UBC Theses and Dissertations
Geology of the Snip Mine, and its relationship to the magmatic and deformational history of the Johnny Mountain area, northwestern British Columbia Rhys, David A.
The Snip mine consists of an auriferous southwest-dipping shear vein system in a north-dipping Triassic greywacke-siltstone sequence altered with abundant vein let and pervasive biotite. The deposit, termed the Twin zone, is the largest of numerous shear veins in the mine workings. It contains just under 30 tonnes of gold. The Twin zone is composed of four ore types that are mineralogical end members of two distinctive styles of mineralisation. They are: (i) carbonate and (ii) chlorite-biotite ore consisting of laminated schistose veins of calcite, chlorite, biotite and pyrite, which display textures indicative of an origin, at least partially by, wall rock replacement; and (iii) dilatant quartz veins and (iv) dilatant pyrite-pyrrhotite sulphide veins. Alteration comprises pale calcite-K-feldspar-silica envelopes surrounding black biotite envelopes adjacent to the veins. A post-ore biotite-altered mafic dyke intrudes the zone. Geologic relations in the Snip mine indicate that the mineralised veins were emplaced progressively in a dynamic tectonic environment characterised by semi-brittle deformation. Numerous kinematic features in the zone record normally-directed simple shear parallel to a westerly plunging elongation lineation. Deformation was heterogeneous and confined mainly to the shear veins. Galena Pb-Pb isotopic signatures suggest an Early Jurassic age for the veins. Abundant shallow easterly-dipping quartz-calcite extension veins formed during a later, probably Tertiary, event. The Red Bluff porphyry, an elongate K-feldspar megacrystic plagioclase porphyritic quartz diorite to tonalite stock, intrudes the greywacke sequence 300-800 metres northeast of the Twin zone. Two successive, intense hydrothermal events are centred on the porphyry and are associated with subeconomic Au and Cu concentrations. These are: (i) early quartz-magnetite-sericite-K-feldspar-biotite (potassic)alteration associated with abundant quartz-magnetite-hematite veins, overprinted by (ii) sericite-pyrite-quartz (phyllic) alteration characterised by pyrite veining. Intrusion, semi-brittle deformation, and alteration and mineralisation within the large hydrothermal system of the Red Bluff porphyry were closely related spatially and temporally. A genetic relationship is supported by: similarities in structural fabrics and alteration histories in the Twin zone and the porphyry, alteration and metal zoning of vein systems distributed concentrically around the porphyry, and apparently concordant Early Jurassic zircon U-Pb age from the porphyry with galena Pb-Pb signatures from the Twin zone and surrounding vein systems. Two significant structurally-controlled Au deposits in the area, Inel and Stonehouse, have similar age, mineralogy, structure, alteration and spatial relationships to Early Jurassic intrusions. They represent contemporaneous, possibly genetically related, hydrothermal systems.
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