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-inquartz 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°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 ~0·3wt % F, and record crystallization conditions of 780-800°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 η=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.
CITATION STYLE
Zhang, D., & Audétat, A. (2017). Chemistry, mineralogy and crystallization conditions of porphyry Mo-forming magmas at Urad-Henderson and Silver Creek, Colorado, USA. Journal of Petrology, 58(2), 277–296. https://doi.org/10.1093/petrology/egx016
Mendeley helps you to discover research relevant for your work.