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The southwest zone breccia-centered silica-undersaturated alkalic porphyry Cu-Au deposit, Galore Creek, B.C : magmatic-hydrothermal evolution and zonation, and a hydrothermal biotite perspective Byrne, Kevin

Abstract

Situated in northwest B.C Canada, the Southwest Zone Cu-Au breccia-centered deposit is one of twelve mineralized centers in the Galore Creek alkalic porphyry district. Formed in an island arc-setting outboard of ancestral North America in the lateTriassic, deposits in the Galore Creek district have a combined measured and indicated resource of 785.7 Mt at 0.52 per cent Cu, 0.29 g/t Au and 4.87g/t Ag. Mineralisation in the Southwest Zone is centered in narrow hydrothermally cemented breccias. Composite dikes of megacrystic orthoclase-phyric syenite and megacrystic orthoclase and plagioclase-phyric monzonite are cut by polylithic, poorly sorted pebblecobble clast size matrix-rich breccias. Hydrothermally cemented breccias, characterised by the cement (infill) assemblage phlogopite ± K-feldspar ± magnetite ± anhydrite ± diopside ± suiphide, overprint the western contact between matrix-bearing breccias and megacrystic porphyries. Coeval with cemented breccia formation, intrusions of biotite phyric monzodiorite occur at the matrix-bearing breccia-wall rock contact and in the matrix-bearing breccias. Biotite-phyric monzodiorite and the principal cemented breccia domains are co-spatial and syn-Cu-Au. Drilling has outlined a zone of >0.3% hypogene Cu approximately 20-100m thick, 500m wide and 400m in length that strikes 100, dips 45-60°S, and has a semiellipsoidal morphology. This mineralisation is coincident with potassic (stage D) alteration and infill. Cu-poor, diopside-dominated (calc-[potassic]) alteration formed contemporaneously with, and locally flanks, potassic-D infill. Sulphide minerals are zoned from a core of chalcopyrite-bomite, to chalcopyrite>pyrite, to pyrite>chalcopyrite out to pyrite only. Garnet-bearing peripheral propylitic alteration overlaps with a pyrite and Au-halo and locally overprints potassic and calc-Q,otassic) assemblages. Based on electron microprobe analysis, systematic spatial variations in Ti-content and Fe³⁺/Fe²⁺ of infill biotite are evident. Increases in Ti-contents and Fe³⁺/Fe²⁺ overlap with positive gradients in Cu concentration, taken with interpreted alteration reactions, this suggest Cu-deposition is caused by decreasing fO₂ coupled with an increase in pH at 420-475°C. Low log(fH₂/fHF)₂, determined from infill biotite, distinguish potassic fluids in the Southwest Zone, and other alkalic porphyry deposits, from fluids in calcalkalic systems and reflects the contrasting magmatic composition.

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