Abstract

The Otjua tungsten prospect, the largest scheelite-fluorite replacement skarn (250,000 metric tons at 0.53 wt % WO 3 and 8 wt % CaF 2 ) in Namibia, is developed within Late Proterozoic Okawayo Formation calcite marble in the aureole of the late tectonic, highly fractionated, but unmineralized, Pan-African Otjua leucogranite (Rb-Sr whole-rock date of 478 + or - 4 Ma). The contact between the Otjua leucogranite (71.7-73.4 wt % SiO 2 ; 5.78 wt % K 2 O; Rb/Sr = 4.66; ( 87 Sr/ 86 Sr) I = 0.7196; delta 18 O SMOW = 15.1ppm) and the metasediments is not exposed nor has it been intersected in drilling. Three zones or facies of skarn, successively, vesuvianite facies (with minor scheelite), scheelite-bearing garnet facies and tungsten-barren pyroxene facies skarn, represent increasing degrees of replacement of calcite marble. Garnet-facies skarn constitutes more than 90 percent of the ore zone. Systematic changes in skarn mineralogy, pyroxene and scapolite chemistry, and whole-rock compositions of the facies of skarn show that the steeply dipping calcitic marble acted as a reactive conduit for a channelized mineralizing fluid that emanated from a central source at depth. Variations in scapolite composition are closely related to scheelite distribution within the skarn system. Available evidence suggests that the metasomatic event was a constant volume process. The marble that hosts the skarn mineralization has the isotopic composition of a metamorphosed marine carbonate (delta 18 O SMOW = 15.5ppm, delta 13 C PDB = 0.1ppm), whereas interstitial calcite from the skarn has a different isotopic signature (delta 18 O SMOW = 15.3ppm, delta 13 C PDB = -5.1ppm). A channelized magmatic fluid was responsible for the depletion in the delta 13 C value; the similarity in marble and skarn oxygen isotope compositions is accounted for by the abnormally heavy oxygen isotope composition of the Otjua pluton. Skarn formation was undoubtedly related to the emplacement of the leucogranite, but the presence of widely distributed, probably preexisting, tungsten-scheelite mineralization in the metamorphosed marls (scheelite-bearing granofels) of the Late Proterozoic Damaran sequence suggests that the intrusion rather than being the source of the metals may merely have acted as a heat-engine and mobilized fluids from the metasediments within its thermal aureole.

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