Geochemical and mineralogical distribution of germanium in the Khusib Springs Cu–Zn–Pb–Ag sulfide deposit, Otavi Mountain Land, Namibia

Abstract

The Khusib Springs deposit in the Otavi Mountain Land, Namibia, is a small though high-grade carbonate-hosted Cu–Zn–Pb–Ag–(As–Sb–Ge) deposit of the “Tsumeb-type” containing approximately 300,000 t of ore grading 10% Cu, 1.8% Pb and 584 ppm Ag. A lens-shaped sulfide orebody, up to 10 m thick and elongated parallel to bedding, replaces limestone, locally brecciated, of the lower Tsumeb Subgroup (Damara Supergroup) in the northern limb of a large NE-trending synclinal structure. A prominent zone of dolomite alteration surrounds the orebody. In a drill core through the orebody, the uppermost 2 m are enriched in Zn (up to 5%) and Ag (up to 3350 ppm), but lower in Ge (< 30 ppm) than the central and lower parts of the orebody, which are in turn enriched in Ge (up to 200 ppm) and Pb (up to 7%). Massive ore consists of a texturally complex mineral assemblage dominated by Zn-rich tennantite, enargite, galena and pyrite. Fe-poor sphalerite is abundant only in the uppermost part of the orebody and in the footwall where it forms a prominent zone of disseminated sphalerite ± chalcopyrite ± galena ± pyrite. The upper zone of the massive ore is further characterized by a general lack of enargite, and the occurrence of a variety of late-stage phases including tetrahedrite, Ag-bearing tennantite, pearceite–polybasite and native silver. The central and lower parts of the orebody are composed of tennantite–enargite–galena ore with variable proportions of pyrite, but lacking sphalerite. Most Ge is hosted in Ge-bearing colusite [Cu 26V 2(As,Ge) 6S 32] forming small (usually < 50 μm in size) grains along the margins of ore layers and veinlets. With the notable exception of enargite, the major sulfides contain only trace amounts of Ge (< 100 ppm). Trace element analyses reveal a wide range of Ge concentrations in enargite, averaging to 500 ppm. Frequently, colusite is associated with deformed trails of phengitic, F-rich muscovite, rutile, apatite, tourmaline, dolomite and pyrite. Such trails are interpreted as former stylolites in the carbonate host rock, which has been replaced by sulfide ore. Colusite carries on average 4.0 wt.% Ge, up to 4.7 wt.% W and up to 4.4 wt.% Sn, and is occasionally accompanied by small amounts of Mo–W sulfides. Additional minerals identified in the ore include W-bearing germanite, Ge-bearing stannoidite, chalcopyrite, bornite, stromeyerite, minerals of the chalcocite-group, and covellite. Geochemical and mineralogical features of sulfide ores from Khusib Springs resemble some sulfide ores of the pipe-shaped polymetallic Ge-rich Tsumeb deposit. Similar to the latter, ore formation probably occurred during D 2 deformation and metamorphism and involved highly saline, hot brines (up to 370 °C [Chetty, D., Frimmel, H.E., 2000. The role of evaporites in the genesis of base metal sulfide mineralization in the Northern Platform of the Pan-African Damara Belt, Namibia: geochemical and fluid inclusion evidence from carbonate wall rock alteration. Mineralium Deposita 35, 364–376]) carrying sulfur derived from evaporite sequences. A polyphase mineralization involving three successive stages is proposed: early sphalerite–pyrite–(chalcopyrite–bornite) mineralization is replaced by tennantite, enargite, galena and Ge-bearing colusite. A late stage mineralization overprints parts of the orebody at lower temperatures leading to precipitation of ‘schalenblende’ (Cu- and Ag-rich), digenite, tetrahedrite and pearceite–polybasite. The occurrence of small, but high-grade Ge-rich Cu–Zn–Pb–Ag deposits in the lower Tsumeb Subgroup has important consequences on the mineral potential of the area, because prospecting for Cu-rich ores has been traditionally performed in the uppermost Tsumeb Subgroup only, in relation to paleokarst structures.