The Bisha Volcanic-Associated Massive Sulfide Deposit, Western Nakfa Terrane, Eritrea

Abstract

The Neoproterozoic Bisha volcanic-associated massive sulfide deposit (VMS) is a large (>39 million metric tons (Mt)) relatively high grade Zn-Cu-Au-Ag deposit in a newly discovered VMS district in western Eritrea. The host stratigraphic section comprises a large, possibly coeval and cogenetic intrusion known as the Bisha Gabbroic Complex, ~3 km downsection, a series of mafic to felsic tuffs with minor flows and minor sedimentary rocks in the proximal footwall, and felsic flows, fine tuffs, and volcanic-derived siliciclastic rocks in the proximal hanging wall. Minor siliceous exhalite units extend for several kilometers along strike and are present upsection. Two other VMS deposits are present along strike: Bisha Northwest, 1.5 km to the north-northwest, and Harena, 9.5 km to the south-southwest; additionally, a large, lower grade VMS deposit, Hambok, is present ~15 km to the southwest. The Bisha deposit is a single, nearly inclusion and dike-free lens, 1.2 km along strike, that has been folded and affected by significant, near-surface oxidation and supergene processes. The deposit and host strata form an overturned synform within a probable, west-over east, kilometer-scale nappe structure. It would appear that the strata have been folded and thrust against the gabbroic complex which behaved as a competent buttress during deformation. The oxidation and supergene enrichment have led to the development of four mineralogically and spatially distinct ore types or zones. The hematite-goethite-quartz gossan, from 0- to ~30-m depth, has high gold, silver, and lead contents, with native gold, argentiferous gold, electrum, anglesite, and lesser cerrusite. The kaolinite-quartz-sulfate zone occurs at ~25- to 35-m depth, and also is enriched in gold, silver, and lead, and has the same ore mineralogy as the gossan; additionally it has minor chalcocite and minor to trace amounts of unusual Zn-Pb-Fe carbonate and sulfate minerals. The supergene sulfide zone is present from ~35- to 65-m depth and has significant copper enrichment, with predominant chalcocite, and lesser digenite, covellite, and bornite, along with pyrite, minor pyrrhotite, anglesite, and gangue minerals. The primary massive sulfide, from ~65- to more than 450-m depth, has a significant volume of zinc-rich ore and a typical VMS sulfide assemblage of pyrite, sphalerite, chalcopyrite, and minor to trace galena, pyrrhotite, tetrahedrite, tennantite, arsenopyrite, barite, and gangue. The average ore and trace metal contents of the four ore types are calculated from an extensive assay and whole-rock geochemical database. The average metal contents for primary massive sulfide are comparable to that of other bimodal-siliciclastic deposits using an extended, primitive mantle-normalized plot. The ore metals, Cu, Pb, Zn, Au, and Ag, are 2.5 to 3 orders of magnitude higher than average continental crust, whereas the trace metals are more variable. Manganese is significantly depleted in comparison to the average crustal value. Moderate to strong chloritic alteration and stringer sulfide veins are present in footwall felsic and intermediate tuffs along the entire strike length, immediately east of the eastern massive sulfide lens. This is high-lighted using a chlorite alteration index, in wt percent: (MgO + FeO)*100/(MgO + FeO + CaO + K2O + Na2O). In addition, the chloritic footwall is depleted in silica, alkali, and alkaline earth elements, and europium. In contrast, these elements are enriched in the hanging-wall tuffs, indicating that the Bisha hydrothermal system continued after deposition of the tuffs. Primary geochemistry of mafic to felsic volcanic and nearby intrusive rocks, including the Bisha Gabbroic Complex, indicate a bimodal igneous suite, with evidence for cogenetic and possibly coeval volcanic and intrusive rocks. The suite has tholeiitic to transitional calc-alkalic affinities, with mantle plume-influenced, primitive arc signatures for least altered basalts. Lead isotope ratios of galena and anglesite mineral separates from the Bisha, Bisha Northwest, and Harena deposits have model ages that range from 790 to 770 Ma. This is in contrast to a lead model age of ~720 Ma for the Adi Nefas VMS deposit in the eastern Nakfa terrane and to galena from a nearby gold vein deposit at ~490 Ma. The model ages for the Bisha area VMS deposits indicate that these deposits and their host rocks formed early during the development of the 850 to 550 Ma Nakfa terrane.