VOLATILE TRANSPORT AND DEPOSITION OF MO, W AND RE IN HIGH-TEMPERATURE MAGMATIC FLUIDS

Molybdenite (MoS2) and wolframite (FeWO4) sublimates are typically observed in high-temperature (650-500°C), reduced (fO2 near NNO or QFM) volcanic gases. The molybdenite shows considerable Re enrichments (up to 11.5 wt%) which is the highest reported natural concentration of this rare element in a mineral phase. This reflects large enrichments of Re relative to Mo in volcanic gases. Thermodynamic gas calculations used to model the volatile transport of Mo and W in a high temperature (>500°C) magmatic fluid show that molybdic acid (H2MoO4) and tungstic acid (H2WO4) are by far the most abundant volatile species of Mo and W for a variety of gas compositions and O2 fugacites ranging from QFM to HM buffers. The oxychlorides (MoO2Cl2, WO2Cl2) are present in significant concentrations only at low temperatures (<400°C) or for very high HCl fugacites (10 mole %). Variations in HF, HBr and total S do not have a significant influence on the volatile transport of these elements. By analogy with Mo, HReO4 is believed to be the volatile species of Re at high temperature. The deposition temperatures of molybdenite or Re sulfide from the gas phase increase with increasing H2S fugacity. This will restrain the ability of S-rich magmatic systems to transport volatile Mo at lower temperatures. The saturation temperatures calculated for 1 ppm Mo and 0.1 ppm W concentrations in a S-poor magmatic fluid are 680 and 780°C, respectively. Most (99%) of the metal burden of Mo and W in the fluid is deposited in a 100-150° temperature interval, just below the magmatic temperature. © 1990.