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

• Published in: Mineralium deposita Vol. 35; no. 8; pp. 741 - 753
• Main Authors: Stein, H. J., Raith, J. G.
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01.11.2000
• Subjects: Geochemistry; Geological time; Geology; Granite; Isotopes; Metamorphism; Mineralization; Mineralogy; Orogeny; Sulfur; Tungsten; Veins (geology)
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s001260050276

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 molybdenite) occurs in foliation-parallel quartz veins in high-grade (750°C, 5-6kbar) metapelites of the Wolfram Formation, and less commonly in small bodies of silicified 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±5Ma. These molybdenites define a well-constrained ^sup 187^Re-^sup 187^Os isochron with an age of 1019±6Ma, which is interpreted as the age of W(Mo) mineralization. This age is significantly 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 1020Ma, thereby precluding the 1060Ma Concordia granite as the source for mineralizing fluids. A narrow range of positive δ^sup 34^S compositions (+3.6 to +4.5[per thousand]) for eight molybdenites from five W(Mo) mines is consistent with a SO^sub 2^-rich fluid 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 Namaqualand. Therefore, the effects of retrograde Namaquan metamorphism extend at least to 1020Ma or, alternatively, these W(Mo) veins were affected by a poorly constrained later event (e.g. early Pan-African).[PUBLICATION ABSTRACT]