GEOCHEMISTRY OF FLUIDS IN THE CERRO COLORADO PORPHYRY COPPER DEPOSIT, PANAMA

The Cerro Colorado porphyry copper deposit in western Panama is one of the youngest (5.9 Ma) porphyry copper deposits known. Hypogene mineralization consists of pyrite, chalcopyrite, minor bornite and and molybdenite. Sulfide ore occurs in the altered Rio Escopeta granodiorite as well as the older (17 Ma) andesitic country rocks into which the granodiorite was intruded.;The temporal and spatial changes in the chemical and physical characteristics of hydrothermal fluids associated with alteration and mineralization are reflected in changes in vein mineralogy and fluid inclusion type. The purpose of this study is to examine such changes and to correlate changes with episodes of alteration and sulfide mineralization. The chronological sequence of vein formation as determined by this study is: barren quartz (A veins); quartz + sulfide (B veins); quartz + sericite (C veins); quarts + sericite + sulfide (D veins); massive sulfide (E veins); late stage gypsum and/or carbonate (secondary veins). Fluid inclusions in vein quartz and quartz phenocrysts fall into three broad categories: (1) high salinity, high K/Na (HSA, HS, HSU, HSC, HSP inclusions); (2) high salinity, low K/Na (H, HC, HP inclusions); (3) low salinity, low K/NA (L and V inclusions). The high K/Na fluids appear to be early, as they are found only in quartz phenocrysts. The high salinity low K/Na fluids were associated with vein formation, as they are found in vein quartz as well as in fracture planes in quartz phenocrysts. The low salinity fluids, which commonly show evidence of boiling, appear to be mostly secondary inclusions in phenocrysts and vein quartz.;Homogenization temperatures in all inclusion types range from about 250(DEGREES) to 450(DEGREES)C. Hydrogen isotope equilibration temperatures for fluid inclusion water-sericite pairs range from 400(DEGREES) to 600(DEGREES)C. A large pressure correction of homogenization temperatures is therefore unnecessary.;Estimates of inclusion trapping depth through use of salinity and boiling temperature of L and V inclusions, a method developed by Haas (1971), suggest the Rio Escopeta granodiorite intruded within about 5km of the surface.;Filling pressures range from close to hydrostatic (about 300 bars at 5km) for inclusions which homogenize by vapor bubble disappearance to lithostatic (1300 bars at 5km) for inclusion which homogenize by halite dissolution. Fluctuations in pressure from lithostatic to hydrostatic are associated with vein formation and mineralization.