Time Scales of a Geothermal System from Actinolite Fe-Mg Zoning

Abstract

Volume diffusion provides a mechanism to assess the duration, and hence the timescales of thermal events. In this study, interdiffusion of Fe and Mg across a sharply zoned interface in actinolite was studied to determine timescales of hydrothermal activity associated with the Santos deposit in Punta del Cobre, Chile. Petrographic analysis to identify textures, mineral relations, and mineral orientation was conducted on several thin sections of drill core from the Santos deposit. Mineral phase relations, fluid inclusion results from the nearby Candelaria deposit, and comparison with the modern Salton Sea Geothermal System allow estimation of the maximum temperatures during ore formation. SEM analyses were performed on the samples to identify strongly zoned actinolite crystals. Microprobe stage scan analyses were conducted to create a diffusion profile and determine the diffusion coefficient and time. Although there is little known about diffusion in actinolite, orthopyroxene (OPX) was chosen as a proxy because it is a chain silicate and has Fe-Mg diffusion in both of the M sites and thus broadly analogous to actinolite. Using OPX diffusion coefficients maximum timescales from 10⁴ to 10⁶ years for maximum temperatures of 350 to 450˚C. These times match data from some modern geothermal systems.