| Oxygen isotope fractionation factors among SO(,4)('2-), WO(,4)('2-), and MoO(,4)('2-) ions are computed using the harmonic oscillator approximation. It is assumed that the fractionations between XO(,4)('2-) and CaXO(,4) are the same for X = S, W, Mo. Published experimental data on anhydrite-water fractionation are used to determine the fractionations between water and the minerals scheelite (CaWO(,4)) and powellite (CaMoO(,4)) in the range 100-550(DEGREES)C.;The distribution of protonated monomeric and polynuclear W(VI) species in NaCl solutions along the liquid-vapor curve from 100-300(DEGREES)C is investigated by potentiometric titration in a high-temperature concentration cell with hydrogen electrodes. H(,2)WO(,4), HWO(,4)('-), and WO(,4)('2-) are shown to be the dominant tungsten species in most ore solutions. Equilibrium quotients relating these species are used to extract solubility products from published scheelite solubility data. A quantitative model for the transport and deposition of scheelite in 0-6 molal NaCl brines from 100-500(DEGREES)C and vapor saturation to 1200 bars is presented. Major controls on scheelite solubility include (in decreasing order of importance) proton and calcium concentration, ionic strength, and temperature. Pressure has little effect on scheelite solubility.;The results discussed above are combined with field, petrographic, and stable O-C-S-H isotope data and published fluid inclusion temperatures to elucidate the processes of scheelite skarn formation at King Island, Tasmania. Early hypersaline, 370 to > 650(DEGREES)C magmatic fluids derived from crystallizing Devonian quartz monzonite intrusives entered the metasedimentary aureole along major faults and fractures. Fluids were further channelized by impermeable metapelite layers into interbedded marbles. Neutralization and increase in calcium concentration of these fluids, by reaction with both carbonate and previously deposited calcsilicates, led to the deposition of andraditic garnet, salitic clinopyroxene, and molybdenum-rich scheelite ((+OR-) wollastonite, idocrase) in massive skarn replacement bodies. Later, less saline, 230-380(DEGREES)C external fluids (evolved meteoric, connate, or evolved magmatic) entered the ore zones, altering the permeable skarns to hydrous minerals and depositing quartz, calcite, and iron sulfides. Molybdenum was leached out of the early scheelites and deposited as molybdenite, largely as a result of decreasing temperature. The isotopic signature of the late-stage fluids extends up to 5 kilometers from known ore zones. |
