Metasomatic processes in contact aureoles of porphyry copper deposits: A case study of the Binham District, Utah

The northeast contact aureole of the Bingham stock contains mineral assemblages previously unknown in the district. The alteration/mineralization patterns associated with ore types developed in two stages. The Main Stage, developed as a halo around the porphyry orebody, is coeval with potassic alteration and Cu-Au ores in the stock, and consists of biotite-actinolite with chalcopyrite + bornite + nukundamite in quartzite, and andradite-salite skarn with chalcopyrite + pyrite + magnetite in limestone. The Late Stage, coeval with sericitic alteration and focussed along contacts and steeply dipping north-northeast-striking faults and fold axial planes, converted skarn to carbonate-quartz assemblages, formed sericite in quartzite accompanied by bornite + digenite + pyrite and pyrite + chalcopyrite, and replaced limestone with massive sulfide (pyrite + chalcopyrite) and hematite + talc + chalcopyrite + pyrite.; Nukundamite (Cu_3.38Fe_0.62S₄), recognized as a minor phase in sericitic assemblages in a few other deposits, is widespread in the Main-Stage potassic assemblages in quartzite on the northeastern contact. Nukundamite is stable in a highly limited range of a(Fe 2+)/a(H+)2, fS₂, and fO₂ between 501 and 224°C at very high sulfidation states. These high sulfidation states were achieved as magmatic brine and vapor rose through several kilometers of quartzite along the northeast contact. Due to lack of mineral redox buffers in quartzite, the cooling brine became increasingly oxidized and sulfidized at constant fS₂, buffered by magmatic H₂S_(g) ≈ SO_2(g). It also maintained relatively high concentrations of S and K, and precipitated nukundamite + K-feldspar + phlogopite at <400°C.; Comparison of active volcanic and magmatic-hydrothermal vapor compositions with geochemical environments deduced from petrologic study of ore deposits reveals common evolutionary trends. Compositions of volcanic fumaroles straddle the H₂S_(g) ≈ SO_2(g) buffer from >800 to 400°C at fS₂ and fO ₂ values consistent with intermediate sulfidation states of ores in skarn and potassic alteration of the porphyry environment. At lower temperature, active magmatic-hydrothermal fluids achieve high sulfidation and oxidation states through boiling and cooling, consistent with sulfide assemblages in sericitized quartzite and massive sulfide replacement ores. Below 200°C, these reactive fluids become reduced and neutralized by reaction with wall rock.