CYCLICAL P-T-X EVOLUTION OF MAGMATIC-HYDROTHERMAL FLUIDS IN THE BINGHAM PORPHYRY CU-AU-MO DEPOSIT, UTAH

The Bingham Canyon porphyry copper deposit is one of the largest and best-exposed copper-gold-molybdenum orebodies in the world. Almost 100 years of open pit mining and deep drilling provide greater than 2 km of vertical exposure through the deposit. 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, the most abundant quartz veins and the most intense potassic alteration. Vein truncation relations coupled with abrupt changes in copper-gold grades, sulfide ratios and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive 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 barren quartz veins up to 1.5 km below the porphyry-hosted copper-gold orebody record the presence of a supercritical CO₂-rich magmatic-hydrothermal fluid containing ~ 8000 ppm copper. This fluid underwent phase separation approximately 500m below the base of the orebody at estimated paleodepths of 2.5 km (lithostatic pressure). Brine and vapor ascended 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^oC 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 as temperatures declined to 400-350^oC and at near-hydrostatic pressures of around 200 bars.