Chemistry, Mineralogy and Crystallization Conditions of Porphyry Mo-forming Magmas at Urad-Henderson and Silver Creek, Colorado, USA

The metal content, volatile content and crystallization conditions of ore-related rhyolite samples from the Climax-type porphyry Mo deposits at Urad-Henderson and Silver Creek (Rico) were reconstructed based on the composition of melt inclusions, mineral inclusions, and titanium-in-quartz thermobarometry. Additional melt inclusion data were obtained from contemporaneous mafic dikes. Crystallized melt inclusions were either directly analyzed by laser ablation inductively coupled plasma mass spectrometry, or re-homogenized and then analyzed by electron microprobe. Inter-mineralization rhyolite melts at Urad-Henderson were highly evolved (four times more fractionated than average granite of the continental crust), contained 10-15ppm Mo, 6-7 wt % H2O and 0.5-0.7wt % F, and record crystallization conditions of 730-770 degrees C, 1-3 kbar and an oxygen fugacity about two log units above the fayalite-magnetite-quartz buffer (FMQ+2). Melts from two presumably syn-mineralization rhyolite dikes at Silver Creek were slightly less evolved and contained 3-5ppm Mo, 7-8 wt % H2O and similar to 0.3wt % F, and record crystallization conditions of 780-800 degrees C, 2-5 kbar. Both datasets are characterized by distinctly lower fluorine contents but higher temperatures and higher water contents than corresponding values reported for ore-related rhyolites at Climax. Calculated melt viscosities (log eta = 3.5-4.9 Pa s) are lower than those of average granitic melts at the same temperature, which may have facilitated crystal-melt segregation and thus accumulation of large volumes of fractionated, crystal-poor melts in shallow magma chambers, a process that seems to be critical for the formation of Climax-type porphyry Mo deposits.