EXPLORING CHEMICAL SIGNATURES OF RESERVOIR ROCKS USING TOURMALINES IN ACTIVE GEOTHERMAL SYSTEMS

Expansion of lower carbon energy sources requires increasing alternative energy supplies such as that provided by geothermal systems. Chemical characterization of these systems is necessary, in part, to establish temperatures attained, to track fluid flow paths and fluid composition, and to investigate mineral-fluid interactions that may inhibit or enhance fluid pathways for heat extraction. Many active and fossil geothermal systems contain the mineral tourmaline (tur), XY₃Z₆[T₆O₁₈](BO₃)₃V₃W, an excellent chemical monitor of the system environment. This study examines newly obtained chemical data from tur in active, vapor-dominated geothermal systems from the Darajat, Indonesia, and The Geysers, CA, geothermal fields, supplemented with previously obtained compositional data from these and other fields, to elucidate chemical signatures of tur in geothermal environments and their potential importance in geothermal system development.

Major and minor chemical data for tur, obtained via EMP, demonstrate the varying compositions amongst the samples. Fine-grained turs occur primarily as radial sprays in two Darajat samples. Tur compositions have intermediate X_Mg = 0.35 - 0.60 with X-site vacancies < 0.45, F <0.01 apfu, and are schorl, dravite, and oxy-schorl species. Geysers tur, restricted to veins within the felsite, is oscillatory zoned, more Fe-rich, X_Mg = 0.01- 0.45, with a wider compositional range, X-site vacancies = 0.1 – 0.7 and F < 0.2 apfu. Species are foitite, oxy-foitite, schorl and oxy-schorl. Compositional variations can be described primarily by the exchange vectors (Al)(NaMg)_-1 and (AlO)[Mg(OH)]_-1 combined with Fe²⁺Mg_-1. Darajat data suggests minor Fe³⁺Al_-1. Nearly all geothermal tur in active systems reported to date are oxy- or OH species, suggesting that F is not a major component in these geothermal fluids. Most tur are alkali or vacancy dominant species. Based on Na_tur-fluid partitioning data, Geysers fluids contain ~0.44 mol/l Na, K ~0.09 mol/l; Darajat fluids contain 0.47 Na mol/l, b.d. K. These chemical data suggest that tur in geothermal systems form through fluid interactions with the host rocks and that tur chemistry reflects the host rock in which it forms rather than having a unique geothermal signature. Turs in geothermal systems can reveal key chemical information about fluid-rich systems that can produce low carbon energy.