outcrop-scale, should be sought during future field work. 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A multi-spectroscopic study of luminescence sensitivity changes in natural quartz induced by high-temperature annealing, Journal of Physics D: Applied Physics, v. 34, n. 5, p. 722-731. 53 Appendix A: Thin Section Scans LS16-69_88.1m (XPL) 54 LS16-69_88.1m (XPL) 55 LS16-69_88.8m (PPL) 56 LS16-69_88.8m (XPL) 57 LS16-69_89.0m_A (PPL) 58 LS16-69_89.0_A (XPL) 59 LS16_69_89.0m_B (PPL) 60 LS16-69_89.0m_B (XPL) 61 LS16-69_89.4m (PPL) 62 LS16-69_89.4m (XPL) 63 LS16-69_89.6m (PPL) 64 LS16-69_89.6m (XPL) 65 LS16-69_90.4m (PPL) 66 LS16-69_90.4m (XPL) 67 Appendix B: Assays of LS16-69 drill core, interval 87.40m – 92.00m Hole ID From_m To_m Length (m) TotAu (GM/T) Mo (PPM) Cu (PPM) Pb (PPM) Zn (PPM) LS16-69 87.40 88.71 1.31 0.16 1.6 16.1 14 61 LS16-69 88.71 89.12 0.41 51.49 0.2 36 925.4 264 LS16-69 89.12 89.51 0.39 0.34 0.3 10.9 3.4 26 LS16-69 89.51 91.00 1.49 0.01 1.5 14.5 21.7 39 LS16-69 91.00 92.00 1.00 0.01 0.6 12.2 13.2 20 Hole ID From_m To_m Ag (PPM) Ni (PPM) Co (PPM) Mn (PPM) Fe (%) As (PPM) LS16-69 87.40 88.71 0.2 8.9 5.8 466 1.49 1 LS16-69 88.71 89.12 28.7 3.5 1.4 178 1.48 0.5 LS16-69 89.12 89.51 0.4 3.8 5.3 280 0.86 1.4 LS16-69 89.51 91.00 0.2 8.9 7.4 164 1.48 1 LS16-69 91.00 92.00 0.2 8.9 6.7 138 1.62 1.1 Hole ID From_m To_m U (PPM) Au (PPB) Th (PPM) Sr (PPM) Cd (PPM) Sb (PPM) LS16-69 87.40 88.71 3.1 110.4 9.8 53 0.7 0.2 LS16-69 88.71 89.12 3.3 57861.3 9.6 35 5.4 1.1 LS16-69 89.12 89.51 2.3 139.2 5.9 48 0.2 0.9 LS16-69 89.51 91.00 2.7 4.5 8.3 21 0.6 0.4 LS16-69 91.00 92.00 1.9 9.8 7.8 26 0.3 0.4 Hole ID From_m To_m Bi (PPM) V (PPM) Ca (%) P (%) La (PPM) Cr (PPM) LS16-69 87.40 88.71 0.1 4 1.77 0.036 9 7 LS16-69 88.71 89.12 1.9 2 0.44 0.006 6 4 LS16-69 89.12 89.51 0.1 3 0.58 0.036 8 7 LS16-69 89.51 91.00 0.1 3 0.48 0.046 8 7 LS16-69 91.00 92.00 0.1 3 0.58 0.046 6 6 Hole ID From_m To_m Mg (%) Ba (PPM) Ti (%) B (PPM) Al (%) Na (%) LS16-69 87.40 88.71 0.57 205 0.002 1 0.66 0.027 LS16-69 88.71 89.12 0.17 55 0.001 1 0.3 0.042 LS16-69 89.12 89.51 0.24 472 0.002 1 0.44 0.018 LS16-69 89.51 91.00 0.22 58 0.002 1 0.45 0.042 LS16-69 91.00 92.00 0.15 44 0.002 1 0.34 0.043 68 Hole ID From_m To_m K (%) W (PPM) Hg (PPM) Sc (PPM) Tl (PPM) S (%) LS16-69 87.40 88.71 0.23 0.2 0.01 1.4 0.1 0.8 LS16-69 88.71 89.12 0.17 0.1 0.3 0.6 0.1 1.31 LS16-69 89.12 89.51 0.27 0.1 0.02 0.9 0.1 0.31 LS16-69 89.51 91.00 0.26 0.1 0.01 1 0.1 1.3 LS16-69 91.00 92.00 0.22 0.1 0.01 0.9 0.1 1.58 Hole ID From_m To_m Ga (PPM) Se (PPM) Te (PPM) LS16-69 87.40 88.71 2 0.5 0.2 LS16-69 88.71 89.12 1 1.8 13.4 LS16-69 89.12 89.51 1 0.5 0.2 LS16-69 89.51 91.00 1 0.5 0.2 LS16-69 91.00 92.00 1 0.5 0.2
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University of British Columbia. EOSC 449
Mechanisms of Post-Hydrothermal Deformation in Orogenic Gold Deposits : A Case Study of the Klondike Gold District, Yukon
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