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

      Mahaffey, Zachary B.; Newberry, Rainer; Schrader, Christian; Regan, Sean (2021-08)
      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 to Co) and As (maximum 5.6%, generally correlates with Co). Compositions of Co-pyrite lacking significant As (up to 8.5% Co) and late cobaltite suggest temperatures of 400-500℃, consistent with Upper Blueschist to Greenschist conditions. Pyrite compositions can be extremely variable within a single sample and even within a single grain: nearly half of all EPMA pyrite analyses yield < 0.25% Co. Clearly metamorphic (porphyroblastic) pyrite displays concentric oscillatory Co + As zonation. Average Co content in pyrite generally increases with Cu grade, which suggests Devonian Co and Cu deposition occurred simultaneously. Co deportment is a function of location in the deposit, which correlates with Cu grade and Co mineral zonation. Based on metal balance calculations, more than half (on average 57%) of the Co in South Reef intervals with > 0.5% Cu is due to Co-pyrite. In the Lower Reef the proportion is much higher: 55-93%. High Cu grade intervals typically yield increased carrollite abundance and decreased cobaltite. Carrollite consistently reports to the Cu concentrate, however, cobaltite recovery is variable and can report to the tails with Co-pyrite. Thus, maximizing Co recovery from Bornite will require producing a pyrite concentrate in addition to the Cu concentrate.