Abstract

The Dimako–Mboscorro lode gold deposit has two styles of mineralization: quartz veins (4.5–54 ppm Au) and altered wallrock (up to 2.6 ppm Au). The veins occur in steeply dipping brittle–ductile shear zones in a foliated biotite metagranite. The alteration envelop around the gold-bearing quartz veins shows lateral zoning comprising a silicified/ ferruginized and mylonitic zone (SFZ) at the centre, through a transitional sericite zone (SZ), to an outer, slightly sheared, but mineralogically less altered, though bleached, metagranite zone (BZ) at the shear zone boundaries. Hydrothermal alteration in these zones is characterized by silicification (SFZ, SZ), hematite formation (SFZ), alkali metasomatism with muscovite/sericite formation (SFZ, SZ, to a much lesser extent BZ), and pyritization (SFZ, SZ). The auriferous quartz veins are concordant to the host structure but discordant to the foliation in the metagranite. The main ore minerals in the veins and in the SFZ and SZ zones are gold, hydrothermal hematite, sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, chalcocite, covellite and colloidal supergene hematite. Gold occurs as inclusions in sphalerite (15.7±0.4 wt% Ag; 83.6±0.2 wt% Au), as coarse grains associated with euhedral hematite pseudomorphs (18.5±1.0 wt% Ag; 82.0±1.1 wt% Au), as stringers within fractures in quartz (13.3±0.1 wt% Ag; 87.1±0.5 wt% Au), as well as fine to coarse grains related to limonite, goethite and colloidal hematite (8.1±1.8 wt% Ag; 92.1±2.2 wt% Au). A hydrothermal metal association of Ag, Au, As, Ba, Pb, V, W, and Zn is identified in the wallrock and quartz veins. These metals, together with K-enrichment, are potential pathfinder elements for wider lithogeochemical exploration in this region, especially when combined with the lateral wallrock zonation pattern. Preliminary fluid inclusion data from the auriferous quartz vein samples indicate that gold was probably deposited from low salinity (<1 to 8 wt% eq. NaCl, with mean at 5.52 wt% eq. NaCl) H2O–CO2 fluids that have total homogenization temperatures of 263–335°C.

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