Abstract

Breithauptite (NiSb) generally occurs in magmatic Ni-Cu-PGE ores and in hydrothermal veins associated with Ni and Co arsenide minerals. Its presence in the Rajpura-Dariba–Sindesar-Khurd–Bethumni metallogenic belt, Rajasthan, India, may be ascribed to metamorphic remobilization as observed elsewhere, e.g., Sulitjelma, Norway. From the coexistence of breithauptite with probable Sb-Ni melt phases at Sindesar-Khurd, a temperature of approximately 930–970 °C is indicated from the phase relations in the Ni-Sb binary phase diagram. However, this temperature is substantially above the peak metamorphic temperature of 580–600 °C estimated from equilibrium silicate phase assemblages in the rocks around the Rajpura-Dariba ore body, which is situated six kilometers to the south of Sindesar-Khurd. We have conducted experiments at 600 °C in the Ni–Sb system in the presence of Ag to understand if Ag is responsible for such a drastic decrease of the temperature of melting. The presence of other phases including freibergite [(Ag,Cu,Fe,Zn)12Sb4S13], Ag-rich tetrahedrite [(Cu,Ag,Fe,Zn)12Sb4S13], ∼2FeS.Ag2S, and ∼CuPbSbS3.Ag2S along with breithauptite at Sindesar-Khurd indicates the selective incorporation of Ag. In a second assemblage nisbite (NiSb2) instead of breithauptite was observed along with 3PbS.Ag2S, 2Ag2S.FeS, (Fe,Ag)S, a Sb-Ni-Ag-Fe-S phase, and a Ag-Sb-Ni-Cu-Fe-S phase. Nisbite is stable only below 628 °C, suggesting that these assemblages formed below this temperature, and the presence of several additional phases of variable compositions further indicates disequilibrium and probable rapid crystallization from a melt. Our results suggest that Ag significantly depresses melting temperatures in the Ni–Sb system, making possible the existence of an Ag-Sb-Ni melt at 600 °C. In the presence of S and other elements such as Fe, Pb, Cu, etc. the melting temperature would be further depressed. Thus the removal of Ag from the melt through formation of Ag-bearing phases, e.g., 2FeS.Ag2S, 3PbS.Ag2S, 2Ag2S.FeS, or CuPbSbS3.Ag2S, may result in a residual metastable melt at the ambient temperature. Such a melt may quench to a fine-grained assemblage containing breithauptite and nisbite-like Sb-Ni solid phases, such as encountered at Sindesar-Khurd. The second assemblage, which is nisbite-bearing, could result from a relatively Ag-rich Ni-poor melt fraction that becomes extremely Sb-rich due to the removal of Ag.

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