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Tectonic setting of mineralization in the southern Kootenay Arc and Purcell Anticlinorium, southeastern British Columbia Ewan R. Webster & David R.M. Pattison Department of Geoscience, University of Calgary, Alberta, Canada Introduction Geology of the Southern Kootenay Arc and Purcell Anticlinorium Metamorphic Geology and Mineral Zones Contact Metamorphic Bathozones and Mineral Subassemblages The geoglogically complex region of southeast British Columbia between Nelson, Salmo and Creston is situated Nelson Midge ISOGRAD PATTERN Kootenay Lake 117°10'0"W 117°0'0"W 116°50'0"W 116°40'0"W 116°30'0"W at the interface between three tectonic domains: The Purcell Anticlinorium, Kootenay Arc and the Priest River LAYERED ROCKS & DRIFT Creek Two elongate domains of regional middle am- JURASSIC METAMORPHISM Stock Plutonic Rocks Complex (Figure 1). This tectonic juxtaposition and contemporaneous magmatism, metamorphism, and defor- Drift Based on the abundant Midge phibolite facies rocks merge in the northern part of mation occurred during Cordilleran orogenesis in a time interval spanning the Jurassic to Eocene. This study Hall Formation Nelson 116 Drewry Creek staurolite+andalusite assemblages, con- Lower and Batholith Nelson Eocene the study area, forming a southward facing forked aims to elucidate on the complex tectonothermal history of the area during orogenesis as a means of putting the Upper ELise Formation Point stock Drewry Middle Lower Elise Formation batholith isograd pattern that is separated by a large area of tact metamorphism associated with the areas mineral deposits in context Jurassic q LITHOLOGY Point Cretaceous Archibald Formation greenshcist facies metamorphism. Mine and Wall stocks occurred at 3.5–4 t pluton q The study area is situated mainly within de- l 51 u kbar (Figure 7, yellow domain). Ymir Group P The structure of the rocks in the southern Kootenay Arc and western limb of the Purcell Anticlinorium is affected a { u formed and metamorphosed sedimentary Jurassic F Active Formation r by at least three phases of penetrative deformation, spanning the time interval from the Middle Jurassic to c k REGIONAL METAMORPHIC ZONES Lower Cambrian e rocks of the: Purcell Supergroup, Winder- 111 e Nelway Formation e l Eocene (Fyles, 1964, 1967; Leclair, 1988; Glover, 1987; Brown, 1995). In general there is an increase in struc- to Middle r P The Porcupine Creek stock is situated 4 161 T Three regional amphibolite-facies metamorphic C u r Ordovician Upper Laib Formation mere Supergroup and Paleozoic clastics and e 92 r tural complexity and intensity of deformation going from the west of the area to the east and north, correspond- e km northwest of the Wall stock and has { c n g e zones have been identified within the map area: Laib Formation carbonates. 43 c Mount Skelly d l ing to progressively deeper structural levels. An exception to this overall pattern are the Purcell Trench fault i l Mount Skelly h Regional Metamorphic Zones Midge Creek fault a lower pressure (~3.0–3.5 kbar) con- T f pluton garnet, kyanite and sillimanite. These metamorphic Badshot Formation M a (PTF) and Midge Creek fault (MCF), across which there are marked differences in structural style and metamor- r Pluton e 49°20'0"N u 49°20'0"N tact aureole with cordierite+andalusite Reno Formation A A’ n lt zones are typical of Barrovian metamorphism, with phic grade (Figure 2,3 & 5). c 90 Lower h Garnet (Figures 5, 7). Quartzite Range Formation F the highest metamorphic grade in the footwall of Cambrian Bayonne a Batholith GRANITOIDS 51 Bayonne batholith Hamill Quartzite Baldy u the PTF and MCF, and progressively lower grades The regional metamorphic grade in the study area is dominantly greenschist facies with two discrete, elongate { lt The metasediments described above are host Kyanite CRETACEOUS METAMORPHISM Upper Three Sisters Formation Pluton with increasing distance from the fault zone. domains of amphibolite facies metamorphic rocks (Figure 5). In the northern part of the field area the amphibo- to numerous granitoid intrusions that range in lite facies metamorphism forms a southward facing forked isograd pattern. The western fork is parallel to strike Proterozoic Steeple The mid-Cretaceous, post-kinematic Monk Formation Steeple Porcupine Sillimanite (Windermere age from Middle Jurassic to Eocene and are Mountain 69 Lost Creek pluton (LCP) and Summit and is truncated by the Midge Creek fault. This fork is a continuation of the metamorphic high mapped north of { Irene Formation Porcupine Mountain Creek 55 Supergroup) part of larger intrusive suites extending stock pluton CONTACT METAMORPHISM the west arm of Kootenay Lake by Moynihan and Pattison (2008). The eastern fork is approximately parallel to Toby Formation Creek 141 51 stock (SS) have imparted lowest pres- Stock The study area has been intruded by numerous Ju- the Purcell Trench fault and transects the strike of lithological units. The eastern fork continues south into the Mount Nelson Formation across southeastern BC. 157 sure contact aureoles (2.2-3.2 kbar, Wall rassic and Cretaceous plutons (Figures 2, 5) that Figure 7. Schematic P-T phase diagram showing the relationship of mineral assemblage domains U.S.A and merges with amphibolite facies metamorphism in the Priest River Complex. Dutch Creek Formation Salmo stock 127 69 Shaw Contact Metamorphic Zones to key sub-assemblages. Contact aureoles from the field area are highlighted on the diagram with figure 7 blue domain) on the surround- Wall Next Creek Fault 72 Shaw stock 102 Creek have imparted contact aureoles to the surrounding their corresponding colours. As observed on the P-T diagram these mineral assemblages are very Van Creek Formation Salmo Stock 100 147 ing country rock. Creek stock country rock. The contact aureoles typically have a sensetive to pressure and help constrain the emplacement depth of the intrusions. The Jurassic intru- The MCF juxtaposes greenschist-facies phyllite of the Milford Group against sillimanite-zone schist in the foot- Middle Kitchener Formation Stock Garnet Proterozoic Stock FAULTING sions were genrally emplaced at deeper structural levels relative to the Cretaceous intrusions. Dia- wall. There is also a large contrast in metamorphic grade across the PTF (Figure 5): pelite in the footwall has the Middle Creston Formation Salmo Sheep Creek 155 granoblastic texture, unique mineralogy and a (Purcell Two large Eocene normal faults crosscut the Mining Camp 127 gram modified from Pattison and Vogl (2005). Abbreviations are from Kretz (1983). mineral assemblage sillimanite±kyanite+garnet+muscovite+biotite+plagioclase+quartz+rutile, whereas pelite in Supergroup) Lower Creston Formation 49°10'0"N 49°10'0"N Staurolite±andalusite coarser grain size that make them distinguishable study area: the Purcell Trench fault and the 55 the hangingwall has the assemblage biotite+muscovite+chlorite+albite+quartz (Figure 6). Based on these min- Upper Aldridge Sheep Creek Mine stock 52 Mining Camp 81 from the regional metamorphism. In summary, the Jurassic intrusions were emplaced at 3.5–4 kbar, which, for a pressure of 2.7 g/cc, corresponds to eral assemblages, the contrast in peak metamorphic conditions across the fault is >2 kbar and >150°C. Middle Aldridge Midge Creek fault, across which there are 162 Sillimanite B 156 78 Lower Aldridge marked differences in structural style and an 11–13 km emplacement depth, considerably deeper than the Cretaceous intrusions at 7–11 km. This implies { Mine Stock Summit Huscroft Andalusite±cordierite METAMORPHIC CONTRASTS approximately 5 km of aggregate exhumation between emplacement of the Jurassic intrusions and the Cretaceous The study area has been intruded by numerous Jurassic (I-type) and Cretaceous (S-Type) plutons that have im- INTRUSIVE ROCKS metamorphic grade. stock fault Across the PTF and MCF there are marked differ- intrusions. parted contact aureoles to the surrounding country rock. Based on mapped mineral assemblages and isochemical Eocene Intrusions Salmo Salmo 209 96 Mining Camp Mining Camp Lost Creek 60 104 Andalusite phase diagram modeling the Jurassic intrusions were emplaced at deeper levels in the crust than the Cretaceous Cretaceous Granitic Rocks 58 ences in structural style and metamorphic grade. pluton intrusions (Figure 7). The historic Salmo mining camp is well known for its extensive deformed MVT Pb-Zn de- Mid-Jurassic Granitic Rocks B’ Summit Prospective One exception to the large temperature and pres- STRUCTURE Area posits and Cretaceous W-Mo-Au skarns. Recent work has delineated a large, previously unidentified contact au- Moyie Sills Stock 102 sure contrast across the PTF is an area of garnet- Lost Creek As a result of different episodes of deforma- reole extending around the Summit stock and Lost Creek pluton. Several skarn deposits on the western margin ROCKS OF UNCERTAIN AFFINITY K-Ar data zone rocks in the hangingwall of the Huscroft fault Pluton Prospective tion that are confined to specific regions of of the Lost Creek pluton (Jumbo, Molly, MUT) suggest a potential for further Mo-W mineralization in the ther- Undivided sedimentary rocks Area Creston itself in the footwall of the PTF. The upper- the study area, the structural features are di- Links to Mineralization mally metamorphosed area extending east to the Summit stock (Figure 5). Meta rocks of uncertain affinity Blazed Creek fault Mt Rykert block Blazed Creek Fault greenschist-facies rocks in the hangingwall of the C vided into a western, northern and eastern 101 52 Muscovite K-Ar age (Ma) “Mixed Unit” pelite intruded by 54 Rykert batholith Huscroft fault contrast with the sillimanite-zone granite and pegmatite domain. 49°0'0"N 49°0'0"N Biotite K-Ar age (Ma) C’ rocks beneath the fault, implying a large amount of A 50–75 km wide arcuate belt of Cretaceous intrusions, known as the Bayonne magmatic suite, extends from the FAULT displacement on the fault. Because the PTF cuts Hornblende K-Ar age (Ma) Canada–United States border to north of Quesnel Lake, crosscutting the study area.

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