Experimental mobility of sulfides along hydrothermal gradients

High mobility of the metals Fe, Zn, Pb, and Cu but not Ag and Mo has been demonstrated in hydrothermal transport experiments involving mixtures of the metal sulfides as nutrients in 1-m KCl solutions, with pH values buffered by K feldspar q- mica q- quartz, at temperatures up to 450 degrees C, and temperature gradients of 4.6 degrees to 9.6 degrees C/cm. Strong chloride complexing controls the solubility and transport of the mobile metals in these experiments. Up to 4 g of sulfide dissolution and transport was achieved during convection-driven experimental runs of 12 to 19 days duration, but despite doubled temperature gradients, a maximum of 0.4 g of sulfide dissolution and negligible sulfide transport occurred during 20 to 30 days, in diffusion-driven experiments. Replacement textures in the nutrient sulfides at the hottest end of the reaction vessel were conspicuous among chalcopyrite, galena, pyrrhotite, Ag 2 S, and to a lesser extent, sphalerite. Neither dissolution nor replacement was evident in nutrient molybdenite. Dominant replacing phases (less soluble) were two compositional varieties of intermediate solid solution (iss) of the Cu-Fe-S system, Ag 2 S, galena, native copper, and very minor silver.Well-defined parageneses of (1) early galena, sphalerite, and minor pyrrhotite, (2) intermediate-stage pyrrhotite, sphalerite, and galena + pyrite, and (3) late-stage intermediate solid solution + fayalite were precipitated as mineral crusts at the cool end of the reaction vessel. Late-stage intermediate solid solution replaced early sphalerite in these crusts with textures analogous to chalcopyrite disease. The paragenesis of precipitated sulfides is due to differing mobilities of the metals from the nutrient sulfides at the hot end of the vessel to the warm site of deposition. Their mobilities depend upon differing stabilities of the chloride complexes of Pb, Zn, Fe, and Cu and their levels of saturation throughout the vessel. The immobility of Mo and Ag in these experiments shows that they do not form important chloride complexes in these solutions.