Wall-Rock Alteration, Structural Control, and Stable Isotope Systematics of the High-Grade Copper Orebodies of the Kennecott District, Alaska

Abstract

The Kennecott district, Alaska, contained three important copper-(silver) orebodies, Bonanza-Mother Lode, Jumbo, and Erie, and numerous other small occurrences. From 1911 to 1938, the Kennecott Copper Corporation recovered some 1.18 billion pounds (Blbs) of copper and around 9 million ounces (Moz) of silver from these deposits. The host rocks for the Kennecott deposits are part of the type section Wrangellia terrane. Ore was hosted in the Triassic (Carnian) Chitistone Limestone, most of it located 20 to 40 m above the contact with the native copper-bearing Triassic (Ladinian) Nikolai Greenstone. The Bonanza-Mother Lode and Jumbo orebodies are upward-tapering veins having wide bases controlled by one or more, often subtle, bedding-parallel fault(s); they plunge downdip to the northeast with long axes approximately parallel to the direction of regional shortening. The veins hosting the orebodies occupy NE-striking oblique-slip faults with dextral and normal kinematics that were probably formed between 155 to 110 Ma during the docking of Wrangellia with North America. The deposits consist of a paragenetically early and volumetrically minor assemblage of chalcopyrite-bornite-luzonite-covellite that precipitated between approximately 180° and 110°C, a dominant stage of chalcocite-djurleite that precipitated in the range of 110° to 90°C, and a minor late assemblage of covellite-digenite-(spionkopite/yarrowite) that probably precipitated at even lower temperatures. Wall-rock alteration includes premineral hydrothermal dolomitization and synmineral dedolomitization. Hydrothermal dolomite occurs in replacement zones one to tens of meters wide along favorable structures and stratigraphic horizons, whereas dedolomite is generally restricted to thin (<1 m) selvages along structures, within breccias, and adjacent to orebodies. Sulfur isotope measurements on samples collected from the Bonanza and Jumbo mines, as well as from numerous smaller deposits and prospects in both the Chitistone Limestone and Nikolai Greenstone throughout the region, indicate that δ^(34)S generally becomes enriched through time and paragenesis. Sulfur for the deposits was probably derived from sulfate in both the Nikolai Greenstone and the basal Chitistone Limestone that was reduced during burial and diagenesis via thermochemical sulfate reduction (TSR). The extremely broad range (54.1‰) of sulfur isotope compositions, including in the main-stage chalcocite-djurleite assemblage (from −16.0 to +17.4‰), is probably due to TSR operating on two separate sources of sulfate and limited incorporation of bacteriogenic sulfide, which was sourced from either early bacterial sulfate reduction (BSR) or from dissolution of diagenetic pyrite found in the lower Chitistone Limestone. Field and isotopic data support a model in which an oxidized fluid generated by dehydration reactions during prehnite-pumpellyite metamorphism of the Nikolai Greenstone scavenged copper and possibly sulfur from the greenstone and then moved along faults into the overlying Chitistone Limestone where it mixed with an anoxic basinal brine containing reduced sulfur. Copper deposition was controlled fundamentally by redox reactions with main-stage precipitation occurring at low temperatures (<110°C). Important field criteria for the recognition of Kennecott-style deposits include any or all of the following: baroque dolomite, dedolomite, and hydrothermal breccia, particularly in association with faulted carbonate host rocks.