Hydrothermal Feldspars in the Broadlands-Ohaaki Geothermal System, New Zealand

Abstract

Petrographic, microprobe and XRD studies were made to characterise hydrothermal feldspars in twenty-two cores of Ohaaki rhyolite and Rautiwiri breccia from the Broadlands-Ohaaki geothermal system. Albite and adularia coexist in the deep central upflow zone (>600 m depth). Albite occurs alone in the deep and peripheral parts of the system in areas undergoing sodic metasomatism, whereas adularia occurs alone in the shallow upflow zone in areas undergoing potassic metasomatism. Their compositions are nearly pure and unaffected by temperature. Hydrothermal feldspars appear to replace precursor andesine crystals through a coupled dissolution-precipitation process at microscopic scale. The slight differences in structural ordering reflected in adularia XRD data and its absence from well scales suggests this feldspar precipitates at different rates under different conditions.