RELICT AND MODERN EPIDOTE: LESSONS FROM RANTAU DEDAP, SUMATERA

Abstract

Epidote is an important hydrothermal alteration mineral; it is readily visible in drill cuttings, and generally indicates where temperatures are sufficient for setting production casing in geothermal wells. At the Rantau Dedap project in Sumatera, relict epidote was identified at the surface prior to drilling the first well. The relict epidote forms small, dull brownish crystals that persist in most cuttings samples, even where overprinting by the active geothermal system has produced low temperature minerals like smectite, halloysite, opal and chalcedony. At depth, coarser, yellow epidote crystals occur, some of which may be in equilibrium with the current geothermal system.
Determining which epidote is in equilibrium with current conditions is essential if epidote is to be used as a geothermometer for guiding drilling decisions, but is only possible with detailed petrographic studies. In systems where it is suspected there might be relict epidote, rapid and accurate petrology is strongly recommended.