Controls on gold solubility in arc magmas: An experimental study at 1000 °C and 4 kbar |
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Authors: | Sé bastien Jé go,Michel Pichavant |
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Affiliation: | a Institute of Earth Sciences, Academia Sinica, 128 Academia Road, Sec. 2, Nankang, Taipei 11529, Taiwan, Republic of China b Institut des Sciences de la Terre d’Orléans (ISTO), CNRS: UMR6113, INSU, Université d’Orléans, France c Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia |
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Abstract: | In order to (1) explain the worldwide association between epithermal gold-copper-molybdenum deposits and arc magmas and (2) test the hypothesis that adakitic magmas would be Au-specialized, we have determined the solubility of Au at 4 kbar and 1000 °C for three intermediate magmas (two adakites and one calc-alkaline composition) from the Philippines. The experiments were performed over a fO2 range corresponding to reducing (∼NNO−1), moderately oxidizing (∼NNO+1.5) and strongly oxidizing (∼NNO+3) conditions as measured by solid Ni-Pd-O sensors. They were carried out in gold containers, the latter serving also as the source of gold, in presence of variable amounts of H2O and, in a few additional experiments, of S. Concentrations of Au in glasses were determined by LA-ICPMS. Gold solubility in melt is very low (30-240 ppb) but increases with fO2 in a way consistent with the dissolution of gold as both Au1+ and Au3+ species. In the S-bearing experiments performed at ∼NNO−1, gold solubility reaches much higher values, from ∼1200 to 4300 ppb, and seems to correlate with melt S content. No systematic difference in gold solubility is observed between the adakitic and the non-adakitic compositions investigated. Oxygen fugacity and the sulfur concentration in melt are the main parameters controlling the incorporation and concentration of gold in magmas. Certain adakitic and non-adakitic magmas have high fO2 and magmatic S concentrations favorable to the incorporation and transport of gold. Therefore, the cause of a particular association between some arc magmas and Au-Cu-Mo deposits needs to be searched in the origin of those specialized magmas by involvement of Au- and S-rich protoliths. The subducted slab, which contains metal-rich massive sulfides, may constitute a potentially favorable protolith for the genesis of magmas specialized with respect to gold. |
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