Strong redox control on the liquid/vapor partitioning of sulfur and its implications for magmatic-hydrothermal ore genesis
Abstract
Porphyry Cu(±Mo, Au) deposits are the largest positive sulfur anomalies in the Earth's crust. The common wisdom is that the main stage of ore mineral precipitation in these deposits occurs when magmatic SO2 disproportionates to sulfide and sulfate with decreasing temperature (T). Furthermore, liquid-vapor immiscibility has been suggested to play a critical role in the genesis of these deposits, with the liquid phase acting as temporary repository for a significant fraction of the ore metals. Sulfur is commonly thought to strongly partition into the vapor phase, which raises the question where the condensed liquid (=brine) gains S from to precipitate ore sulfides.
We conducted experiments to study the partitioning of S between liquid and vapor as a function of oxygen fugacity (fO2). The experiments were conducted in the pressure (P) and T range of 100 - 130 MPa and 725 - 900 oC. Rapid-quench Molybdenum - Hafnium Carbide pressure vessels were used, and the liquid and vapor phases were trapped as synthetic fluid inclusions (SFI) in quartz, which was fractured in situ during the experiments by applying thermal shock after equilibrium had been attained. The composition of the SFI, including S concentrations, were subsequently determined by LA-ICP-MS. The results show that at fO2 one log unit below the Ni-NiO buffer (NNO-1), S strongly partitions into the vapor as expected, with liquid/vapor partition coefficients (Dl/v) of about 0.2 at a liquid/vapor density ratio of ≈3.7. However, with increasing fO2, S increasingly partitions into the liquid phase. Even at the moderately high fO2 of the Re-ReO2 buffer (NNO+1.9), characteristic of many arc magmas, Dl/v values of 0.6±0.1 to 7.2±1.7 (1σ) were determined at a liquid/vapor density ratio of ≈3.7 and T=900 oC. The value of Dl/v increases with increasing alkali metal/S ratio in the starting solutions at a given fO2. The partitioning of S into the liquid is even stronger at the fO2 of the hematite-magnetite buffer. These observations can be explained by a change in S speciation from hydrogen sulfide to sulfate species, with sulfate species becoming stable in the liquid phase already at moderately high fO2. Thus brines condensing from oxidized magmatic fluids can sequester a significant amount of S; however, this subsequently needs to be reduced to sulfide to allow ore mineral precipitation.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V13C0176Z
- Keywords:
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- 1042 Mineral and crystal chemistry;
- GEOCHEMISTRY;
- 1043 Fluid and melt inclusion geochemistry;
- GEOCHEMISTRY;
- 1065 Major and trace element geochemistry;
- GEOCHEMISTRY;
- 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGY