Mantle-Derived Carbonate Fluid Alteration and Gold Mineralization in Southern Granulite Terrain, Wynad, India

Abstract

The Wynad Gold Field, unlike Archaean greenstone-hosted gold mineralization that is commonly found in India, comprises a network (length >100 m, thickness ~ 1.5 m) of much younger (~450 Ma) auriferous hydrothermal quartz veins within migmatitic tonalitic gneisses of the Southern Granulite Terrain, north of the Moyar shear zone. The gold mineralization occurs closely associated with probable mantle-derived quartz-carbonate dykes. We have used field criteria, geochemistry of pyrite-hosted fluids and Rb-Sr geochronology of the quartz-carbonate dykes to constrain source(s) to these auriferous veins. Gold bearing quartz-pyrite veins, emplaced within faults and shears, represent late-stage hydrothermal veins originating from ~0.5 m thick, undeformed, pink quartz-carbonate dykes. Both hypidiomorphic and subvolcanic textures indicated by carbonate and muscovite megacrysts, set within a fine-grained groundmass, are observed. The groundmass assemblage is comprised of quartz + pyrite + phengite + chlorite + ankerite + ferroan calcite + albite + hematite + zircon. The auriferous veins contain additional pyrrhotite and chalcopyrite. The gold-bearing pyrites are essentially arsenian with relatively low Fe (Fe 0.82–0.90 S2 to Fe0.89–1.0 S2) as compared to ideal pyrite. Invisible gold varying in concentration from 0.11 to 0.17 wt% were analyzed within pyrite grains having abundant inclusions of calcite and KCl crystals, and CO2. Isotopic ratios from the carbonate dykes yielded an Rb-Sr age of 446 ± 26 (2σ), MSWD=2 with Sri=0.7029 ± 0.0004 (2σ). From field observations of direct continuity between the auriferous quartz veins with the quartz-carbonate dykes, low initial Sr isotopic composition (0.7029) and earlier reported mantle-derived carbon in fluid inclusions in auriferous vein quartz, we infer a distinct mantle origin for these dykes. This strongly suggests that there is at least one component that was directly extracted from the mantle which carried the gold. This mantle component probably was initiated due to structural and thermal events, synchronous with or post-dating the Pan-African orogeny, along the Moyar shear zone.