Tungsten deposits in the Wolfram Schist, Namaqualand, South Africa; strata-bound versus granite-related genetic concepts

Abstract

Several tungsten deposits, from which ca. 1,800 metric tons (t) of concentrate at 65.3 percent W03 was produced, occur in the Okiep copper district, South Africa, which is part of a high-temperature, low-pressure granulite terrane (M 2 ; 750 degrees -850 degrees , 5-7 kbars). The 1.2 to 1.0 Ga high-grade metamorphism of Proterozoic supracrustal and older intrusive rocks is polymetamorphic and was accompanied by crustal melting resulting in formation of several early to postorogenic granitoid suites. The W-Mo deposits are spatially and genetically associated with the 1.1 Ga Concordia granite, a silica-rich (70-76 wt %), peraluminous leucogranite with subalkaline monzonitic magma characteristics. More fractionated cogenetic intrusions are interpreted as separated residual granitic liquids. They are enriched in large ion lithophile (K 2 O, Rb) and some high field strength (W, Nb, etc.) elements. Mineralized pegmatites, replacement orebodies (strongly silicified granitic rocks), and exo- and endogranitic veins are distinguished. Ferberite and minor scheelite are associated with Mo, Cu, and Bi sulfides and U-Th, Nb, REE silicates, oxides, and phosphates. Metapelites (Wolfram schist) hosting S 2 subparallel exogranitic veins are characterized by M 2 upper amphibolite-lower granulite facies assemblages (biotite-sillimanite + or - garnet, + or - cordierite plus quartz, alkali feldspar, and plagioclase). Schistose host rocks were derived from normal clastic sedimentary material, slightly peraluminous lithologies probably from more pelitic kaolinite-rich precursors. The facts that late to postorogenic granites and the W-Mo deposits are deformed, metamorphic ore textures (e.g., garnet coronas around ore minerals) are preserved, and ore deposit, specific hydrothermal low- temperature alteration is absent, suggest that the second M 3 late-Kibaran metamorphic overprint postdated mineralization. Oxygen isotope temperatures of ca. 600 degrees to 660 degrees C for coexisting garnet and quartz from mineralized veins and host rocks indicate that high-grade conditions were again reached during this late Kibaran overprint. Retrograde greenschist facies metamorphism is of Pan-African age (ca. 560-590 Ma) and is unrelated to mineralizing processes. The W-Mo deposits are interpreted as metamorphosed, deep-seated granitic ore deposits occurring in a polymetamorphic high-grade terrane. Two high-grade events of Kibaran age (M 2 and M 3 ) and a low-grade event of Pan-African age (M 4 ) are distinguished.