DID THE GIANT STRATIFORM BROKEN HILL PB-ZN-AG DEPOSIT, CURNAMONA PROVINCE (AUSTRALIA) MELT?

Recent published research has proposed that some metamorphosed massive sulfide deposits, including the Broken Hill deposit (BHD), have partially melted. Here we discuss the evidence for and against this process. The deposit is located within a rift in an upward-coarsening sequence of clastic metasediments into which Paleoproterozoic mafic and felsic melts have intruded. The deposit was metamorphosed to granulite facies conditions (750-800 ^oC and 5-6 kb) and was subjected to at least five periods of deformation. At peak P-T conditions, a melt phase was produced in the systems Cu-As-S, Ag-Pb-S, Ag-Sb-As-S, Cu-Sb-Pb-S, Sb-As-S, Cu-Sb-S, and Fe-As-S. However, the proportion of minerals in these systems is insignificant (i.e., <0.0001 volume %) when compared to the system Pb-Zn-Fe-Mn-S, which is the most relevant to BH ores. P-T conditions were never high enough to produce a melt phase in this system. It has been argued in the literature that Mn-rich lithologies (garnetite and quartz garnetite), which are intimately associated with the Pb-rich ores at the BHD, were produced by a reaction between Mn-rich sphalerite and aluminous wall rocks. However, to produce such rocks would require Mn contents of sphalerite compositions that are unrealistically high and yet to be found in nature. Moreover, the implication of the melt model is that wherever Mn-rich garnet rocks are found sphalerite should be located next to them. This is clearly not the case. The presence of polyphase sulfide inclusions in garnet in garnetite have also been considered by some in the literature to be sulfide melt inclusions with the implication that the enclosing garnet was a product of melting. However, we consider these sulfides to be products of hydrothermal processes during retrograde metamorphism. It is impossible to form garnetite and quartz-garnetite in rocks within the Curnamona Province (CP) that have formed at upper greenschist-lower amphibolite facies conditions by partial melting. The P-T conditions were too low. We consider that the Fe and Mn component of garnetite and quartz garnetite are products of exhalation and inhalation at or near the sea floor, and that these rocks are meta-exhalites or meta-inhalites. Some garnet-rich rocks also formed by metasomatic processes throughout the protracted metamorphic history that affected the CP.