Gold and copper partitioning in magmatic-hydrothermal systems at 800 °C and 100 MPa

Abstract

Porphyry-type ore deposits sometimes contain fluid inclusion compositions consistent with the partitioning of copper and gold into vapor relative to coexisting brine at the depositional stage. However, this has not been reproduced experimentally at magmatic conditions. In an attempt to determine the conditions under which copper and gold may partition preferentially into vapor relative to brine at temperatures above the solidus of granitic magmas, we performed experiments at 800 °C, 100 MPa, oxygen fugacity ( f O 2 sys ) buffered by Ni–NiO, and a S 2 sys fixed at either 3.5 × 10 −2 by using intermediate solid solution–pyrrhotite, or 1.2 × 10 −4 by using intermediate solid solution–pyrrhotite–bornite. The coexisting vapor (∼3 wt.% NaCl eq.) and brine (∼68 wt.% NaCl eq.) were composed initially of NaCl + KCl + HCl + H 2O, with starting HCl set to <1000 μg/g in the aqueous mixture. Synthetic vapor and brine fluid inclusions were trapped at run conditions and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Our experiments demonstrate that copper and gold partitioned strongly into the magmatic volatile phase(s) (MVP) (i.e., vapor or brine) relative to a silicate melt over the entire imposed range of a S 2 sys . Nernst style partition coefficients between coexisting brine (b) and melt (m), D b/m (±1 σ), range from 3.6(±2.2) × 10 1 to 4(±2) × 10 2 for copper and from 1.2(±0.6) × 10 2 to 2.4(±2.4) × 10 3 for gold. Partition coefficients between coexisting vapor (v) and melt, D v/m range from 2.1 ± 0.7 to 18 ± 5 and 7(±3) × 10 1 to 1.6(±1.6) × 10 2 for copper and gold, respectively. Partition coefficients for all experiments between coexisting brine and vapor, D b/v (±1 σ), range from 7(±2) to 1.0(±0.4) × 10 2 and 1.7(±0.2) to 15(±2) for copper and gold, respectively. Observed average D b/v at an a S 2 sys of 1.2 × 10 −4 were elevated, 95(±5) and 15 ± 1 for copper and gold, respectively, relative to those at the higher a S 2 sys of 3.5 × 10 −2 where D b/v were 10(±5) for copper and 7(±6) for gold. Thus, there is an inverse relationship between the a S 2 sys and the D b/v for both copper and gold with increasing a S 2 sys resulting in a decrease in the D b/v signifying increased importance of the vapor phase for copper and gold transport. This suggests that copper and gold may complex with volatile S-species as well as Cl-species at magmatic conditions, however, none of the experiments of our study at 800 °C and 100 MPa had a D b/v ⩽ 1. We did not directly determine speciation, but infer the existence of some metal–sulfur complexes based on the reported data. We suggest that copper and gold partition preferentially into the brine in most instances at or above the wet solidus. However, in most systems, the mass of vapor is greater than the mass of brine, and vapor transport of copper and gold may become more important in the magmatic environment at higher a S 2 sys , lower f O 2 sys , or near the critical point in a salt-water system. A D b/v ⩽ 1 at subsolidus hydrothermal conditions may also occur in response to changes in temperature, f O 2 sys , a S 2 sys , and/or acidity. Additionally, both copper and gold were observed to partition into intermediate solid solution and bornite much more strongly than into vapor, brine or silicate melt. This suggests that, although vapor and brine are both efficient at removing copper and gold from a silicate melt, the presence of Cu–Fe sulfides can sequester a substantial portion of the copper and gold contained within a silicate melt if the Cu–Fe sulfides are abundant.