‘Derived’ and observed sulphosalt-sulphide phase assemblages compared — A case study from Rajpura-Dariba,India

Log a_S2-T diagrams for several sulphosalts (and sulphides) are constructed from published thermochemical data. Additionally, extension of published data on the Cu-S system (Barton, 1973) to the univariant phase equilibria involving fahlore and high-chalcocite in the Cu-Sb-As-S system (Luce et al., 1977) permits construction of approximate As-isopleths for fahlore in log a_S2-T space. Phase relationships thus derived are compared with the sulphosalt-sulphide assemblages present in the biphase, polymetallic mineralization at Rajpura-Dariba where geology, genesis and mineral-chemistry are well documented. A gratifying correspondence between the two is observed. Biphase origin of the orebody, inferred from geological data, is confirmed, and rigorous narrow limits of variation of a_S2 and T could be set for some assemblages belonging to the second phase of mineralization. The phase diagrams also indicate that the observed pyrargyrite-proustite textural relation and the decomposition fabric of some fahlores, independently, point to an intermittent increase in the activities of S₂ and As during the second phase mineralization.