Partial melting of sulfide ore deposits during medium- and high-grade metamorphism

Abstract

Minor elements, such as Ag, As, Au and Sb, have commonly been remobilized and concentrated into discrete pockets in massive sulfide deposits that have undergone metamorphism at or above the middle amphibolite facies. On the basis of our observations at the Broken Hill orebody in Australia and experimental results in the literature, we contend that some remobilization could be the result of partial melting. Theoretically, a polymetallic melt may form at temperatures as low as 300°C, where orpiment and realgar melt. However, for many ore deposits, the first melting reaction would be at 500°C, where arsenopyrite and pyrite react to form pyrrhotite and an As-S melt. The melt forming between 500° and 600°C, depending on pressure, will be enriched in Ag, As, Au, Bi, Hg, Sb, Se, Sn, Tl, and Te, which we term low-melting point chalcophile metals. Progressive melting to higher T (ca. 600°-700°C) will enrich the polymetallic melt progressively in Cu and Pb. The highest-T melt (in the upper amphibolite and granulite facies) may also contain substantial Fe, Mn, Zn, as well as Si, H2O, and F. In our model, we suggest that the presence of polymetallic melts in a metamorphosed massive sulfide orebody is recorded by: (1) localized concentrations of Au and Ag, particularly in the presence of low-melting-point metals, (2) multiphase sulfide inclusions in high-T gangue minerals, (3) low interfacial angles between sulfides or sulfosalts suspected of crystallizing from the melt and those that are likely to have been restitic, (4) sulfide and sulfosalt fillings of fractures, and (5) Ca- and Mn-rich selvages around massive sulfide deposits. Using these criteria, we identify 26 ore deposits worldwide that may have melted. We categorize them into three chemical types: Pb- and Zn-rich deposits, either of SEDEX or MVT origin. Pb-poor Cu-Fe-Zn deposits, and disseminated Au deposits in highgrade terranes.