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Experimental determination of the solubility of gold (Au) in intermediate arc magmas.

Abstract : Cu-Au-Mo deposits represent major metal resources known to be spatially and temporally associated with intrusive arc magmatism, and in particular with adakites. Although the bulk of the ore metals seem to originate from the magmas, there is as yet no consensus on processes responsible for their concentration. The fact that primary ore minerals are predominantly sulfides has led to the suggestion that sulfur may play an important role in metal enrichment processes at the magmatic stage. Here, we conducted highpressure/ temperature experiments (0.4 GPa, 1000°C) on 3 natural dacitic magmas from the Philippines (two adakites and one typical calc-alkaline composition) over a wide range of fO2 (~NNO-1 to ~NNO+3), as measured by solid sensors. The experiments were carried out in pure Au capsules, the latter also serving as the source of gold, in the presence of variable amounts of H2O. Both S-free and S-bearing (~1 wt% S added) experiments were performed. Charges consist of dominant silicate melt quenched to glass plus minor silicate phases (mostly Cpx). All S-bearing charges are saturated with either sulfides or sulfates, depending on the fO2. Our data show that Au solubility in S-free charges is very low (30-240 ppb) but increases with fO2. In S-bearing charges Au contents reach much higher values, ranging from ~500 to 5200 ppb depending on the fO2, the most reduced charges being the gold-richest. Also, the behaviour of Au w.r.t. sulfur appears to be radically different according to the fO2. No significant difference of Au solubility is observed between adakitic and non-adakitic melts in both S-free and S-bearing charges. A major consequence of this study is that sulfide crystallization does not scavenge all Au present in the magma, but allows gold-enriched residual magmas to be emplaced in the upper crust. Additional experimental runs are currently being performed at higher pressures (> 1 GPa), that should lead to important supplementary data and results.
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Contributor : Nathalie Pothier <>
Submitted on : Monday, August 31, 2009 - 11:13:50 AM
Last modification on : Wednesday, September 25, 2019 - 2:56:01 PM

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  • HAL Id : insu-00411922, version 1

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Sébastien Jégo, Michel Pichavant, J.A. Mavrogenes. Experimental determination of the solubility of gold (Au) in intermediate arc magmas.. 19th Annual VM Goldschmidt Conference, Jun 2009, Davos, Switzerland. pp.A588. ⟨insu-00411922⟩

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