Abstract

In the absence of high-temperature experimental solubility data, thermodynamic calculations are used to predict the speciation of arsenic in hydrothermal solutions and the solubility of arsenic minerals. The calculations suggest that at neutral to acid pH the aqueous species H 3 AsO 03 can account for the transport of adequate concentrations of As to explain the associations of many hydrothermal As minerals. Extrapolations of thermodynamic data to high temperatures made using constant heat capacity of reaction for equilibria involving "balanced identical like charges" were combined with experimentally determined solubility data for pyrite. This is valid provided the calculations are restricted to the pH-f (sub O 2 ) range where H 2 S 0 is the dominant aqueous sulfur species. The As mineral associations (1) orpiment + realgar + native arsenic + pyrite, (2) arsenopyrite + pyrite, and (3) arsenopyrite + or - pyrrhotite and loellingite + or - pyrrhotite are predicted to reach arsenic solubilities high enough to account for hydrothermal transport at progressively higher temperatures in systems containing excess iron sulfide. The solubility of arsenopyrite is more strongly dependent on oxygen fugacity than most other ore minerals.Calculated arsenopyrite solubility in relation to calculated and experimental solubilities of other ore minerals predicts the following depositional sequences in aluminosilicate-hosted cassiterite-sulfide deposits. Cassiterite is predicted to be followed by pyrite + arsenopyrite + base metal sulfides for a hydrothermal solution which remains buffered with regard to pH and oxygen fugacity by excess oxide and silicate minerals in the host rocks throughout its cooling history. Host-rock buffers are likely to have dominated the chemical evolution of fluids in disseminated breccia ores showing penetrative wall-rock alteration.In contrast, the commonly observed depositional sequence cassiterite + arsenopyrite preceding pyrrhotite + base metal sulfides is favored by very low oxygen fugacities. These may be attained during the chemical evolution and cooling of fluids of similar starting composition but with restricted opportunity for reacting with host-rock redox buffers. This situation is likely to occur where fluids are channeled into narrow veins and pipes in rocks of very low permeability, where they deposited localized concentrations of hydrothermal minerals involving only limited wall-rock alteration. Here, precipitation of arsenopyrite by reduction of As(III) complexes may couple with oxidation of Sn(II) complexes to precipitate cassiterite:3SnCl 02 + 2H 3 AsO 03 + 2FeCl 02 + 2H 2 S 0 = 3SnO 2 + 2FeAsS + 10Cl (super -) + 10H (super +) .Coprecipitation of cassiterite + arsenopyrite by such an oxygen-conserving redox reaction would provide an efficient mechanism to extract tin from solution over a short cooling interval. This may account for the correlation of high tin ore grades with high arsenopyrite abundance as observed in some cassiterite deposits.

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