Is gold solubility subject to pressure variations in ascending arc magmas?

Abstract

Magmas play a key role in the genesis of epithermal and porphyry ore deposits, notably by providing the bulk of ore metals to the hydrothermal fluid phase. It has been long shown that the formation of major deposits requires a multi-stage process, including the concentration of metals in silicate melts at depth and their transfer into the exsolved ore fluid in more superficial environments. Both aspects have been intensively studied for most of noble metals in subsurface conditions, whereas the effect of pressure on the concentration (i.e., solubility) of those metals in magmas ascending from the sublithospheric mantle to the shallow arc crust has been quite neglected. Here, we present new experimental data aiming to constrain the processes of gold (Au) dissolution in subduction-linked magmas along a range of depth. We have conducted hydrous melting experiments on two dacitic/adakitic magmas at 0.9 and 1.4GPa and ~1000°C in an end-loaded piston cylinder apparatus, under fO 2 conditions close to NNO as measured by solid Co-Pd-O sensors. Experimental charges were carried out in pure Au containers, the latter serving as the source of gold, in presence of variable amounts of H 2 O and, for half of the charges, with elemental sulfur (S) so as to reach sulfide saturation. Au concentrations in melt quenched to glass were determined by LA-ICPMS. When compared to previous data obtained at lower pressures and variable redox conditions, our results show that in both S-free and sulfide-saturated systems pressure has no direct, detectable effect on melt Au solubility. Nevertheless, pressure has a strong, negative effect on sulfur solubility. Since gold dissolution is closely related to the behaviour of sulfur in reducing and moderately oxidizing conditions, pressure has therefore a significant but indirect effect on Au solubility.The present study confirms that Au dissolution is mainly controlled by fO 2 in S-free melts and 3 by a complex interplay of fO 2 and melt S 2-concentration in sulfide-saturated melts, at given temperature. In addition, we propose that the transition from sulfide (S 2-) to sulfate (S 6+ ) species in melt is shifted towards more oxidizing conditions when pressure and the degree of melt polymerization increase. If this is true, this may have important consequences during mantle melting and magma ascent. In particular, if mantle melting occurred in moderately oxidizing conditions, a small degree of partial melting would allow the primary melts to become Au-enriched but the relatively high pressure would move the sulfide-sulfate transition to more oxidizing conditions, making the primary melts saturated with sulfide phases that would sequester gold from the melt. During magma ascent, the decreasing pressure would favour the destabilization of sulfides and the release of gold to the silicate melt. However, at shallow levels, decreasing pressure, magma evolution, and varying redox conditions would be continuously competing to concentrate or fractionate gold.

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

“…The Au micro-particles embedded within the glass have irregular shapes (Tassara et al, 2017), pointing to partial dissolution in the surrounding melt. This is in good agreement with Au solubility experiments (Botcharnikov et al, 2011;Jégo et al, 2016) and indicates that the studied silicate melt had an enhanced capability for transporting significant amounts of Au (Figure 7).…”

“…Hence, sulfide breakdown will not only release sulfur but also its contained ore metals (Lorand and Luguet, 2016), such as Au (Figure 6D), which under certain circumstances could lead to metal enrichment in the percolating silicate melts. The solubility of Au in sulfur-bearing silicate melts is maximized when sulfur is dissolved as both sulfide and sulfate species (Cooper et al, 1996) and when the sulfide concentration is near the sulfide-saturated threshold (Botcharnikov et al, 2011;Li and Audétat, 2012;Zajacz et al, 2012;Jégo et al, 2016). Figure 7 shows that the estimated f O 2 of the silicate melt after reacting with the peridotite wall-rock is between FMQ 0 and +1.2.…”

Saline aqueous fluid circulation in mantle wedge inferred from olivine wetting properties

“…We used the same starting materials as those used in the experiments at Bayerisches Geoinstitut. The starting materials were packed into an Au-lined machinable ceramics cylinder (MACOR, MgO + Al 2 O 3 + F-micas), which was also used in the experiments by Jégo et al 73 . A thin Au disk was then placed on top of the Au capsule, and the Au capsule inside the cylinder was shut following the initial pressurization of the run.…”

Gold in Mineralized Volcanic Systems from the Lesser Khingan Range (Russian Far East): Textural Types, Composition and Possible Origins

“…In subduction-related environments, gold is preferentially partitioned into sulfide (monosulfide solid solution, or MSS) phase, especially under oxidized and sulfur-rich conditions [28,31,32,[49][50][51][52][53][54][55][56]58]. Other factors contributing to either gold concentration or fractionation in magmatic environment include pressure and salinity of melts and associated fluids [104][105][106][107]. For example, gradual decrease in pressure in crustal magma conduits will result in sulfide decomposition and partitioning of gold into the silicate liquid [106].…”

“…Other factors contributing to either gold concentration or fractionation in magmatic environment include pressure and salinity of melts and associated fluids [104][105][106][107]. For example, gradual decrease in pressure in crustal magma conduits will result in sulfide decomposition and partitioning of gold into the silicate liquid [106]. At shallow levels, in sub-volcanic environment, other factors such as redox conditions may play the most important role in determining overall gold budgets in mineralized volcanic systems [108][109][110][111][112][113].…”