Stratigraphy and hydrothermal alteration of the Ming Cu-Au volcanogenic massive-sulphide deposit, Baie Verte Peninsula, Newfoundland

The Cambro-Ordovician bimodal-mafic Ming Cu-Au-(Zn-Ag) volcanogenic massive-sulphide deposit is a type example of an Appalachian precious metal-enriched volcanogenic massive-sulphide deposit. The footwall of the deposit comprises at least three distinct felsic volcanic and volcaniclastic units. The immediate hanging wall is lithologically heterogeneous, comprising a highly silicified volcaniclastic rock and a magnetite-rich volcanogenic siltstone. Three generations of mafic to intermediate sills and dykes intrude the deposit and have distinctive lithogeochemical signatures; they are interpreted to be genetically related to the mafic rocks in the ophiolitic cover sequence that overlies the deposit. The Ming deposit has distinct hydrothermal alteration mineral assemblages including: chloritequartz-epidote, sericite–quartz-green mica, quartz-pyrite, and Mn-garnet–calcite. Other minor secondary phases include biotite, tremolite, and magnetite. A Cu-rich zone consisting primarily of chalcopyrite, pyrrhotite, and pyrite with minor Bi-Te sulphosalt minerals and sphalerite in a strongly chlorite-quartzepidote-altered felsic volcanic rock occurs 50 m to 100 m below the main sulphide lens, and represents the high-temperature discharge zone of the Ming hydrothermal system. An overprint of metamorphic biotite is ubiquitous throughout the felsic footwall rocks and represents metamorphosed K-Fe-(Mg) alteration to upper greenschist facies. Despite local remobilization of the sulphide minerals due to deformation and metamorphism, the relationship between the host rocks, the alteration assemblages and their spatial distribution, and the ore strongly favour a syngenetic origin for the sulphide zones and their base and precious metals, suggesting intrinsically precious metal–enriched volcanogenic massive-sulphide ore-forming fluids at the Ming mine.