An Archean Felsic Stock With Peripheral Gold And Copper Occurrences Abotossaway Township, District Of Algoma, Ontario

A stock of trondhjemite intrudes an Archean succession metamorphosed to the greenschist facies at Gutcher lake, 30 km north of Wawa, Ontario. The stock is 4 km(\u272) in plan and is partly enveloped by an aureole of epidote-hornblende hornfels up to 1 km wide. Within the contact aureole chlorite is pseudomorphous after biotite; chlorite, calcite, plus quartz are pseudomorphous after hornblende plus epidote; poikiloblastic hornblende has rims of actinolite; and iron content in epidote decreases from core to rim. The stock has chlorite pseudomorphous after biotite; and feldspar is mottled with white mica, quartz, plus calcite and has clear rims of albite. Fractures filled with quartz, ankerite, calcite, chlorite, plus muscovite transect the stock and its contact aureole.;Chalcopyrite and gold coexist with chlorite, quartz, biotite, pyrite, nd pyrrhotite as stringers and lenses in layered rocks within or adjacent to the contact aureole. Gold also occurs with pyrite, pyrrhotite, chalcopyrite, and sphalerite in quartz-carbonate veins transecting the contact aureole and margin of the stock. These veins are fringed by an albite-pyrite-muscovite-chlorite-calcite-quartz assemblage.;Wallrocks about gold-bearing veins are enriched in Si, Fe, K, H(,2)O + CO(,2), S, plus Rb; depleted in Na; and reduced with (Fe(\u27+2)/Fe(,t)) = 0.9, compared to (Fe(\u27+2)/Fe(,t)) = 0.7 in background rocks. Veins have (delta)(\u2718)O(,Qtz) = 11.5(\u27o)/oo to 14.1(\u27o)/oo. The interpretation is that alteration of the stock, retrogression of its contact aureole, and formation of gold-bearing veins were contemporaneous with regional greenschist metamorphism. Gold-bearing veins precipitated out of metamorphogenic fluids at T = 325(DEGREES)C to 450(DEGREES)C, and with (delta)(\u2718)O(,H(,2)O) = 6(\u27o)/oo to 9(\u27o)/oo