Note on the Occurence and Compositions of Calcite from the Te Mihi Sector of the Wairakei Geothermal System, New Zealand

Abstract

Four types of calcite occur in cores and cuttings recovered from 14 wells drilled into the Te Mihi section of the Wairakei geothermal field, (a) vein and open calcite which contains up to 3 wt (b) bladed calcite of stoichiometric composition, (c) calcite, with about 0.5 wt pseudomorphing primary plagioclase, and (d) calcite present in Calcite is much more abundant - 15%)in samples wells drilled between 1985and 1987 than it is in cores wells drilled before (0 - 3%calcite). This shows that calcite has formed in the shallow parts of the reservoir as a result of boiling from the pressure drop accompanying water drawdown. Drawdown has also allowed shallow ground waters, into which derived the alkali chloride water has condensed, to infiltrate the reservoir. Calcite deposited in veins and vugs as thiswater was heated by its contactwith hotter rocks. Calcitethat plagioclase deeper in the reservoir formed by slow reaction between primary feldspar and the alkali chloride reservoir fluid of the Wairakei system.