The Occurrence of Acid-Leached Graywacke at the Geysers, Sonoma County, California

Abstract

Ac id-I eached graywacke has been Identlfled In 15 of 19 GEO wells In the Northwest Geysers. The altered gray w a c k e I s characterized by a bleached appearance and a hlgh degree of secondary dlssolutlon porosity / permeablllty. Mlnerals deposlted In vugs and velns Include large (up to 30mm) doubly termlnated quartz crystals, bladed calclte, pyrite, pseudohexagonal pyrrhotlte, alblte, potasslum feldspar and kaollnlte. A l u n l t e. trace amounts of Iron oxldes, opallne slllca and posslbly sulfur are also observed. The dlssolutlon features and the morphologles of the replacement minerals suggest that these rocks were subject to a complex history of bolllng and low pH d l ssolutlon. The d l S 8 O l U t l O n features Include the l each l n g and replacement of detrltatalblte and selected matrlx phylloslllcates. The acld - leached zones are generally encountered above the steam-bearlng reservolr rocks and may represent the shallowest levels of the geothermal system.