RESIDENCE OF GE IN THE SOUTH REEF OF THE BORNITE CU-CO DEPOSIT, AMBLER DISTRICT, ALASKA

The Devonian carbonate-hosted Cu-Co Bornite deposit is hosted in the Bornite sequence in the western Brooks Range, with NI43-101 inferred resources of 5450 Mlbs of Cu and 77 Mlbs of Co. Recent reconnaissance geochemical data from the South Reef orebody confirm locally elevated concentrations of Ge (10s to up to 125 ppm) in select mineralized drill core samples. Micro-XRF analyses of 41 samples, followed by petrographic and automated mineralogy analyses (n=25) indicate multiple stages of carbonate formation and brecciation, culminating in the deposition of copper as breccia fill, replacement, and vein ores. Sulfide assemblages are dominated by chalcopyrite and pyrite, with bornite and chalcocite being the dominant Cu-bearing phases in high grade intercepts. Cobalt occurs in cobaltiferous pyrite, carrollite, and rare cobaltite, the latter two of which are intimately associated with Cu-sulfides. Other minor sulfides include galena and sphalerite, along with tennantite.

Germanium predominantly occurs as sparse, up to 100 μm-diameter germanite/renierite inclusions in chalcocite and bornite and only rarely in chalcopyrite. This association is similar to previously reported Ge distribution and sequestration in the Number One orebody in the Bornite deposit. Mapping μXRF analyses indicate low unquantified concentrations in galena, as well as the alteration minerals K-feldspar and muscovite/illite where it may be substituting for geochemically similar silicon. Ge was not detected in sparse sphalerite.

The source and depositional mechanism of Ge remain uncertain, although the association of renierite/germanite with high sulfidation state Cu sulfides (e.g., chalcocite and bornite) is shared with other Ge-bearing carbonate-hosted Cu deposits around the globe. An abundance of organic matter (represented by anthraxolite), and paucity of barite but presence of cymrite (Ba-silicate) in the Bornite deposit ore zones suggest a reduced Ge-bearing ore forming fluid. Thermodynamic considerations indicate that ore deposition was strongly controlled by a shift in pH and oxygen fugacity where a reduced, acidic, metalliferous fluid was buffered to near neutral pH and an oxygen fugacity near the hematite-magnetite buffer as a consequence of fluid-rock interaction at the site of deposition.