Fluid-silicate melt Cl partition and its implications on magmatic fluid exsolution and hydrothermal ore genesis

Partition coefficients for Cl between felsic melts and a supercritical aqueous fluid (∼4–16 wt.% NaCl_eq) were experimentally determined to better constrain Cl behavior during magmatic fluid exsolution in upper-crustal magma chambers. Experiments were conducted at 850 °C, 200 MPa, and oxygen fugacity near NNO + 0.5, using a range of melt and fluid compositions. At constant total chlorinity of 1 mol/kg H₂O, values range from 11.3 to 21.1, negatively correlated with both the melt’s aluminum saturation index (ASI) and the HCl/total Cl ratio in the fluid. For a fixed melt composition (ASI = 1.02), values increase linearly from 18.7 to 60.1 as total chlorinity rises from 1 to 4 mol/kg H₂O. Rayleigh fractionation modeling of fluid exsolution from upper-crustal magmas using these data indicates that during progressive crystallization, chlorinity of exsolved fluids rapidly decline before stabilizing at ∼1 mol/kg H₂O (∼4 wt.% NaCl_eq), regardless of initial fluid chlorinity or H₂O content in melt. This implies that the majority of exsolution fluids released from felsic magmas in the upper crust are of low salinity (∼1 mol/kg H₂O). Copper transfer modeling further suggests that efficient metal extraction occurs in Cl- and H₂O-rich magmas, particularly where early H₂O saturation is achieved, thus favoring the formation of high-grade porphyry copper deposits.