Magmatic-hydrothermal fluids and copper-gold ore formation at Bingham Canyon, Utah

Five separate porphyry intrusions have been mapped on the basis of crosscutting relations between dikes and veins. Each intrusion was followed by a cycle of quartz vein formation, potassic alteration and copper-gold deposition. The first porphyry contains the highest-grade copper-gold ore and the most abundant quartz veins and intense potassic alteration. The mass of introduced copper and gold decreased significantly during later porphyry intrusive-hydrothermal cycles. Molybdenite-quartz veins formed after termination of dike emplacement, and are in turn cut and offset by quartz-sericite-pyrite veins. The Cu-Au-Mo orebody is therefore the product of successive ore-forming events one superimposed on the other.; Fluid inclusions in quartz veins show that initially homogeneous CO ₂-rich magmatic-hydrothermal fluids (containing ∼8000 ppm Cu) underwent phase separation approximately 500m below the base of the orebody at estimated paleodepths of 2.5 km (lithostatic pressure). Brine and vapor continued to ascend and formed early high-temperature biotite veinlets, andalusite-sericite-biotite-K-feldspar-corundum veins, and multiple generations of quartz veins. Early barren quartz veins from the base of the orebody formed at temperatures of >550 to 400°C and pressures of 500 to 200 bars, consistent with fluctuations between lithostatic and hydrostatic pressure at around 2 km paleodepth. Cathodoluminescence petrography shows that bornite-digenite-chalcopyrite were deposited with minor quartz and K-feldspar in fractures and irregular dissolution vugs within early quartz veins at temperatures of 400 to 350°C and at near-hydrostatic pressures of around 200 bars.; In the porphyry-hosted ore, sixty five percent of gold grains, (5 to 20 microns in diameter) occur within or attached to copper-iron sulfides (mainly bornite), the remainder are enclosed in quartz or K-feldspar. Secondary-ion mass spectrometry (SIMS) analyses indicate that copper-iron sulfides contain concentrations of submicroscopic gold that are several orders of magnitude lower than the solubility of gold in these sulfides at their depositional temperatures. The data are consistent with the incorporation of gold into the structure of copper-iron sulfides, followed by exsolution of native gold and its migration over sub-millimeter distances upon cooling. The high gold grades and gold:copper ratios in bornite-rich ore appear to result from the propensity of bornite to sequester significantly more gold than chalcopyrite.