Pressure gradients and boiling as mechanisms for localizing ore in porphyry systems

Abstract

Fluid inclusions in ore zones of porphyry systems indicate that extensive boiling of hydrothermal fluids accompanies deposition of ore and gangue minerals. The boiling commonly accompanied a change from a lithostatic to a hydrostatic environment during evolution of an epizonal stock. Pressure gradients near the margin of the stock can determine whether ore or only a diffuse zone of mineralization is formed. A sharp drop in pressure in an epizonal environment is more likely to cause extensive boiling than a comparable change in a deeper environment, as the slope of the boiling curve steepens with an increase 'in pressure. The drop in pressure causes the hydrothermal fluids to boil and creates a crackle (stockwork) breccia, which hosts the veinlets of gangue quartz and ore minerals. The boiling selectively partitions CO 2 , H 2 S, and HCl into the vapor phase, changing the pH, composition, ionic strength, and thus the solubility product of metal complexes in the remaining liquid and causing the ore and gangue to come out of solution. Fluid inclusions trapped from boiling solutions can exhibit several forms, depending on the physical and chemical conditions of the hydrothermal fluid from which they were trapped. In one case, inclusions when heated can homogenize to either liquid or vapor at the same temperature, which is the true boiling temperature. In another case, homogenization of various inclusions can occur through a range of temperatures. The latter case results from the trapping of mixture of liquid and vapor. Variations in salinity can result from boiling of the hydrothermal fluid, or intermittent incorporation of high-salinity fluids from the magma, or trapping of fluids of varying densities at pressure-temperature conditions above the critical point of the fluid. In places, paleopressure-temperature transition zones can be recognized by fluid-inclusion homogenization temperatures and phase relationships and by the presence of anhydrite daughter minerals. Boiling of a hydrothermal fluid in the porphyry environment affects light stable isotopes. Hydrogen is preferentially fractionated into the vapor phase from water boiling below 223°C; above this temperature deuterium is selectively enriched in the vapor phase. In certain environments boiling creates a vapor-dominated system in which the condensate is swept away by meteoric waters and the H/D in the residual fluids is progressively increased through time.