Experimental Study of the Partitioning of Au, Ag, Mo, W, Zn, Mn, and Ti among Pyrrhotite and Immiscible Fe-S-O and Rhyolitic Melts

Porphyry and related ore deposits are associated in space and time with shallow level intrusions. Identifying mechanisms of chemical enrichment or depletion during the events preceding shallow emplacement of an evolved arc magma may allow for better exploration for economically viable deposits. The effect of ore metal partitioning amongst a rhyolitic melt (sm), pyrrhotite (po), magnetite (mt), and an immiscible Fe-S-O melt (Fe-S-O) has been evaluated in this experimental study for Au, Ag, Mo, W, Zn, Mn and Ti. Capsules were loaded with mt, po, sm, and either Mo + W, or Au + Ag. Other metals (Zn, Mn, and Ti) were introduced with the natural starting materials. Experiments were run at 1036°C (Au, Ag) or 1042°C (Mo, W) in sealed silica tubes, for durations up to 2000 minutes. The coexistence of mt and po allows for the determination of oxygen and sulfur fugacities. fO2 was between the FMQ and NNO buffers, and log fS2 ~ -1 bar. Run products were analyzed by using EMPA and LA-ICP-MS. Experiments yielded a partition coefficient, D(po/sm)±1σ(SDM) =120±50, for Au, which compares well with the value determined by Jugo et al., 1999 (140±40) for hydrous rhyolitic melts in equilibrium with po at 850°C and 100 MPa. The experiments also yielded D(po/sm) and D(Fe-S-O/sm), respectively for Ag 58±8, 120±20; Mo 35±3, 90±10; W 1.2±0.6x10-3, 9±3; Zn 3.4±0.4, 10±1; Mn 1.1±0.1, 1.9±0.3; and Ti 0.030±0.002, 0.28±0.05. During the ascent of magma through the crust, crystal fractionation will remove these elements from the melt to different extents. Assuming po constitutes 0.1wt% of the crystalline assemblage, and the initial magma fractionates 90%, pyrrhotite crystallization can result in removal of: Au by 24%, Ag by 13%, Mo by 8%; W, Zn, Mn and Ti would remain largely unchanged. Small blebs of Fe-S-O melts may also be fractionated at higher temperatures, removing up to 24% Ag and 19% Mo, again leaving W, Zn, Mn and Ti largely unaffected. The metals removed by fractionation would be unavailable for ore formation; sub-economic deposits or the absence of significant geochemical anomalies could be the end result. Crystal fractionation of po may be augmented by the addition of sulfide to the magma by assimilation of reduced sulfide-bearing sedimentary rock. This process may lead to excess po (or, at high enough temperatures, Fe-S-O) precipitation and increased ore metal sequestration, further removing ore metals from the magmatic system, and thereby reducing the probability of porphyry-ore formation by later, high-level, magmatic-hydrothermal processes.