Fluid inclusions as a guide to porphyry copper deposits

Abstract

Fluid-inclusion studies of 36 porphyry copper deposits, mainly in theUnited States, demonstrate that all but 3 evolved through a hydrothermal stage characterized by very high salinities, generally in excess of about 35 weight percent NaC1 equivalent, and boiling temperatures of these fluids ranged from about 250° to 700° C for various stages and deposits. Highsalinities, shown by halite-bearing inclusions, and boiling, suggested by coexisting gas and liquid-rich inclusions, are considered diagnostic of epizonal intrusions which are the most favorable parents for porphyry copper mineralization. Depth of emplacement of many copper-bearing stocks is deduced from fluid inclusions to have been about 6,000 to 10,000 feet (1,800 to 3,000 metres); fluid pressures during mineralization are interpreted generally to be less than 500 bars. Moderate-salinity (less than about 12 percent) and-moderate-temperature (<350°C) fluids are noted in all porphyry copper deposits and were responsible for the deposition of most copper and molybdenum in deposits such as Bagdad, Esperanza, Mineral Park, Morenci, Ray, Sierrita, in the southwestern United States and several in southern British Columbia. However, with only three exceptions, highly saline fluids apparently were present at an early stage and also deposited metals. The relative amounts and economic importance of copper and molybdenum deposited from high and moderate salinity fluids varies within the porphyry deposit class. There is compelling geologic and geochemical evidence that chloride ions are important for transport of metals in hydrothermal solutions, but the porphyry coppers stand out as a class associated with fluids of especially high salinity during at least one stage of their formation. Halite cubes in fluid inclusions are an effective although rough indicator of those salinities and can be conveniently monitored during petrographic study of thin sections. It is suggested that fluid inclusions be used to characterize favorable intrusions, metal anomalies, and cap rocks as an additional method of detecting possible disseminated porphyry copper-type metallization. The presence of halite-bearing inclusions, especially coexisting with gas-rich inclusions, is considered a favorable geochemical anomaly in the search for porphyry ore.