High gold concentrations in sulphide-bearing magma under oxidizing conditions

Botcharnikov, Roman E.; Linnen, Robert L.; Wilke, Max; Holtz, François; Jugo, Pedro J.; Berndt, Jasper

doi:10.1038/ngeo1042

Cited by 189 publications

(152 citation statements)

References 21 publications

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“…NNO-0.7) to FMQ + 2 (ca. NNO + 1.3) the dominant sulfur species rapidly changes from S 2− to S 6+ (Jugo et al 2010;Botcharnikov et al 2011). Therefore, the low S content of apatite of the gabbrodiorite complex is also consistent with previous work showing that it corresponds to a magma in which S occurred mostly in its reduced form at NNO ± 1 (Chelle-Michou et al 2014, 2015a.…”

Section: Sulfur Concentration Of the Meltsupporting

confidence: 78%

“…10) and are discussed independently below. 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).…”

Section: Sulfur Concentration Of the Meltmentioning

confidence: 99%

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

Chelle-Michou

Chiaradia

2017

Contrib Mineral Petrol

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Chlorine and sulfur are of paramount importance for supporting the transport and deposition of ore metals at magmatichydrothermal systems such as the Coroccohuayco Fe-Cu-Au porphyry-skarn deposit, Peru. Here, we used recent partitioning models to determine the Cl and S concentration of the melts from the Coroccohuayco magmatic suite using apatite and amphibole chemical analyses. The pre-mineralization gabbrodiorite complex hosts S-poor apatite, while the syn-and post-ore dacitic porphyries host S-rich apatite. Our apatite data on the Coroccohuayco magmatic suite are consistent with an increasing oxygen fugacity (from the gabbrodiorite complex to the porphyries) causing the dominant sulfur species to shift from S 2− to S 6+ at upper crustal pressure where the magmas were emplaced. We suggest that this change in sulfur speciation could have favored S degassing, rather than its sequestration in magmatic sulfides. Using available partitioning models for apatite from the porphyries, pre-degassing S melt concentration was 20-200 ppm. Estimates of absolute magmatic Cl concentrations using amphibole and apatite gave highly contrasting results. Cl melt concentrations obtained from apatite (0.60 wt% for the gabbrodiorite complex; 0.2-0.3 wt% for the porphyries) seems much more reasonable than those obtained from amphibole which are very low (0.37 wt% for the gabbrodiorite complex; 0.10 wt% for the porphyries). In turn, relative variations of the Cl melt concentrations obtained from amphibole during magma cooling are compatible with previous petrological constraints on the Coroccohuayco magmatic suite. This confirms that the gabbrodioritic magma was initially fluid undersaturated upon emplacement, and that magmatic fluid exsolution of the gabbrodiorite and the pluton rooting the porphyry stocks and dikes were emplaced and degassed at 100-200 MPa. Finally, mass balance constraints on S, Cu and Cl were used to estimate the minimum volume of magma required to form the Coroccohuayco deposit. These three estimates are remarkably consistent among each other (ca. 100 km 3 ) and suggest that the Cl melt concentration is at least as critical as that of Cu and S to form an economic mineralization.

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Section: Sulfur Concentration Of the Meltsupporting

confidence: 78%

Section: Sulfur Concentration Of the Meltmentioning

confidence: 99%

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

Chelle-Michou

Chiaradia

2017

Contrib Mineral Petrol

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“…In Fig. 3a, melt Au contents are plotted versus oxygen fugacity and compared with data from previous studies at 0.4 GPa (Jégo et al, 2010), 0.2 GPa (Botcharnikov et al 2011), and ambient pressure/high temperature (Borisov and Palme, 1996). All new data plot as a cluster at fO 2~N NO, showing no particular variation with fO 2 .…”

Section: Sulfur Concentrations In S-bearing Glassesmentioning

confidence: 97%

“…4b), and high melt S contents, emphasize the role of sulfur -not only fO 2 -and the importance of considering the interplay between different parameters in the mechanism of gold dissolution in silicate melts, as discussed in detail by Botcharnikov et al (2011).…”

Section: Gold Concentrations In S-bearing Glassesmentioning

confidence: 99%

“…However, Botcharnikov et al (2010) show significantly lower Au solubility values (from 300 to 2500 ppb) compared to Jégo and Pichavant (2012), and Zajacz et al (2012;2013) also report similarly lower Au concentrations (from 220-1550 ppb Au and from ~60-3200 ppb Au, respectively) in reducing to moderately oxidizing conditions at 1000°C and 2 kbar. Nevertheless, another study by Botcharnikov et al (2011) reports much higher gold concentrations (from 250 to 8000 ppb Au) in basaltic to andesitic melts over an fO 2 range going from NNO-1 to ~NNO+2, which emphasize the role of fO 2 in controlling the incorporation of gold in melt. Under more oxidizing conditions (fO 2 ≥ NNO+3), Jégo and Pichavant (2012) show that gold dissolution is much less effective -with solubility values around 600 ppb Au -and seems independent on the melt S content, yet very high.…”

Section: Introductionmentioning

confidence: 99%

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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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Planetary, Geological and Environmental Sciences

Farges

Wilke

2016

X‐Ray Absorption and X‐Ray Emission Spectroscopy

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High gold concentrations in sulphide-bearing magma under oxidizing conditions

Cited by 189 publications

References 21 publications

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

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

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

Planetary, Geological and Environmental Sciences

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