Source and Evolution of Molybdenum in the Porphyry Mo(-Nb) Deposit at Cave Peak, Texas

The 37 +/- 1 Ma breccia-hosted porphyry Mo(-Nb) deposit at Cave Peak is genetically related to a mafic, alkaline intrusion nearby (the Marble Canyon Stock), thus providing an excellent opportunity to study the influence of mafic magmatism on porphyry Mo formation. Laser ablation inductively coupled plasma mass spectrometry analysis of crystallized melt inclusions allowed reconstruction of pristine concentrations of Mo and other fluid-mobile elements in melts ranging from phonotephrite (similar to 50 wt % SiO2) to rhyolite (similar to 75 wt % SiO2) in composition. Incompatible trace element abundances in these melt inclusions define smooth, linear trends reaching up to 100 times average upper continental crust, implying that the felsic magmas evolved from a mafic parent by fractional crystallization. The mafic parent has a geochemical signature typical of primitive melts generated in within-plate tectonic settings. Molybdenum increases from 4 ppm in the mafic endmember to 12 ppm in felsic melts immediately before Mo mineralization. After breccia formation, Mo concentrations in the residual melt decreased gradually to 5 ppm, suggesting that Mo was removed from the residual melts by exsolving fluids. This interpretation is supported by fluid-melt partition coefficients (D-Mo,D- (fluid/melt) = 17-20) measured on assemblages of coexisting fluid and melt inclusions. A high sulfur fugacity in the mafic input magma is indicated by early pyrrhotite saturation. To account for the 0 center dot 4-1 center dot 5 x 10(6) t Mo present in the deposit a volume of similar to 30-100 km(3) alkali feldspar granite magma is needed, or up to 10 times more of the mafic melt. Significantly less magma is required to provide all the sulfur present in the deposit. The results of this study provide direct evidence for derivation of metals (and sulfur) in porphyry Mo deposits from associated mafic, alkaline magmas through fractional crystallization.