Alkali metals control the release of gold from volatile-rich magmas

Zajacz, Zoltán; Seo, Jung Hun; Candela, Philip A.; Piccoli, Philip M.; Heinrich, Christoph A.; Guillong, Marcel

doi:10.1016/j.epsl.2010.06.002

Cited by 122 publications

(113 citation statements)

References 56 publications

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“…The ionic strength of experimental solutions was fixed at 1 M. In the present study, the data of [3] were corrected for ionic strength for the calculation of the thermodynamic value of Au solubility constant (Supplementary Material Section 2). Experimental Au solubility data reported by Ryabchikov and Orlova [7] (750 °С, 1500 bar, Supplementary Material Section 3.1), Guo et al [9] (800 °С, 2000 bar, Supplementary Material Section 3.2), and Zajacz et al [8] (1000 °С, 1500 bar, Supplementary Material Section 3.3) were The differences between the experimental and calculated Au solubility values (last two columns of Table 1) do not exceed 0.2 log units and are independent of NaCl concentration in the wide range of fluid salinities (from 0.1 to 3 mol·(kg H2O) −1 ), HCl concentrations, and redox conditions (Figure 3). This confirms the high accuracy of our method of the calculation of activity coefficients (in particular, the constant value of the ion size parameter o a = 4.5 Å), including the activity coefficient of H2°(aq), which was calculated ignoring the salting-out effect even in concentrated NaCl solutions.…”

Section: Au Solubility Constant At 25-1000 °C and Pressures Up To 5 Kbarmentioning

confidence: 99%

Stability of AuCl2− from 25 to 1000 °C at Pressures to 5000 bar and Consequences for Hydrothermal Gold Mobilization

et al. 2018

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Abstract:Gold is transported in high-temperature chloride-bearing hydrothermal fluids in the form of AuCl 2 − . The stability of this complex has been extensively studied, but there is still considerable disagreement between available experimental data on the temperature region 300-500 • C. To solve this problem, we measured the solubility of gold in HCl/NaCl fluids (NaCl concentration varied from 0.1 to 3 mol·(kg·H 2 O) −1 ) at 450 • C and pressures from 500 to 1500 bar (1 bar = 10 5 Pa). The experiments were performed using a batch autoclave method at contrasting redox conditions: in reduced experiments hydrogen was added to the autoclave, and in oxidized experiments the redox state was controlled by the aqueous SO 2 /SO 3 buffer. Hydrogen pressure in the autoclaves was measured after the experiments in the reduced system. The gold solubility constant, Au (cr) + HCl • (aq)

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Section: Au Solubility Constant At 25-1000 °C and Pressures Up To 5 Kbarmentioning

confidence: 99%

Stability of AuCl2− from 25 to 1000 °C at Pressures to 5000 bar and Consequences for Hydrothermal Gold Mobilization

et al. 2018

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“…We did not examine here the partitioning of gold between melt and fluid because the experimental setup of the present study was not designed to permit the 20 determination of the fluid composition. Nevertheless, gold partitioning between fluid and melt has been investigated in a number of previous studies (Frank et al, 2002;Simon et al, 2003;Zajacz et al 2010;2012).…”

Section: Experimental Products and Compositionsmentioning

confidence: 99%

“…In S-bearing charges, a vapour phase is considered to be systematically present even if the melt is strictly-speaking H 2 Oundersaturated (aH 2 O <1) under the experimental conditions: this is due to the relatively low solubility of S in silicate melts that systematically induces -besides the formation of a S-rich mineral phase at sulfide saturation (i.e., pyrrhotite) -the formation of a S-rich vapour phase (containing mainly H 2 S + SO 2 + S 2 + H 2 O) at equilibrium, whatever the aH 2 O value calculated in the corresponding S-free system (see section 3.4.4). Then, the presence of a vapour phase may lead to significant partitioning of gold from liquid to vapour (e.g., Ulrich et al, 1999;Sun et al, 2004;Simon et al, 2007;Zajacz et al 2010;2012). Given the amount of H 2 O initially loaded in the capsules, the calculated melt H 2 O contents, and the approximate mass of the silicate charges, this vapour phase is present in very low amounts (probably less than a couple wt% of the total charge).…”

Section: Experimental Products and Compositionsmentioning

confidence: 99%

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

Jégo

Nakamura

Kimura

et al. 2016

Geochimica et Cosmochimica Acta

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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.

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“…These fluids, which have created the major part of economic gold resources on Earth (1-4), may carry much higher Au concentrations, of tens to hundreds of parts per million, as reported from rare fluid inclusion analyses (4, 6, 9, 10) and a few laboratory experiments of Au solubility (17)(18)(19)(20). How gold is transported by such fluids remains, however, controversial, and a variety of other species with H 2 S, Cl, As, and alkali metal ligands (3,14,(17)(18)(19)(20) or Au nanoparticles (4,6,12) were suggested. Thus, a consistent picture of Au speciation and transport in deep and hot crustal fluids is lacking, hampering our understanding of geochemical fluxes of gold across the lithosphere and the formation of gold economic resources.…”

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confidence: 99%

Sulfur radical species form gold deposits on Earth

Pokrovski

Kokh

Guillaume

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

121

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Current models of the formation and distribution of gold deposits on Earth are based on the long-standing paradigm that hydrogen sulfide and chloride are the ligands responsible for gold mobilization and precipitation by fluids across the lithosphere. Here we challenge this view by demonstrating, using in situ X-ray absorption spectroscopy and solubility measurements, coupled with molecular dynamics and thermodynamic simulations, that sulfur radical species, such as the trisulfur ion S − 3 , form very stable and soluble complexes with Au + in aqueous solution at elevated temperatures (>250°C) and pressures (>100 bar). These species enable extraction, transport, and focused precipitation of gold by sulfur-rich fluids 10-100 times more efficiently than sulfide and chloride only. As a result, S − 3 exerts an important control on the source, concentration, and distribution of gold in its major economic deposits from magmatic, hydrothermal, and metamorphic settings. The growth and decay of S − 3 during the fluid generation and evolution is one of the key factors that determine the fate of gold in the lithosphere.gold | sulfur | ore deposit | hydrothermal fluid | trisulfur ion

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Alkali metals control the release of gold from volatile-rich magmas

Cited by 122 publications

References 56 publications

Stability of AuCl2− from 25 to 1000 °C at Pressures to 5000 bar and Consequences for Hydrothermal Gold Mobilization

Stability of AuCl2− from 25 to 1000 °C at Pressures to 5000 bar and Consequences for Hydrothermal Gold Mobilization

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

Sulfur radical species form gold deposits on Earth

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