Abstract

Homoclinal Upper Jurassic to Cretaceous inlier consisting of sedimentary, pyroclastic, and volcanic rocks plus intrusions overlain by younger, unconformable andesitic to basaltic flows and flow breccias. Four stratigraphic conformable subdivisions, termed the Goosly Sequence, are recognized in the inlier. A quartz monzonite stock with an approximate age of 58 m.y. and a gabbro-monzonite complex with an approximate age of 49 m.y. intrude the Goosly Sequence. Copper-silver-antimony sulfides and sulfosalts with associated gold occur as tabular zones with attitudes grossly conformable to the Goosly Sequence. Sulfides were deposited as disseminations, open-space fracture fillings, veins, and crackle and breccia zones with an associated advanced argillic alteration suite. Potassium-argon age dating indicates a main pulse of mineralization and hydrothermal alteration around 58 m.y. and a younger postmineral event at around 49 m.y. Mining of the Southern Tail orebody commenced in April 1980 with a current mill feed rate of 5,400 metric tons/day. Production to December 1982 totaled 4.3 million metric tons of ore grading 135 g/metric ton Ag, 0.45 percent Cu, and 1.3 g/metric ton Au. Antimony and arsenic are leached from concentrate and recovered as by-products. The Main Zone orebody, with reserves of 21.6 million metric tons grading 109 g/metric ton Ag, 0.35 percent Cu, and 0.85 g/metric ton Au, commenced production in late 1983. Alteration and mineralization are compatible with an advanced argillic alteration, possibly related to base leaching associated with fluid circulation at a high level in a developing porphyry system.--Modified journal abstract.

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