Descriptive and Geoenvironmental Model for Cobalt-Copper-Gold Deposits in Metasedimentary Rocks

Abstract

A compilation of global data on cobalt-copper-gold (Co-Cu-Au) deposits hosted by metasedimentary rocks refines previous descriptive models for their occurrence and provides important information for mineral resource assessments and exploration programs. As defined herein, the Co-Cu-Au deposits contain 0.1 percent or more by weight of Co in ore or mineralized rock, consisting of disseminated to semi-massive Co-bearing sulfarsenide and sulfide minerals with associated Fe- (iron) and Cu-bearing sulfides, and local gold, concentrated predominantly within rift-related, siliciclastic metasedimentary rocks chiefly of Proterozoic age. Most of the deposits contain abundant As (arsenic) and have atomic Co/As ratios that approach 1. Some deposits have appreciable Ag ± Bi ± W ± Ni ± Y ± REE ± U (silver, bismuth, tungsten, nickel, yttrium, rare earth elements, and uranium). Deposit geometry includes stratabound and stratiform layers, lenses, and veins, and (or) discordant veins and breccias. The geometry of most deposits is controlled by stratigraphic layering, folds, axial-plane cleavage, shear zones, breccias, or faults. Ore minerals are mainly cobaltite, skutterudite, glaucodot, and chalcopyrite, with minor gold, arsenopyrite, pyrite, pyrrhotite, bismuthinite, and bismuth; some deposits have appreciable tetrahedrite, uraninite, monazite, allanite, xenotime, apatite, scheelite, or molybdenite. Magnetite can be abundant in breccias, veins, or stratabound lenses within ore or surrounding country rocks, but is absent in some deposits. Common gangue minerals include quartz, biotite, muscovite, K-feldspar, albite, chlorite, and (or) scapolite; many deposits contain minor to major amounts of tourmaline. Altered wall rocks generally contain abundant biotite or albite. Based on geoenvironmental data for the Blackbird district in central Idaho, weathering of minerals containing abundant Fe (iron), S (sulfur), As, Co, and Cu in the deposits produces acidic waters, particularly in pyrite-rich deposits. Mine runoff has concentrations of Fe, Cu, and Mn (manganese) that exceed U.S. drinking water or aquatic life guidelines.

First posted December 30, 2013

Revised March 14, 2014