The role of the melt aluminum saturation index in controlling gold and molybdenum proportions in porphyry copper deposits: An experimental investigation

Partition coefficients for Cu, Au and Mo between an aqueous fluid (∼ 4–16 wt% NaCl eq.) and felsic melts, were determined experimentally to better constrain the enrichment of these metals during magmatic fluid exsolution in porphyry copper systems. The experiments were conducted with a variety of melt and fluid compositions at 850 ℃, 200 MPa and an oxygen fugacity approximating that of the Ni-NiO buffer. The results show that at a total chlorinity of 1 mol/kg H2O, the values of Dfluid/meltCu, Dfluid/meltAu and Dfluid/meltMo are ∼ 7.5–27.7, ∼ 1.2–41.2 and ∼ 0.5–2.7, respectively, and that these values vary as function of the melt aluminum saturation index (ASI). For a fixed starting glass composition with an ASI of 1.02 and a fluid with a total chlorinity that increases from 1 to 4 mol/kg H2O, the Dfluid/meltCu and Dfluid/meltAu values increase from 27.7 to 68.8 and from 26.1 to 45.6, respectively, whereas the Dfluid/meltMo value increases from 0.6 to 2.6. Rayleigh fractionation modeling of fluid exsolution from an oxidized (fO2＞∼ FMQ + 2) and sulfide-poor arc magma using these data shows that the ratios involving Cu, Au and Mo in porphyry copper deposits are controlled by the ASI of the melt. Exsolution of a low chlorinity magmatic fluid from a metaluminous to weakly peraluminous magma (ASI= ∼ 1–1.1) will produce a fluid with a high Au/Cu ratio fluid and a Cu-Au porphyry deposit, whereas a strongly peraluminous (ASI＞1.1) magma will produce a low Au/Cu ratio fluid and a Cu porphyry deposit. In contrast, both metaluminous and strongly peraluminous silicate magmas (ASI ∼ 1.0 or ASI＞1.2) will exsolve fluids with high Mo/Cu (or Mo/Au) ratios. Varying the initial chlorinity of the exsolved fluid from ∼ 1 to 4 mol/kg H2O and the initial H2O content of the magma have little effect on the selective partitioning of Cu, Au and Mo into the exsolved fluid, and also an insignificant effect on the bulk Au/Cu and Mo/Cu ratios of the hydrothermal ore systems.