Quartz veins in porphyry copper deposits record the physiochemical evolution of fluids in subvolcanic magmatic-hydrothermal systems. We have combined cathodoluminescence (CL) petrography with fluid-inclusion microthermometry to unravel the growth history of individual quartz veins and to link this history to copper ore formation at Bingham, Utah. Early barren quartz veins with K-feldspar + biotite (potassic) alteration selvages occur throughout the 2 km vertical exposure of quartz monzonite porphyry stock. At depths of 500 m to at least 1350 m below the orebody, fluid inclusions in these barren veins trapped a single-phase CO2-bearing fluid containing similar to2-12 wt% NaClequiv. Within and to depths of 500 m below the orebody, early quartz veins contain abundant hypersaline liquid (38-50 wt% NaClequiv) and vapor-rich inclusions trapped together at temperatures of 560-350 degreesC and pressures of 550-140 bar, consistent with fluctuations between lithostatic and hydrostatic pressure at paleodepths of 1.4 to 2.1 km. CL petrography shows that bornite and chalcopyrite were deposited together with a later generation of quartz and K-feldspar in microscopic fractures and dissolution vugs in early barren quartz veins and wall rock. This late quartz contains hypersaline liquid (36-46 wt% NaClequiv) and vapor-rich inclusions trapped at 380-330 degreesC and at 160-120 bar hydrostatic pressure. We conclude that a single-phase magmatic-hydrothermal fluid underwent phase separation to hypersaline liquid (or brine) and vapor similar to500 m below the base of the orebody at a paleodepth of similar to2.5 km. Brine and vapor continued to ascend and formed multiple generations of barren quartz veins with potassic selvages. Thermal decline to temperatures below 400 degreesC was the main driving force for copper-iron sulfide deposition, given the lack of evidence of mixing of brines with low-salinity waters, the lack of correspondence of the ore zone with the initiation of phase separation, and no change in wallrock alteration style.

The central and southern White Mountains constitute a well-exposed section of the middle to late Mesozoic convergent margin of western North America. New 40Ar/39Ar analyses were obtained for 8 Ca-amphibole and 13 biotite specimens from igneous and metamorphic wall rocks (8 rock samples from the Barcroft pluton, a single McAfee Creek-type aplite, 7 mafic dikes/metadikes, and two Andrews Mountain quartzites). Most of the spectra are very complicated. Except for one relatively fresh and four apparently fresh diabase dikes, all 9 granitoids, the 2 metadiabasic dikes, and both contact metamorphic quartzites partially lost (or gained) argon during heating and/or deuteric-hydrothermal alteration over ~100 m.y. of igneous arc evolution. New granitoid ages are similar to—or younger than—published U/Pb ages for the Middle Jurassic Barcroft and the Late Cretaceous McAfee Creek plutons, and are similar to published K-Ar and 40Ar/39Ar ages of the Cottonwood/Beer Creek and other plutons in the central and southern White Mountains. Contact metamorphic ages of wall rocks approximate nearby granitoid emplacement ages. Mafic dikes invaded the area episodically; most yield disturbed 40Ar/39Ar spectra exhibiting a range of cooling ages. Some, metamorphosed by the Barcroft pluton, must be older than 165 Ma; others give apparent ages of 144 (Independence dike swarm), 115-118, and 98 Ma. Clearly, multiple stages of mafic dike injection accompanied mid- and late-Mesozoic accretion of this sector of the North American continental margin.

Nukundamite is an important ore mineral in a geologic resource of approximately 90 million metric tons (Mt) of 0.8 wt percent Cu in quartzite on the northeast contact of the Bingham stock. In contrast with other deposits, where nukundamite occurs as a minor phase in relatively low temperature, sericitic assemblages, nukundamite at Bingham is widespread in higher temperature, potassic assemblages. The coexistence of this high-sulfidation sulfide with potassic assemblages suggests an unusual evolutionary path for magmatic-hydrothermal fluids in quartzite wall rocks.Nukundamite-bearing copper ore, restricted to a 240-m-thick quartzite unit of the Pennsylvanian Bingham Mine Formation, extends 170 m from the northeast margin of the mineralized stock along the strike of beds and plunges 1 km down the dip of beds (30° NNW). The quartzite unit is cut off at depth by a thrust fault, below which beds of the Bingham Mine Formation are vertical and cut by abundant northeast-striking, steeply dipping faults, dikes, and isoclinal folds, the Fortuna zone.Ore-forming hydrothermal assemblages in quartzite can be classified as Main and Late stage. Main-stage minerals are predominantly disseminated quartz + phlogopite ± K feldspar, with nukundamite replaced by chalcopyrite and bornite, and no pyrite. Replacement of nukundamite by chalcopyrite and bornite reduced its abundance from possibly as high as 90 to 10 vol percent of total sulfides at the end of the Main stage. Late-stage assemblages consist of quartz + sericite ± kaolinite with abundant pyrite, lesser amounts of bornite, digenite, enargite, and traces of nukundamite.Based on experimental data and retrieved thermodynamic properties, nukundamite is stable within a highly limited range of a (Fe+2)/ a (H+)2, f S2, and f O2 at temperatures between 501° and 223°C at very high sulfidation states. Values of a (Fe+2)/ a (H+)2 in the nukundamite field are also appropriate for the stable assemblages chalcopyrite + bornite and chalcopyrite + pyrite, but nukundamite requires higher values of f S2 and f O2 than the latter, more common, assemblages. Phase equilibria indicate that nukundamite-bearing sulfide assemblages of the Main stage are metastable (supersaturated with respect to pyrite).We hypothesize that early brine and vapor, known to be of magmatic origin on the basis of isotopic and fluid inclusion evidence, traveled upward through several kilometers of quartzite along the stock contact and the Fortuna zone and then up to the south-southeast along bedding. At its source region the brine had SO2( g )/H2S( g ) > 1, typical of degassing magmas of intermediate composition. On cooling, and due to lack of redox buffers in quartzite, the brine became increasingly oxidized and evolved to SO2( g )/H2S( g ) ≅ 0.1 at 450°C at constant f S2. Precipitation of nukundamite with K feldspar + phlogopite at temperatures below 400°C (SO2( g )/H2S( g ) ≅ 0.01) was possible due to relatively high concentrations of S and K in the brine (ΣS( aq ) = 0.2 m , [K+] = 0.5 m ), also resulting from lack of exchange reactions with quartzite.Subsequent replacement of nukundamite by bornite + chalcopyrite during the late Main stage required a different fluid at lower sulfidation and oxidation states, and slightly higher a (Fe+2)/ a (H+)2. We suggest that this late Main-stage fluid, rising within the faulted northeast margin of the stock, had been enriched in iron and depleted in sulfur during Main-stage potassic alteration and ore deposition in the stock.

The Gibbs free energy of formation of nukundamite (Cu 3.38 Fe 0.62 S 4 ) was calculated from published experimental studies of the reaction 3.25 Cu 3.38 Fe 0.62 S 4 + S 2 = 11 CuS + 2 FeS 2 in order to correct an erroneous expression in the published record. The correct expression describing the Gibbs free energy of formation (kJ·mol −1 ) of nukundamite relative to the elements and ideal S 2 gas is Δ f G° nukundamite, T(K) = −549.75 + 0.23242 T + 3.1284 T 0.5 , with an uncertainty of 0.6%. An evaluation of the phase equilibria of nukundamite with associated phases in the system Cu–Fe–S as a function of temperature and sulfur fugacity indicates that nukundamite is stable from 224 to 501°C at high sulfidation states. At its greatest extent, at 434°C, the stability field of nukundamite is only 0.4 log f (S 2 ) units wide, which explains its rarity. Equilibria between nukundamite and bornite, which limit the stability of both phases, involve bornite compositions that deviate significantly from stoichiometric Cu 5 FeS 4 . Under equilibrium conditions in the system Cu–Fe–S, nukundamite + chalcopyrite is not a stable assemblage at any temperature.