Partitioning of Cu, Sn, Mo, W, U, and Th between melt and aqueous fluid in the systems haplogranite-H₂O−HCl and haplogranite-H₂O−HF

Abstract

The partition coefficients K_D=c_fluid/c_melt of Cu, Sn, Mo, W, U, and Th between aqueous fluid and melt were measured in the systems haplogranite-H₂O−HCl and haplogranite-H₂O−HF at 2kbars, 750°C, and Ni−NiO buffer conditions using rapid-quench cold seal bombs, with many reversed runs. Concentrations of trace elements (1–1000 ppm) in the quenched aqueous fluid and in the glass were determined by plasma emission spectrometry (DCP). K_D of F is close to 1 in the system studied. K_D of Cu and Sn strongly increases with increasing Cl concentration due to the formation of chloride complexes in the aqueous fluid, while HF has no effect. However, in 2M HCl, K_D of Cu approaches 100, while K_D of Sn is below 0.1 under the same conditions. The partition coefficients of Mo and W are high if water is the only volatile present (Mo: 5.5, W: 3.5), but strongly decrease with increasing HCl and HF, due to the destabilization of hydroxy complexes. K_D of U and Th is very low in the absence of complexing agents, but strongly increases with increasing HF concentration. K_D of U also increases with increasing HCl concentration and with increasing CO₂ concentration in the system haplogranite-H₂O−CO₂, indicating the stability of chloride and carbonate complexes of U at magmatic temperatures. The data suggest a stoichiometric ratio of Cl: U=3:1 and of F:U=2:1 in these complexes. Cl-rich fluids are responsible for the formation of porphyry Cu deposits, but are much less effective in the transport of Sn. F appears not to be essential for the concentration of Mo and W in fluids evolving from a granitic magma. The different complexing behavior of U and Th in aqueous fluids may account for their fractionation during magma genesis.