Micro-PIXE studies of low-level gold solubility in natural magmas: Experiments on two felsic magmas†

The limits of gold solubility in granitic magmas are poorly constrained, yet may be important for understanding the formation of several gold deposit types. This is largely due to the fact that this parameter cannot be readily measured in natural rocks. Experimental approaches are also difficult, as an analytical technique is required that combines a high level of spatial resolution (typically <5 µm) with the capability to measure gold in low concentrations. In an attempt to address this problem, experiments were conducted to investigate gold solubility in sulphur- and chlorine-bearing granitic and syenitic magmas, and the products were analysed by micro-PIXE (particle-induced x-ray emission). Starting glasses were synthesized from aluminosilicate gels fired at 1000°C, and were sealed within gold capsules, with 4.4 wt% H2O and variable proportions of either chlorine or sulphur as potential gold complexing agents. The initially gold free glasses were equilibrated with the pure gold capsules at conditions of 1.5 MPa, 750°C and at fO2≈NNO buffer, for a period of at least 150 h. All experiments produced quenched granitic and syenitic glasses containing a disseminated, high-temperature silicate mineral assemblage, in addition to oxide and/or sulphide and small gold crystals. The gold, sulphur and chlorine concentrations of the glasses were analysed, and under the specific analytical conditions used in this study micro-PIXE was able to detect gold in the glasses at levels as low as 2 ppm. The PIXE data demonstrate that, under the conditions used in this investigation, sulphur is a more effective complexing ligand than chlorine for incorporating gold into the melt structures, and that surprisingly high gold concentrations of up to 4400 ppm were attained in the syenitic magma with 1 wt% sulphur added. Thus, micro-PIXE analysis has been shown to be capable of constraining the concentration of gold at the generally low values encountered in solubility studies on felsic natural magma compositions, thereby laying the groundwork for future research in this field using the PIXE technique. Copyright © 2004 John Wiley & Sons, Ltd.