Mass Transfer During Hydrothermal Alteration at the Tauhara Geothermal Field

Abstract

Cores recovered from drillhole TH3 (1094m) and THMl (430m deep) in the Tauhara Geothermal Field, near Wairakei, show extensive hydrothermal alteration. The observed mineral assemblages reflect the prevailing temperatures, fluid compositions and hydrology. Minerals present include epidote, prehnite, laumontite, wairakite, heulandite, calcite, barite, sphene, leucoxene, adularia, albite, pyrite, pyrrhotite-, and rare metal-bearing phases. Clay minerals are common, comprising chlorite, smectites, interlayered illite-smectite and discrete illite; where acidic waters occur, kaolinite (and alunite) are present. Chemical changes in the host rocks have accompanied the observed alteration: silica shows pronounced enrichment relative to the unaltered rocks but alumina has redistributed variably. Nagnesium redistribution matches the presence and abundance of chlorite and smectite, and to some extent corrrlates with iron concentrattons in the cores. Na20 and CaO are depleted to differing degrees whereas K20 is usually enriched in the altered rocks.