Mineralogy and mineral chemistry of the metamorphosed and precious metal-bearing Ming deposit, Canada

Abstract

The Ming deposit in the northwestern Canadian Appalachians is a metamorphosed, bimodal-mafic, precious-metal (Au, Ag)-bearing and Cu-rich volcanogenic massive sulfide (VMS) deposit consisting of several, spatially proximal lenses in a sericite- to chlorite-altered rhyodacitic footwall. The various lenses (1807 Zone, 1806 Zone, Ming South Up Plunge, Ming South Down Plunge, and the Lower Footwall Zone) have variations in Cu, Au, Ag, and Zn grades that reflect varying physico-chemical conditions of ore formation. This study describes the complex ore mineralogy of the orebodies and constrains the genesis of the deposit using field methods, mineralogy, whole rock sulfide geochemistry, and micro-analytical methods.The orebodies are predominantly pyrite, chalcopyrite, with lesser sphalerite, pyrrhotite, and trace galena and arsenopyrite, with the exception of the Lower Footwall Zone, which consists of a high temperature (>320°C) chalcopyrite–pyrrhotite–pyrite±cubanite assemblage. The other orebodies (1806, 1807, Ming South Up Plunge and Down Plunge) contain trace amounts of tellurides (hessite, altaite, tsumoite, unnamed bismuthtelluride), sulfosalts (Ag-poor and Ag-rich tennantite–tetrahedrite, meneghinite, AgSb phases, stannite), and precious metal phases (electrum, AgHg±Au alloys). The 1807 Zone is enriched in Te, Bi, and Se, whereas the 1806 Zone is telluride-free and contains As, Sb, Hg, Au, and Ag. Mineral chemistry of sphalerite shows strong variations in Fe content (1.12–11.04wt.%). Intermediate Fe (4.33–6.33wt.% Fe) and Fe-rich (7.327–11.04wt.% Fe) sphalerite are common in all orebodies, whereas Fe-poor sphalerite (1.12–3.57wt.% Fe) occurs exclusively in the 1807 and 1806 zones. Tennantite-tetrahedrite is typically Ag-poor (0.25–2.19wt.%) in the 1807 Zone, but is significantly enriched in Ag (up to 29.3wt.%) in the 1806 Zone. Galena in the 1807 Zone and Ming South orebodies is commonly myrmekitically intergrown with tellurides and has high concentrations of Te, Bi, Se, and Ag. In contrast, galena in the 1806 Zone is less enriched in Te, Bi, and Se, but high in Ag.Variations in mineralogy, epithermal elements (As, Bi, Hg, Sb, Se, Sn, Te) and precious metal (Ag, Au) content, and mineral chemistry between the different orebodies indicate that they were formed from predominantly reduced, acidic hydrothermal fluids with varying ƒTe2/ƒS2, ƒSe2/ƒS2, and mBi/mSb ratios as temperatures steadily decreased from >300°C to <260°C during ore formation. In the 1807 and Ming South orebodies, late-stage deposition of Te-, Bi-, Se-, Ag-rich galena and tellurides occurred prior to precipitation of Ag-poor tennantite-tetrahedrite, whereas in the 1806 Zone hydrothermal fluids low in ƒTe2/ƒS2, ƒSe2/ƒS2, and mBi/mSb favored the precipitation of Te-, Bi-, Se-poor galena and Ag-rich tennantite-tetrahedrite. A magmatic source for epithermal elements and precious metals is suggested and was part of the depositional history of the Ming deposit.