Tourmaline in Geothermal Systems: An Example From Darajat, Indonesia

摘要

The chemical compositions of two separate tourmaline populations (Stage 1 and Stage 3) from well DRJ-S1 at Darajat have been determined. This data, alongside petrologic observations, is used to improve our understanding of the evolution of the Darajat vapor-dominated geothermal system. Stage 1 tourmalines (replacing feldspar) have distinctly higher Fe/(Fe + Mg) and Na/(Na + Ca) ratios than Stage 3 tourmalines (formed in anhydrite veins). Mineral paragenesis and the high Fe content of the tourmalines suggest that Stage 1 formed in a higher temperature, fluid-dominated environment following the emplacement of subvolcanic intrusives. Ca-abundant Stage 3 tourmalines formed after descending steam condensates causing advanced argillic alteration began to neutralize. Continued anhydrite and calcite deposition (due to the heating of descending steam condensates) at shallow levels reduced porosity and permeability, impeding reservoir recharge, resulting in the current vapor-dominated system.