Evolution of a broadlands-type epithermal ore fluid along alternative P-T paths; implications for the transport and deposition of base, precious, and volatile metals

Abstract

Boiling and acidification of hydrothermal solutions are important mechanisms that lead to the deposition of base, precious, and volatile metals in epithermal systems. Computer models of these processes with a Broadlands-type geothermal water are presented in a three-part study: boiling, gas phase metal transport, and acid reactions with metal-bearing waters. Revised and partly rederived thermodynamic data necessary for such a study are presented as an appendix.Boiling of a Broadlands-type water along alternative P-T-enthalpy paths induces the precipitation of base metal sulfides, sulfosalts, and electrum. The precipitation of base metals results from the pH increase due to CO 2 degassing, and the precipitation of electrum results from the pH increase as well as sulfide loss to the gas phase. Heat transfer to or from wall rocks controls the amount of boiling; thus, because the extent of boiling controls pH, heat transfer controls the precipitated mineral assemblage.Oxyacids and hydroxides are generally the dominant aqueous species of arsenic and antimony, respectively. At low temperatures, elevated sulfide concentrations, and intermediate pH, arsenic and antimony thiosulfides become dominant. Upon boiling, the precipitation of copper, lead, and silver sulfosalts instead of sulfides is most likely at low temperatures and low pH values. Calculated low-temperature sulfosalt precipitation is consistent with shallow arsenic and antimony mineralization in epithermal systems and also with the commonly observed late precipitation of these metals as temperature decreases with time. Calculated electrum compositions do not exceed 56 mole percent gold. These compositions vary with temperature but depend mainly on the amount of other silver-bearing minerals competing with electrum.Upon boiling, all mercury fractionates into the gas phase from 300 degrees to 101 degrees C. Numerical condensation by cooling the gas phase below 101 degrees C at a constant pressure induces the precipitation of cinnabar between 80 degrees and 100 degrees C, depending on the mercury concentrations (0.8-800 ppb). Arsenic and antimony, however, do not fractionate significantly into the gas phase as long as an aqueous phase is present. Upon 100 percent boiling of the solution, antimony stays in the dry gas phase down to 220 degrees C where stibnite precipitates, and arsenic down to 130 degrees C where realgar precipitates. The transport of base metals in the dry gas phase requires temperatures above 400 degrees C.The acidification of the boiled Broadlands-type water by acid sulfate waters or by H 2 SO 4 produced from magmatic SO 2 leads to the precipitation of gold-rich electrum or gold with copper sulfides (at 200 degrees , 150 degrees , and 101 degrees C), sulfosalts (at 150 degrees and 101 degrees C), and enargite (at 101 degrees C). Acidification in a hot spring environment at 90 degrees C causes orpiment and stibnite to precipitate. Comparative results between H 2 SO 4 and HCl acidification reactions indicate that the replacement of copper, lead, and silver sulfides by sulfosalts is driven by pH decrease and is favored by higher sulfide and sulfate activities. The enargite-covellite-tetrahedrite-chalcopyrite downward zoning of acid sulfate-type epithermal deposits can be explained by acidification at constant sulfide activity. Also, the formation of shallow epithermal deposits rich in gold, relative to silver, can be explained by the reaction at shallow depths between ascending boiling waters and downward percolating acid sulfate waters.