Germanium occurrence in the bornite deposit, southwestern Brooks Range, Alaska

Abstract

The carbonate hosted Bornite Cu-Zn-Co(-Ge) deposit is located in the Cosmos Hills just south of the western portion of the Brooks Range of north western Alaska. In 2011, a new orebody referred to as the South Reef was found at greater depth. The South Reef is host to substantial amounts of high-grade Cu, Co, and Zn, but also hosts elevated concentration of Ge. With the increased demand for germanium and the lack of literature on its distribution within primary mineral deposits, understanding known Ge-bearing deposit is critical for future exploration efforts. In this study, the mineralogy and paragenesis of the South Reef has been described through detailed optical petrography, scanning electron microscopy, automated mineralogy and mapping micro-X-ray fluorescence analysis. Five paragenetic stages have been delineated: (I) dolomitization, (II) ferroan alteration, (IIIa) Veining, (IIIb) brecciation, (IV) ore formation event, and (V) a post ore forming event. Germanium was found to exist in renierite, white mica, illite, galena and feldspars within the South Reef. Whole-rock geochemistry of samples revealed highest-grade Ge concentrations correlating to the samples containing the highest abundance of bornite and chalcocite. It is proposed here that reduced acidic basinal brines capable of transporting significant Cu interacted with carbonaceous phyllites at the site of deposition. Upon fluid-rock interaction of the reduced acidic basinal brine with organic matter rich (reduced) carbonates and phyllites, the basinal brine was buffered to an intermediate oxidation state (around or below the hematite-magnetite buffer) at a near-neutral pH of around 5-6. Petrographic observations reveal a distinct spatial and temporal precipitation succession of chalcocite, bornite, followed by chalcopyrite. This temporal and spatial succession is interpreted to reflect the evolving fluid accompanied by a decrease in temperature and concomitant decrease in sulfide activity of the mineralizing fluid. While temperature may not have been the predominant precipitation mechanism (a shift in pH towards more alkaline conditions is thought to be the dominant precipitation mechanism), changes in temperature and a decrease in sulfide activity have contributed to the distinct mineral zonation in space and time. Germanium is interpreted to have precipitated in the form of renierite contemporaneously with chalcocite and bornite when the sulfur activity in the fluid was still high. The Bornite deposit shares many similarities with other Ge-bearing carbonate hosted base metal deposits in Africa such as the Tsumeb, Kipushi, Khusib Springs, Kombat, and Kabwe deposits, however it is of lower germanium grade than these deposits.