Abstract

This paper provides a synopsis of our latest understanding of the regional geological setting, mineralisation and ore genesis of the Neoproterozoic Rosh Pinah Zn-Pb deposit in southwestern Namibia. During continental rifting at ~740 Ma, an intracratonic rift (Rosh Pinah Graben) became detached from the main Gariep depository by a basement high. The rift subsequently failed and was filled mainly by a volcanoclastic rift-fill sequence, containing predominantly siliciclastic and subordinate calcareous rocks, and was accompanied by contemporaneous bimodal but dominantly felsic volcanism. Volcanism may have provided the heat engine for contemporaneous sedimentary-exhalative and hydrothermal replacement base metal deposits, which formed within anoxic, carbonaceous mudstone horizons in the deep rift. Mineralisation consists mainly of sphalerite, pyrite, galena and chalcopyrite within mudstone, with interbedded, dolomitic and Ba-rich carbonate lenses, and lesser bornite, tennantite-tetrahedrite, stromeyerite, and native gold. Pervasive hydrothermal alteration has affected the surrounding host rocks, and whole-rock lithogeochemical trends indicate that as the ore body is approached, increases in the elements Mn, Fe, Cu, Zn, Ba and Pb are apparent, with barium being the most extensively dispersed, followed by copper and zinc. Evidence for the presence and locality of a potential hydrothermal vent zone is provided by preliminary metal-zoning studies, which is confirmed by the spatial distribution of dominant brecciation and silicification, and the position of peripheral barium-rich carbonate ore bodies. Vertical zoning is indicated by elevated copper contents in the footwall breccia zone, and increases in lead and zinc towards the massive sulphide zones occurring near or at the hanging wall contact. Fluid inclusion studies reveal the presence of two chemically distinct fluid populations – a pre-orogenic, rift-related Rosh Pinah ore-forming fluid dominated by MgCl2- and CaCl2-rich fluids, probably representing hypersaline brines induced by rift-related magmatism, and a general orogenic fluid dominated by a NaCl-CaCl2-H2O (with minor MgCl2) system. The association of the Rosh Pinah deposit with basin-margin rifting, second-order basins, anoxic sediments, and the finely banded/laminated nature of the ore are indicators of possible sedimentary exhalative (SEDEX) origins for the mineralisation. The close association of the sulphides with silicification and brecciation may also indicate significant post-depositional replacement and/or impregnation.

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