The chemistry of hydrothermal tin(-tungsten) ore deposition

Abstract

A qualitative model for the formation of granite-related mesothermal cassiterite(-wolframite) deposits is derived from a review of geologic evidence and is then tested quantitatively with chemical mass transfer calculations based on published experimental and thermodynamic data.Laboratory experiments and geologic, mineralogic, fluid inclusion, and stable isotope observations indicate that saline fluids of magmatic origin are involved in the formation of most tin ores that occur in veins, breccias, and replacement bodies of aluminosilicate or carbonate rocks. Transport of the ore fluid from a hot granitic source into a cooler depositional environment probably involves structural focusing and prevention of complete chemical reequilibration of the fluid with fresh quartzofeldspathic wall rocks. Under these conditions, a reduced acid fluid can transport high concentrations of Sn(II)-Cl complexes (hundreds of ppm metal) to a site of ore deposition at low temperature. Precipitation of cassiterite, Sn(IV)O 2 , requires oxidation and liberates acidity, which must be balanced by reduction and acid-consuming reactions involving other fluid and wall-rock components for cassiterite enrichment to proceed to economic concentrations. Several geologically likely deposition mechanisms have been tested, which differ in efficiency regarding the maximum tin ore grade that can be achieved in aluminosilicate host rocks. By contrast, wolframite can be precipitated by cooling of an Fe-W-bearing fluid without wall-rock reaction.Single-step acid neutralization of magmatic fluids by feldspar hydrolysis to sheet silicates (phyllic alteration) probably produces subeconomic greisen mineralization, because the maximum tin ore grade is severely constrained by the high acid content of the fluid. Progressive fluid-rock reaction and multistage ore reworking at an advancing alteration front, described by a one-dimensional finite-element reactor model assuming local equilibrium, may be a more efficient and geologically realistic process to form tin-rich greisens and breccia pipes. Loss of H 2 from a reduced tin-rich fluid by vapor separation, and simultaneous reaction with aluminosilicate rocks, is an alternative possibility for the formation of rich greisen-type deposits. Fluid mixing by injection of minor magmatic fluid into a cooler environment of convecting meteoric fluids could be a third, particularly efficient, tin-mineralizing mechanism in vein deposits without extensive wall-rock interaction. In this case and in the deposition of cassiterite by carbonate replacement, there are essentially no chemical limitations on tin ore grade, other than dilution of cassiterite by coprecipitating quartz and sulfides.