EVIDENCE FOR EXTENSIVE MELTING OF THE BROKEN HILL ORE BODY

Recently Spry et al. (2008) (Ore Geology Reviews, 34, 223-241) argued that if the Broken Hill ore body melted, the amount of melt present was insignificant. This caused us to reassess the geochemical, petrologic, and thermobarometric evidence for melting at Broken Hill. A key question regards the peak metamorphic conditions at Broken Hill. A pressure of 5 kilobars is well accepted. The peak temperature is a matter of dispute; estimates range from 740°C to 825°C. We conclude that the most reasonable minimum temperature is in the middle of this range 780°C, which is what Phillips (1980) (Contrib. Min. Pet. 75, 377-386) estimated. The eutectic in the system FeS-PbS-ZnS at 1 bar is 800°C; at 5 kilobars this would be 830°C.  Spry et al. argue that the eutectic temperature is too high for the metamorphism to have produced extensive melting of the Broken Hill ore body. The critical assumption in their argument is that the iron sulfide in the original ore body was troilite (stoichiometric FeS). Because sulfur is a volatile component and is likely to have been lost during metamorphism, we estimate that the pyrrhotite from the parent ore body could not have been less sulfur-rich than pyrrhotite found at Broken Hill today, which is FeS_0.96. Based upon published experiments in the system Fe-Pb-Zn-S, we conclude that, at one bar, the assemblage FeS_0.96-PbS-ZnS would have melted between 740-760°C (770-790°C at 5 kilobars). This is well within the peak metamorphic temperature range Broken Hill. Thus, even discounting the fluxing effect of H₂O, Cl, and minor metals such as Ag, As, Bi, Cu, and Sb, it is difficult to avoid the conclusion that Broken Hill underwent extensive melting during peak metamorphism, during which most of the Pb was incorporated into the melt. This conclusion is supported by our textural studies of the Zn-rich ores showing that the average interfacial angles of galena and chalcopyrite angles against sphalerite-sphalerite boundaries are less than 60°, suggesting that the galena and chalcopyrite in these rocks crystallized out of a phase that wetted the sphalerite-sphalerite grain edges.  This is most likely to have been a polymetallic melt, a conclusion supported by small bismuth-rich alloys within the galena.