Sulphur and lead isotope geochemistry of sulphide minerals from the Zn-Pb-Cu-Ag-Au Lemarchant volcanogenic massive sulphide (VMS) deposit, Newfoundland, Canada

Abstract

The Lemarchant Zn-Pb-Cu-Ag-Au volcanogenic massive sulphide (VMS) deposit, in the Newfoundland Appalachians, Canada, is an important example of a precious metal-bearing VMS deposit. The deposit contains VMS- and sulphosalt-rich mineral assemblages, which are paragenetically distinct. Initial Zn-Pb-sulphosalt mineralization was deposited from low temperature (<250 °C), weakly oxidized (SO4 ≥ H2S), near-neutral fluids with high sulphur activity. Later Cu-rich, polymetallic mineralization was deposited from higher temperature (>300 °C), less oxidized hydrothermal fluids with lower sulphur activity. In situ microanalyses of pyrite, chalcopyrite and galena yield a wide range of δ34S values (−6.4 to +15.1‰). Sulphides from paragenetically early, lower temperature, Zn-Pb-sulphosalt assemblages have the lowest average δ34S values (+4.6‰), whereas the highest δ34S values occur in the late-stage, high-temperature, Cu-rich sulphide assemblages. Variability in δ34S occurs in all styles of mineralization, and is consistent with S derived from thermochemical reduction of seawater sulphate, igneous leaching and, potentially, disproportionated magmatic SO2. Although it is difficult to distinguish explicitly between leached igneous and magmatic sources of sulphur, mineralogical and geochemical data from Lemarchant suggest a possible contribution of magmatic SO2 during the deposition of epithermal-style mineralization at the earlier stages of deposit formation.Lead isotope compositions of galena show little variability with style of mineralization, suggesting similar Pb sources throughout ore deposition. Most Pb at Lemarchant is derived from underlying Neoproterozoic Sandy Brook group volcanic rocks, with potential input of juvenile Pb from the Lake Ambrose basaltic rocks of the Tally Pond group and/or from more juvenile Neoproterozoic basement rocks.