| Abstract |
Rocks altered by acid CI-SO, fluid, exhibit similar silicate alteration as those affected by neutral chloride waters. Silicates, therefore, are not suitable fluid pH indicators in geothermal areas like Mt. Apo and Mahanagdong where such acid fluids exist with neutral brine. In lieu of silicates, sulfide mineralogy is evaluated as a potential fluid pH index in these areas. In Mt. Apo, the sulfide assemblage of acid wells within the Sandawa collapse resembles that of adjacent neutral producers. The suite consists mainly of pyrite (FeS2) and rare chalcopyrite (CuFeSJ with traces of bornite (Cu$eSd, covellite (CdJ khalcocite (Cu-S) and/or sphalerite (Znq. The abundance and occurrence of these sulfides are comparable in acid and neutral wells. Like silicates, therefore, sulfide minerals cannot be employed as index of discharge fluid pH among Sandawa wells. The sulfide phases in Mahanagdong are the same as those in Mt. Apo. Acidic wells in Mahanagdong, however, contain relatively higher amounts of pyrite and chalcopyrite. Veins of pyrite are common, while chalcopyrite veins are present in these acid producers. In contrast, neutral wells in Mahanagdong encountered lesser quantities of pyrite and chalcopyrite. Veins of pyrite are also fewer, while veins of chalcopyrite are absent in these neutral wells. Thus, unlike in Mt. Apo, relative sulfide abundance and presence of sulfide veins can be used to direct the existence of acid CI-SO, fluids in Mahanagdong. |