Arsenic is ubiquitous in all kinds of gold deposits and an indicator for Au, Ag, Cu and Zn deposits in the crust. Understanding the enrichment of As and its association with critical metals requires better constraints of its transport mechanism in the crust, as well as on the dissolution-precipitation of As-bearing minerals at relevant conditions.

Arsenic speciation data is available at shallow crustal conditions, and the solubility of common arsenic minerals is known for sulphur-rich fluids. However, the transport mechanism of arsenic in sulphur-depleted systems as well as at greater depth in magmatic-hydrothermal settings require further investigation.

We apply in situ XAS and XRF techniques coupled with the FAME hydrothermal autoclave at BM16 (FAME-UHD) beamline of the ESRF to determine arsenic speciation and the solubility of realgar (As4S4) in saline hydrothermal fluids (up to 400 °C, 1 kbar). We complement the data by in situ Raman spectroscopy in a hydrothermal diamond anvil cell up to 9 kbar and 750 °C to study the effect of pressure on the solubility of realgar and track changes in As speciation from deep to shallower crustal conditions.

A comparison of our results with previous ex situ studies of orpiment [1] and arsenopyrite [2] solubilities provides new insights into the role of sulphur and chlorine in the transportation of arsenic in hydrothermal fluids.

1. Pokrovski, G. et al. Geochim. Cosmochim. Acta: 60, pp. 737–749. 1996.

2. Pokrovski, G. S. et al. Geochim. Cosmochim. Acta: 66, pp. 2361–2378. 2002.