NUKUNDAMITE (CU5.5FES6.5) AT HIGH SULFIDATION STATES ASSOCIATED WITH POTASSIC ALTERATION IN THE BINGHAM PORPHYRY CU-AU-MO DISTRICT, UTAH

Nukundamite (Cu_5.5FeS_6.5) is an important ore mineral in high grade (1-2 wt. % Cu) quartzite  on the northeast side of the Bingham stock. In contrast with other deposits, where nukundamite occurs in relatively low-temperature, sericitic assemblages, nukundamite at Bingham is found predominantly in higher-temperature potassic alteration assemblages. This occurrence suggests an unusual evolutionary path for magmatic-hydrothermal fluids in quartzite wall rocks.

Coeval with porphyry-hosted Cu-Au ore, Main-Stage hydrothermal activity in quartzite produced 10-30 vol. % phlogopite ± K-feldspar associated with disseminated nukundamite. Replacement of nukundamite by bornite and chalcopyrite late in the Main Stage reduced its abundance from 85-94% to 2-17 % of total sulfides.

Based on thermodynamic calculations, nukundamite is stable at very high sulfidation states, higher than covellite, within a limited range of both a(Fe²⁺)/a(H⁺)² and fS₂.  The restriction of nukundamite to quartzite suggests that this wall rock acted as a non-reactive acquifer during early ore deposition at Bingham. Due to lack of redox buffers in, and exchange reactions with, quartzite, hypersaline liquids which had separated from a magmatic vapor phase retained their relatively high initial SO_2(g)/H₂S_(g) ratio and high concentrations of S and K. This unusual geochemical environment led to the precipitation of nukundamite in equilibrium with K-feldspar and phlogopite.

Replacement of nukundamite by bornite and chalcopyrite late in the Main Stage required a decrease in sulfidation and oxidation state and an increase in a(Fe²⁺)/a(H⁺)² under pyrite-supersaturated conditions. This evolutionary path suggests a change from fluid-buffered to rock-buffered conditions induced by influx of hypersaline brines that had reacted with quartz monzonite porphyry at depth. Iron was leached from the stock by Main Stage hydrothermal fluids during potassic alteration and ore deposition. These rock-buffered fluids, enriched in iron, could have caused the replacement of nukundamite by chalcopyrite and bornite on reaching quartzite at the end of Main Stage, producing a mineral assemblage more typical of early stages of porphyry copper deposits.