The genesis of Climax-type porphyry Mo deposits: Insights from fluid inclusions and melt inclusions

This review aims at summarizing currently available data on fluid inclusions and melt inclusions from Climax-type porphyry Mo deposits. Data are presented from 16 economically mineralized occurrences in North America and China, plus 5 subeconomically Mo-mineralized and 4 barren occurrences in North America, Norway and China. Because melt inclusions preserve original metal- and volatile contents they are very helpful to reconstruct the abundances and evolution of these constituents in magmas. The available data suggest that the mineralizing silicate melts in Climax-type porphyry Mo systems were highly evolved high-silica rhyolites that were variably enriched in F (0.5–4.0 wt%) and H2O (4–9 wt%) but had rather low Mo contents (2–25 ppm). The latter observation implies that Mo had to be extracted from large volumes of magma (at least several tens of km3) to produce a mid-sized deposit. The fluids that exsolved from these magmas were originally single-phase and of relatively low salinity (5–15 wt% NaClequiv), and subsequently condensed a brine phase once the magma solidus was reached. The single-phase fluids contained on the order of 100 ppm Mo and in some places several wt% CO2, whereas pre-mineralization brines contained up to 500 ppm Mo. A comparison between porphyry Mo-mineralized and barren occurrences reveals that the melts and fluids in mineralized systems were not unusually Mo-rich, which suggests that the mineralization potential was controlled by other factors. Likely candidates are the size of the upper crustal magma chamber, and the efficiency by which fluids were extracted from the magma and focused into a smaller rock volume.