Did the giant Broken Hill (Australia) Zn–Pb–Ag deposit melt?

Abstract

Recent published research has proposed that some metamorphosed massive sulfide deposits, including the Broken Hill deposit, Australia, have partially melted. Here we discuss the evidence for and against this process. The Broken Hill deposit is located in the Curnamona Craton within a rift in an apparent upward-coarsening sequence of clastic metasediments into which Paleoproterozoic mafic and felsic melts intruded. The deposit was metamorphosed to granulite facies conditions (750–800 °C and 5–6 kbar) and was subjected to at least five periods of deformation. At peak P– T conditions, a melt phase was produced in silicate rocks, especially granitoids and psammopelites, and in ore characterized by 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 S-bearing systems is insignificant (< 1 wt.%) when compared to the volume of sulfides in the system SiO 2–FeO–MnO–CaO–Al 2O 3–P 2O 5–CO 2–ZnS–PbS–FeS–(FeAsS/FeAs 2), which is the most relevant system to Broken Hill ores. P– T conditions were never high enough to produce a melt phase in this system. Authors arguing for sulfide rock melting suggest that Mn-rich lithologies (garnetite and quartz garnetite), which are intimately associated with the Pb-rich ores at the Broken Hill deposit, 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 quartz garnetite and garnetite are found sphalerite should be located next to them. This is clearly not the case. The presence of polyphase sulfide inclusions within garnet in garnetite has 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 Craton that formed at upper greenschist–lower amphibolite facies conditions by partial melting. 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 Curnamona Craton.