ARTICLE J. G. Raith á H. J. Stein Re±Os dating and sulfur isotope composition of molybdenite from tungsten deposits in western Namaqualand, South Africa: implications for ore genesis and the timing of metamorphism Received: 12 September 1999 /Accepted: 20 April 2000 Abstract Both stratiform/stratabound and granite- related models have been used to explain the genesis of WMo) deposits in the Okiep copper district in western Namaqualand, South Africa. Apparently, stratabound mineralization Fe-rich wolframite with accessory mo- lybdenite) occurs in foliation-parallel quartz veins in high-grade 750 °C, 5±6 kbar) metapelites of the Wolfram Formation, and less commonly in small bodies of silici®ed leucogranites and pegmatites. Six Re±Os ages for molybdenites from four deposits Nababeep Tungsten Far West, Kliphoog, Narrap, Tweedam) range between 1000  4 and 1026  5 Ma. These molybde- nites de®ne a well-constrained 187 Re± 187 Os isochron with an age of 1019  6 Ma, which is interpreted as the age of WMo) mineralization. This age is signi®cantly younger than Proterozoic protolith ages for supracrustal rocks and the emplacement ages for the main intrusive suites, but geologic evidence requires overlap with a period of high-grade metamorphism. We suggest that WMo) mineralization is genetically linked to intra- crustal magmatic processes at 1020 Ma, thereby pre- cluding the 1060 Ma Concordia granite as the source for mineralizing ¯uids. A narrow range of positive d 34 S compositions +3.6 to +4.5&) for eight molybdenites from ®ve WMo) mines is consistent with a SO 2 -rich ¯uid and a granite-related genetic model. Post-peak metamorphic deformation and metamorphism of WMo) ores is most likely related to the retrograde stage of the Namaquan orogeny, which overlaps emplacement of late-orogenic, evolved granites and pegmatites, and the formation of WMo) deposits in western Nam- aqualand. Therefore, the eects of retrograde Nama- quan metamorphism extend at least to 1020 Ma or, alternatively, these WMo) veins were aected by a poorly constrained later event e.g. early Pan-African). Introduction Much controversy exists regarding the genesis of tung- sten deposits. One group of granite-related WSn, Mo, Cu) deposits includes magmatogenic skarn, vein- type, vein-stockwork, greisen, porphyry, breccia pipe and pegmatite-type systems e.g. Kwak 1987 and refer- ences therein). Another group of stratiform and strat- abound deposits, associated with regional metamorphic calc-silicate rocks, metabasites and tourmalinites, has less clear or no genetic anities with intrusive magmatic systems e.g. Plimer 1987). Syngenetic/diagenetic, sedi- mentary-exhalative genetic models, which may include an evaporite-derived ¯uid component, have been pro- posed for the latter HoÈll 1977; Raith 1991; Plimer 1994). A genetically ambiguous group of tungsten deposits occurs in a low-pressure high-temperature granulite fa- cies terrane in western Namaqualand, South Africa. Both epigenetic and syngenetic models have been pro- posed to explain formation of these apparently strat- abound deposits from which 1800 tons of concentrate at 65.3% WO 3 were mined between 1941 and 1956. Molybdenite is a common accessory ore mineral but was never of economic interest. On the basis of ®eld rela- tions, SoÈhnge 1950) was the ®rst to suggest a genetic relationship between the ores and granitic rocks. In the 1970s and 1980s syngenetic sedimentary-exhalative models Stump¯ 1977), perhaps with subsequent en- richment because of anatectic remobilization Bowles 1988), have been advocated. Field relations, mineral parageneses, and petrological and geochemical data for the ores and host rocks have been used by Raith 1995) and Raith and Prochaska Mineralium Deposita 2000) 35: 741±753 Ó Springer-Verlag 2000 Editorial handling: H. E. Frimmel J. G. Raith &) Institute of Geological Sciences, University of Leoben, A-8700 Leoben, Austria e-mail: raith@unileoben.ac.at Tel.: +43-3842-402652; Fax: +43-3842-47016 H. J. Stein AIRIE Program, Department of Earth Resources, Colorado State University, Fort Collins, CO 80523-1482, USA