Wall-rock alteration, structural control and stable isotope systematics of the high-grade copper orebodies of the Kennecott district, Alaska.

The Kennecott district contained three important Cu-(Ag) orebodies, Bonanza-Mother Lode, Jumbo, and Erie, and numerous other small occurrences. From 1911 to 1938, the Kennecott Copper Corporation recovered some 4 000 000 Mt of ore averaging 13% Cu and c.65 g/t Ag from these deposits. The host rocks are part of the type section Wrangellia terrane. The deposits consist of a paragenetically early and volumetrically minor assemblage of chalcopyrite-bornite-luzonite-covellite, precipitated between c.180 and 110 degrees C, a dominant stage of chalcocite-djurleite, precipitated between 110 and 90 degrees C, and a minor late assemblage of covellite and digenite (with spionkopite/yarrowite) that probably precipitated at even lower temperatures. As supported by field and isotopic data, the ore deposits are fundamentally high-angle, fault-controlled, open-space fill and replacement veins, formed by mixing an oxidised Cu- and possible sulphate-bearing fluid from the Nikolai greenstone and a reduced sulphide-bearing connate fluid from the Chitistone limestone, during the waning stage of Wrangellia accretion during c.130-110 Ma. The system was probably driven by a combination of heat produced by low-grade metamorphism and gravitational heat from crustal thickening with attendant uplift and later orogenic collapse. Copper deposition was controlled fundamentally by redox reactions with main-stage precipitation occurring at low temperatures.
The Kennecott district contained three important Cu-(Ag) orebodies, Bonanza-Mother Lode, Jumbo, and Erie, and numerous other small occurrences. From 1911 to 1938, the Kennecott Copper Corporation recovered some 4 000 000 Mt of ore averaging 13% Cu and c.65 g/t Ag from these deposits. The host rocks are part of the type section Wrangellia terrane. The deposits consist of a paragenetically early and volumetrically minor assemblage of chalcopyrite-bornite-luzonite-covellite, precipitated between c.180 and 110 degrees C, a dominant stage of chalcocite-djurleite, precipitated between 110 and 90 degrees C, and a minor late assemblage of covellite and digenite (with spionkopite/yarrowite) that probably precipitated at even lower temperatures. As supported by field and isotopic data, the ore deposits are fundamentally high-angle, fault-controlled, open-space fill and replacement veins, formed by mixing an oxidised Cu- and possible sulphate-bearing fluid from the Nikolai greenstone and a reduced sulphide-bearing connate fluid from the Chitistone limestone, during the waning stage of Wrangellia accretion during c.130-110 Ma. The system was probably driven by a combination of heat produced by low-grade metamorphism and gravitational heat from crustal thickening with attendant uplift and later orogenic collapse. Copper deposition was controlled fundamentally by redox reactions with main-stage precipitation occurring at low temperatures.