The effect of pressure on sulphur speciation in mid- to deep-crustal arc magmas and implications for the formation of porphyry copper deposits

Abstract

. The effect of pressure on sulphur speciation in mid to deepcrustal arc magmas and implications for the formation of porphyry copper deposits. Contributions to Mineralogy and Petrology, 171, [66]. DOI: 10.1007DOI: 10. /s00410-016-1274 Peer reviewed version Link to published version (if available): 10.1007/s00410-016-1274-4Link to publication record in Explore Bristol Research PDF-document This is the author accepted manuscript (AAM). The final published version (version of record) is available online via Springer at http://link.springer.com/article/10.1007/s00410-016-1274-4. Please refer to any applicable terms of use of the publisher. University of Bristol -Explore Bristol Research General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms phases and sulphur speciation in the melt are strong functions of ƒO2, as expected, but also of pressure. At 0.5 GPa the anhydrite stability field is intersected at NNO≥+2, but at 1.0 and 1.5 GPa experiments at the same ƒO2 produce sulphides and the stability field of sulphate moves towards higher ƒO2 by ~1 log unit at 1.0 GPa and ~1.5 log units at 1.5 GPa. As a result, models that appeal to high oxidation state as an important control on the mobility of Cu (and other chalcophiles) during crustal differentiation must also consider the enhanced stability of sulphide in deep-to mid-

REE Redistribution Textures in Altered Fluorapatite: Symplectites, Veins, and Phosphate-Silicate-Carbonate Assemblages from the Nolans Bore P-REE-Th Deposit, Northern Territory, Australia

“…However, as Ce is predominantly incorporated in fluorapatite as Ce 3þ , mass-balance considerations necessitate a source of oxygen for a Ce 4þ -rich silicate to precipitate, and hence the presence of an oxidizing fluid is envisaged. By comparison to the data from natural samples containing magmatic zircon (Ballard et al 2002), whose f O 2 is poorly known but likely formed above the sulfidesulfate transition (Klimm et al 2012, Matjuschkin et al 2016, the f O 2 during thorite-stetindite precipitation was at least two orders of magnitude above FMQ, but likely much higher. Nolans Bore also contains several hundred ppm of U in addition to Th (Huston et al 2016).…”

Unraveling the Effects of Melt–Mantle Interactions on the Gold Fertility of Magmas

“…Given that the S 6+ /S total ratio and log (f O 2 ) follow a near linear trend within this range (Konecke et al, 2019), we infer that the final oxidation state of the silicate melt at apatite formation was between FMQ +0.3 to +0.4 [±0.5, 2σ], which is within the f O 2 estimated for the peridotite wall-rock. Considering that an increase in pressure shifts the stability field of sulfide toward more oxidizing conditions (Matjuschkin et al, 2016) and inferring that apatite follows a similar trend, we conclude that a FMQ +0.3 to +0.4 represents a minimum f O 2 value for the silicate melt.…”

“…Decreasing pressure during ascent could potentially boost the breakdown of the sulfide phases, as the solubility of S in the melt will increase (Mavrogenes and O'Neill, 1999). However, if only this had been the mechanism of sulfide breakdown, no sulfate would be observed as the system would remain reduced (Matjuschkin et al, 2016). Therefore, an oxidation mechanism is still needed to account for the sulfate in the system.…”

Amphibole and apatite insights into the evolution and mass balance of Cl and S in magmas associated with porphyry copper deposits

“…One should note that at upper crustal pressure and at fO 2 ranging from NNO + 1 to NNO + 2, sulfate (S 6+ ) would not be the only S species and that some amount of reduced sulfur may also be present (Jugo et al 2010;Botcharnikov et al 2011; (5) S apatite (wt%) = 0.0629 × lnS melt (wt%) + 0.4513. Matjuschkin et al 2016). Therefore, because apatite mainly incorporates sulfate ions (S 6+ ) in its lattice, the melt sulfur concentration we obtain is likely a minimum value and only accounts for the concentration of the sulfate ions in the melt.…”

“…While sulfide saturation in the lower crust inevitably drives the amount of Cu down during magmatic evolution of calc-alkaline magmatic suites (Chiaradia 2014), the oxidation state of sulfur can dramatically change upon magma ascent and decompression (Matjuschkin et al 2016). Indeed, for a magma having ƒO 2 buffered around NNO + 1 to NNO + 2 such as the ore-forming porphyries at Coroccohuayco, magmatic sulfur is dominantly a reduced species at lower crustal pressures (Matjuschkin et al 2016), but is dominantly in the form of more oxidized sulfates at upper crustal pressures. This contrasts with magmas buffered at around NNO such as those of the gabbrodiorite complex where sulfur is dominantly in reduced form throughout the thickness of the crust.…”