Abstract

The system Cu 2 S-PbS-Sb 2 S 3 has been experimentally studied at 500 degrees , 440 degrees , and 300 degrees C by the evacuated silica tube (+ or - halide flux) technique. Additionally, the ambient sulfur fugacity of equilibrium phase assemblages has been monitored by the two-pyrrhotite indicator method. Contrary to the earlier results of Hoda and Chang (1975), the extents of solid solubility in meneghinite and phase Z are distinctly smaller and the Cu 2 S- and Sb 2 S 3 -rich melt fields at 500 degrees C are more extensive. There is also a marked shift in the position of the Cu 2 S-rich melt field. The meneghinite field gradually tapers down away from its PbS-rich end to the Sb 2 S 3 -rich end and the Cu 2 S-poor boundary moves away from the PbS-Sb 2 S 3 binary with increasing Sb 2 S 3 . The earlier claim that tintinaite is the natural analogue of phase Z is corroborated. Falkmanite appears as a distinct phase at 500 degrees and 440 degrees C. Cu-free meneghinite can possibly be stable at a temperature > or = 550 degrees C. The absence of meneghinite and phase Z at 300 degrees C is a major deviation from the earlier study of Hoda and Chang (1975). Further, robinsonite disappears and semseyite appears leading to the establishment of bournonite-boulangerite, bournonite-semseyite, and chalcostibite-semseyite tie lines. Variations in f (sub s 2 ) at 500 degrees and 440 degrees C within the three-phase fields and on the two-phase boundaries are consistent. The f (sub s 2 ) -imposed experiments in the chalcostibite-zinkenite-stibnite field at 440 degrees C reveal that famatinite forms according to the sulfidation reaction: 3 chalcostibite + 1/2S 2 = famatinite + stibnite.Mineralogy and composition of pertinent natural sulfosalt-bearing assemblages from a wide variety of polymetallic ore deposits are successfully explained in the light of the present experimental results. Significantly, the compositions of natural meneghinites plot at the PbS-rich boundary of the meneghinite field on the Cu 2 S-poor side. The virtual absence of natural Sb 2 S 3 -rich meneghinites can be explained by the requirement of more copper for their stability which is not geologically realistic in a model hydrothermal fluid. Additionally, relatively low f (sub S 2 ) values essential for the stability of meneghinites may be another possibility. Rarity of phases such as chalcostibite and skinnerite can be explained by the extremely low sulfur fugacity values, beyond the Bartonian f (sub S 2 ) -T regime, that are essential for their stability.

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