Geology, structure, petrogenesis, and hydrothermal alteration reconstruction of the Ming volcanogenic massive sulphide deposit, Baie Verte Peninsula, Newfoundland, Canada

Abstract

The Cambro-Ordovician Ming volcanogenic massive sulphide (VMS) deposit, Newfoundland Appalachians consists of stratiform and elongated semimassive to massive sulphide lenses (1807, 1806, Ming North, and Ming South zones) enriched in Cu, Zn, Ag, and Au that are underlain by a discordant Cu-rich stockwork zone (Lower Footwall zone). The host successions consist of highly fractionated FI-/FII-type rhyodacitic coherent to volcaniclastic rocks, which were deposited within a synvolcanic fault-controlled nested basin. The disposition of the sulphide lenses and the stockwork zone were controlled by the same synvolcanic faults. Deformation and greenschist metamorphic overprint have remobilized (≤20 m) less competent sulphide assemblages locally, but original stratigraphic relationships are preserved. Enclosed precious metal-rich sulphide clasts in a mafic polymictic breccia, which immediately overlies the deposit, support a syngenetic introduction of Au and associated tellurides at the Ming deposit. From regional geology and geochemical affinities, the felsic host successions are likely derived from direct partial melting of an island arc tholeiite at depths exceeding garnet stability zones (≥30 km). These types of magmas share many affinities to modern adakites formed in juvenile environments and may be more conducive to Au transport than previously thought. Ten distinct alteration assemblages are associated with the ore-forming hydrothermal fluids, grouped as chlorite-, sericite-, and (Mn-)carbonate-rich facies. The distribution and chemistry of the alteration assemblages are strongly controlled by the hosting lithologies. Most mass changes are restricted to the upper ~100 m of the stratigraphy, within the permeable volcaniclastic lithofacies. The Cu-rich stockwork is hosted by a chlorite-rich assemblage, reflecting high temperature fluids (≥300°C) that mixed with seawater at the coherent-fragmental interface, decreasing Cu solubility and resulting in the precipitation of chalcopyrite. Gold, Ag, sulfosalts and other magmatophile elements are enriched in sulphide zones that are immediately underlain by coherent rocks. The less diffusive potential of the coherent rocks may have increased the efficiency of metal precipitation in these zones and could explain the heterogeneity of metal grades at Ming. The paragenetic evolution indicates that the introduction of precious metals is spatially and temporally associated with the quartz-sericite-sulphides assemblage, which is considered to reflect the overprint of acidic and low temperature (≤250°C) fluids in the waning stage of the hydrothermal system.