Introduction Two major base metal ore deposits, the Rosh Pinah Zn- Pb sulphide and the Skorpion Zn oxide/silicate deposits, are known to be hosted by the Neoproterozoic Rosh Pinah Formation in the Pan-African Gariep Belt in southwestern Namibia. The Skorpion deposit is considered to be of secondary nature and derived from a primary Rosh Pinah-type mineralisation (Borg et al., 2003; Frimmel et al., 2004). Thus, altogether the Rosh Pinah Formation accounts for some 65 million tons of ore with 7 to 11% Zn and 2% Pb, based on past production and known reserves at these deposits (Wartha and Genis, 1992; Borg et al., 2003). A spatial association with felsic volcanism is conspicuous and most workers have speculated that this volcanism had played some role in the ore genesis . At Skorpion, some primary sulphide mineralisation occurs within felsic volcanic rocks, whereas at Rosh Pinah, economic mineralisation is not hosted by volcanic rocks but mainly by silicified black shale, arenite and carbonates. Although metamorphic overprint has caused recrystallisation of most of the sulphide grains, relict framboidal as well as colloform textures, the presence of soft sediment deformation features and graded bedding displayed by sulphide and gangue minerals, as well as concordance with the stratigraphy and rip-up clasts of ore-zone rock types in the overlying strata have been used to infer a syn-sedimentary ore formation (Siegfried and Moore, 1990). On the other hand, intense silicification and brecciation, as evidenced by a dark sulphidic chert in the ore horizon and by stock work brecciation in the arenitic footwall point to post- depositional ore formation during diagenesis. A syn- orogenic origin can be excluded, as the ore bodies have been affected by orogenic deformation, local mobilisation and recrystallisation. Furthermore, the chemistry of the orogenic fluids was found to be not conducive for the transport of large amounts of base metals. Carbonate rocks in the otherwise predominantly siliciclastic and volcanic Rosh Pinah Formation seem to play an important role as the host to mineralisation. Throughout the area of their distribution, carbonate rocks occur as relatively thin beds displaying distinct brown weathered surfaces, thus highlighting their enrichment in Fe. Closer to the sulphide mineralisation the presence of barite highlights enrichment in Ba. In places, dolomite cuts across the siliciclastic rocks, from which a post-depositional, hydrothermal origin of at least some of the carbonate may be inferred. While the lithostratigraphy and tectonic setting of the Rosh Pinah Formation will be described elsewhere (Alchin et al., 2005), this contribution focusses on the geochemistry, including O and C isotopic compositions, of Rosh Pinah Formation carbonate rocks in order to elucidate their origin. Specifically, the question of their being sedimentary, hydrothermal or both will be addressed. Samples from a volcanic setting will be compared and contrasted with those from the Rosh Pinah mine. Furthermore, comparison between samples taken at various distances from sites of mineralisation will help in reconstructing the nature and influence of the mineralising fluid(s). H.E. FRIMMEL AND K. LANE SOUTH AFRICAN JOURNAL OF GEOLOGY,2005,VOLUME 108 PAGE 5-18 5 Geochemistry of carbonate beds in the Neoproterozoic Rosh Pinah Formation, Namibia: Implications on depositional setting and hydrothermal ore formation H.E. Frimmel and K. Lane Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa e-mail: hef@geology.uct.ac.za; katelaneis@hotmail.com © 2005 Geological Society of South Africa ABSTRACT Hydrothermally altered carbonate rocks are an important host to Rosh Pinah-type base metal sulphide mineralisation in the Pan- African Gariep Belt in southern Namibia. In order to elucidate the genesis and environment of formation of these carbonates, as well as the nature of the hydrothermal alteration, a geochemical as well as C and O isotopic study of these rocks was carried out. The results obtained from two localities, one being proximal to an inferred volcanic centre and the other being in a more distal position at the Rosh Pinah mine, revealed a primary sedimentary origin for most of the carbonates. Original limestone beds were altered to variable extent during early diagenesis by a hydrothermal, dolomitising fluid that most likely emanated from local, rift- related felsic magmatism. Enrichment in some base metals, such as Cu, can be related to this magmatism, whereas others, notably Zn and Pb, are not derived from the magma but sourced in the underlying pre-Gariep Mesoproterozoic basement. Sedimentological and isotopic evidence indicates a shallow marine setting in a restricted basin. An anoxic event, required for the formation of massive sulphide bodies, is postulated to have occurred during a cold period and is explained by eustatic drop in sea level, which caused the separation from the open ocean and subsequent starvation of the depositional basin for the Rosh Pinah Formation.