Variations of sulphur isotope signatures in sulphides from the metamorphosed Ming Cu(−Au) volcanogenic massive sulphide deposit, Newfoundland Appalachians, Canada

The Ming deposit is an early Ordovician, bimodal-mafic Cu–Au volcanogenic massive sulphide (VMS) deposit in the Newfoundland Appalachians that was metamorphosed to upper greenschist/lower amphibolite facies conditions and deformed in the Silurian and Devonian. The Ming deposit consists of several spatially proximal ore bodies of which the 1806 Zone, 1807 Zone, Ming South Up Plunge and Down Plunge and the Lower Footwall Zone are the focus of this paper. The ore bodies have similar stratigraphic sequences. The ore bodies can be divided into (1) a silicified horizon that caps the massive sulphides, (2) semi-massive to massive sulphides and (3) sulphide mineralization in a rhyodacitic footwall. Sulphide mineralization in a rhyodacitic footwall includes (a) sulphide stringers immediately below the semi-massive to massive sulphides and (b) chalcopyrite–pyrrhotite–pyrite stringers distally from semi-massive to massive sulphides in the Lower Footwall Zone. Pyrite, chalcopyrite, pyrrhotite, arsenopyrite and galena were analysed by in situ secondary ion mass spectrometry (SIMS) for sulphur isotope compositions. The isotopic signatures of pyrite, chalcopyrite, pyrrhotite and arsenopyrite fall within a limited range of 2.8 to 12.0 ‰ for semi-massive to massive sulphides and sulphide mineralization in the footwall. The silicified horizon capping the semi-massive to massive sulphides has higher δ³⁴S (5.8–19.6 ‰), especially for pyrrhotite (mean, 17.2 ± 2.2 ‰, n = 8). The sulphur isotope composition of galena is more heterogeneous, especially within semi-massive to massive sulphides and sulphide stringers, ranging from 0.8 to 17.3 ‰ (mean, 6.1 ± 4.3 ‰, n = 35) and 7.6 to 17.1 ‰ (mean, 13.7 ± 5.3 ‰, n = 3), respectively. Geothermometric calculations give insufficient formation and metamorphism temperatures for neighbouring mineral pairs, because sulphides were not in isotopic equilibrium while deposited in early Ordovician or re-equilibrated during Silurian–Devonian metamorphism, respectively. Therefore, original isotopic compositions of sulphides at the Ming deposit have been preserved. Modelling of the source of sulphur shows that: (1) reduced seawater sulphate and (2) sulphur leached from igneous wall rock and/or derived from magmatic fluids are the main sources of sulphur in the Ming deposit. The influence of igneous sulphur (igneous wall rock/magmatic fluids) increases with temperature and is an important sulphur source for the semi-massive to massive sulphides and footwall mineralization, in addition to a contribution from thermochemical sulphate reduction (TSR) of seawater. It is difficult to distinguish between sulphur leached from igneous rocks and magmatic fluid-related sulphur, and it is possible that both sources contributed to the ores at the Ming deposit. In addition to igneous sulphur, the heavy isotopes of the silicified horizon are consistent with the sulphur in this horizon being derived only from thermochemical sulphate reduction of early Ordovician seawater sulphate.