Tungsten deposits in the Wolfram schist, Namaqualand, South Africa: Strata-bound versus granite-related genetic concepts
Several tungsten deposits, from which ca. 1,800 metric tons
(t) of concentrate at 65.3 percent WO3 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 C, 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 (K2O, 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 +/- garnet, +/- cordierite plus quartz, alkali
feldspar, and plagioclase). Schistose host rocks were derived from normal elastic 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 M3) and a low-grade event of Pan-African age (M(4)) are distinguished.