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 kaolinitequartz-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 ZnPb-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 sulfid...

Regional setting and characteristics of the Neoproterozoic Wadi Hamama Zn–Cu–Ag–Au prospect: evidence for an intra-oceanic island arc-hosted volcanogenic hydrothermal system

“…12d) and the presence of sulfide minerals in the core samples (Alexander Nubia 2012a, b) signify the role of subaerial weathering in the formation of the gossan of the WH prospect. This is compatible with the conclusion of Barrie et al (2007) that the Tertiary to recent weathering is important for the gossan formation in the VMS deposits of the ANS. Other than the inheritance of primary copper enrichment from the original mineralization, the enrichment of secondary copper minerals in the exploited trenches and pits can be attributed to the supergene process.…”

“…The crustal growth of the ANS was achieved by the accretion of intra-oceanic island arcs to continental margins accompanying the closure of the Mozambique Ocean separating the East and West Gondwana (Stern 1994). Many volcanogenic base metal occurrences and deposits were recorded in the ANS (Schellekens 1986;Al-Shanti et al 1993; Barrie et al 2007;Doebrich et al 2007;Surour and Bakhsh 2013;Harbi et al 2014). In Egypt, the volcanogenic base metal occurrences cluster in the Eastern Desert close to Um Samiuki area, which is considered as the main VMS deposit in Egypt (Abdel Kader and Shalaby 1982;Rasmy et al 1983;El Aref et al 1985;Takla et al 1998;Helmy 1999;Botros 2003;Shalaby et al 2004).…”

Structural setting of Neoproterozoic mineralization, Asmara district, Eritrea

“…In the AAB at Bisha, the massive sulphides and their hosts feature a west-over-east stacking kilometer-scale nappe structure against the gabbroic complex ( Barrie et al, 2007). Though such a structure is likely, the steeply northward and northwestward plunging asymmetric crenulations shown on their Fig.…”

“…The Bisha district consists of Bisha Main, Harena, Northwest Bisha and Hambok (Barrie et al, 2007). Quartz vein-hosted deposits in the old mine at the Augaro locality contain native gold, azurite, and malachite in the oxidation zone and chalcopyrite and pyrite with gold at depth (Jelenc, 1966) probably indicating quartz-sulphide vein style mineralization there.…”

Geologic and Metallogenic Framework of North Africa

“…These include ultramafic cumulates, chromitite, layered gabbro, leucogabbro, and anorthosite, all of which were emplaced within a 3.5-2.9 Ga TTG gneiss terranes. Chromite mineralization, predominantly of podiform type although locally in layered ore, occurs as NE-trending, massive to brecciated Fe-rich chromitite orebodies several meters wide and tens of meters long, hosted in serpentinite, ultramafic cumulates, mafic rocks (coarse-and fine-grained amphibolite), and anorthosite layers (Barrère 1967;Berger et al 2013); these features are typical of Archean ultramafic-mafic-anorthosite complexes worldwide (e.g., Rollinson et al 2010). PGM have only been observed in massive and layered chromitites where laurite (RuS 2 ), as the most abundant PGM, forms small (<5 mm, mostly 2-3 mm) euhedral inclusions within the cores and rims of chromite grains.…”