The mineralogical associations, distribution, and mineral zoning of cobalt in the Bornite deposit, southwest Brooks Range, AK

• Main Author: Mahaffey, Zachary B.
• Other Authors: Newberry, Rainer, Schrader, Christian, Regan, Sean
• Format: Thesis
• Language: English
• Published: 2021
• Subjects: Bornite; Brooks Range; Cobalt; Copper ores; Master of Science in Geoscience; Fairbanks; South Reefs; Alaska
• Online Access: http://hdl.handle.net/11122/12625

Thesis (M.S.) University of Alaska Fairbanks, 2021 The Bornite Cu-Co deposit is predominately hosted in dolomitic marble with interstratified phyllite; the mineral resource is restricted to the Upper, Lower and South Reefs. Cobalt is primarily from carrollite (ideally CuCo₂S₄), cobaltite (ideally CoAsS), and cobaltiferous pyrite ((Fe,Co)S₂). The Co minerals can contain significant Ni, and the Ni rich endmembers, millerite (NiS) and gersdroffite (ideally NiAsS), also rarely occur. Detailed handheld XRF analyses on 15 drill holes, coupled with reflected light petrography, electron microprobe-based compositional maps, and electron microprobe analyses (EPMA) have shown complex compositions, textures, associations, and spatial distribution of the Co minerals in the Bornite deposit. Carrollite compositions and textures vary with the Cu-sulfide assemblage: carrollite with bornite is commonly porphyroblastic and approximated by Cu(Co,Ni)₂S₄, whereas carrollite lacking associated bornite is interstitial and represented by (Cu,Ni)(Co,Ni)₂S₄. Cobaltite occurs in two generations: early As-depleted, Ni-poor, and metastable ((Co,Fe)As₀.₅S₁.₅ to (Co,Fe,Ni₀.₀₆)As₀.₈₈S₁.₁₂), and late near stoichiometric ((Co,Fe,Ni₀.₀₁)As₀.₉S₁.₁ to (Co,Fe,Ni₀.₉₄)As₁S₁). The latter rims and (or) replaces the former. The virtual lack of cobaltite in assemblages containing bornite + pyrite, along with carrollite compositions, suggest a gradient in Cu and Co activity that increased with increasing fS₂. Distinct carrollite zones associated with higher Cu grades are present in the high-grade zone (Number One Orebody) of the Upper Reef and the South Reef. Decreasingly Cu-rich assemblages dominated by chalcopyrite + pyrite and cobaltite are outside the carrollite zones. The Lower Reef has lower Cu grades, lacks a carrollite zone, and variably contains cobaltite. Due to these different Co mineral distributions, the Bornite deposit cannot represent dismemberment of a single homogeneous body. Bornite pyrite can contain Ni and Cu (both inversely correlated ...