ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

The Otjua tungsten prospect, the largest scheelite-fluorite replacement skarn (250,000 metric tons at 0.53 wt % WOo and 8 wt % CaF2) in Namibia, is developed within Late Proterozoic Okawayo Formation calcite marble in the aureole of the late tectonic, highly fractionate & but unmineralized, Pan-African Otjua leucogranite (Rb-Sr whole-rock date of 478 _ 4 Ma). The contact between the Otjua leucogranite (71.7-73.4 wt % SiO2; 5.78 wt % K20; Rb/Sr = 4.66; (87Sr/86Sr)i = 0.7196; b•8OsMow = 15.1%0) 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 arnet facies and tungsten-barren pyroxene facies skarn, represent increasing degrees of replacement of calcite marble. Gar-net-facies skarn constitutes more than 90 percent of the ore zone. Systematic hanges 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