Evidence for Syngenetic Precious Metal Enrichment in an Appalachian Volcanogenic Massive Sulfide System: The 1806 Zone, Ming Mine, Newfoundland, Canada

Abstract

The Ming deposit, Baie Verte, Newfoundland, Canada, is an Early Ordovician bimodal-mafic, Cu-Au-(Zn-Ag) volcanogenic massive sulfide (VMS) deposit. The deposit consists of a number of ore lenses that are stacked in the uppermost part of the Rambler rhyolite. One of the uppermost lenses, the 1806 zone, is enriched in Au and Ag. The deposit has been affected by Silurian-Devonian greenschist to amphibolite grade metamorphism and polyphase deformation and this has led to debates as to whether the Au-Ag enrichment in the deposit is syngenetic or a product of a later metamorphic and structural overprint. The 1806 zone consists predominantly of discordant sulfide stringer, stratabound semimassive to massive sulfides and a weakly mineralized silicified cap zone. The ore is largely hosted within the footwall Rambler rhyolite that is strongly altered to quartz-sericite ± green mica with sporadic chlorite-biotite. Base and precious metal zoning is developed from the down-plunge portion (Cu-Au) to the up-plunge portion (Cu-Zn-Au-Ag-Pb) of the 1806 zone. The ore mineralogy and mineral chemistry are complex and are interpreted to be of an intermediate sulfidation type. Pyrite and chalcopyrite are the dominant sulfide species with minor to accessory sphalerite, galena, pyrrhotite, and arsenopyrite. Sulfosalts including Ag-bearing tennantite-tetrahedrite, stannite, boulangerite, and loellingite, and precious metal-rich phases are common throughout the deposit. Precious metals occur as (1) mercurian electrum (7.93–20.57 wt % Hg) and (2) various Ag phases (miargyrite, pyrargyrite, mercurian stephanite, unnamed AgCuFeS phase, Ag-Hg ± Au alloys) that are more abundant in the up-plunge portion of the 1806 zone. Tellurides and bismuthides are present in trace amounts. Oxides (cassiterite and magnetite) are present exclusively in the down-plunge portion of the 1806 zone. The deposit geometry, metals distribution, complex ore assemblages, abundant sulfosalts, and the significant concentration of the epithermal suite of elements (e.g., Au, Ag, As, Hg, Sb, Bi) in the ore strongly contrast with those of orogenic Au deposits and support a syngenetic or synvolcanic origin for precious metals in the Ming VMS deposit, including a possible magmatic input into the 1806 zone ore-forming system. Evidence for a late, syndeformation precious metal emplacement via orogenic overprinting is lacking at the 1806 zone and all data point to a syngenetic origin for precious metal enrichment, indicating that Ordovician Appalachian VMS deposits are favorable targets for Au. Nevertheless, Silurian-Devonian metamorphism and deformation have resulted in textural recrystallization and local remobilization of precious metals and the main fabrics largely control the current geometry of the host succession and of the Ming orebody.