Partitioning of arsenic between hydrothermal fluid and pyrite during experimental siderite replacement

Arsenian pyrite is the major host mineral of gold in Carlin type gold deposit (CTGD) and forms during the interaction of H2S bearing hydrothermal fluids with Fe-bearing carbonates (e.g., siderite, ankerite, Fe-rich dolomite) in the wall rock. Experimental replacement of siderite at hydrothermal conditions (i.e., 200 degrees C, Psat) was used to study the parameters that control the formation of CTGD pyrite (i.e., pH, time, aH(2)S) and the partitioning of As between the hydrothermal fluid and newly formed pyrite. Hydrothermal pyrite formed via replacement of siderite in experiments mainly occurs as two types: 1) porous, massive pyrite at alkaline pH conditions or 2) clusters of dense euhedral to anhedral pyrite grains at slightly acidic conditions (pH < 6). Longer experimental run times (2 weeks) lead to formation of type 2 pyrite at alkaline conditions and an increase in euhedral grain sizes in all conditions. Concentration of As in newly formed pyrite depends strongly on the As concentration of the hydrothermal fluid used in the experiment. Calculated partition coefficients vary between 300 and 1700 with lowest D values at high As concentrations of the fluid. In the studied system As-1 substitutes for S-2 as indicated by the correlation of high As with low S concentrations. Experimental findings agree well with previous studies in natural CTGD on As substitution and the partitioning of As between ore stage pyrite and hydrothermal fluids trapped as fluid inclusions. The newly constrained partition coefficients for As help to explain the large heterogeneity in natural CTGD pyrite compositions which is caused by changes in the fluid composition due to the strong partitioning of As into the forming pyrite.