The Sulfide Minerals Deposit in the Geothermal Pipes of Dieng Geothermal Field, Indonesia

Abstract

Dieng geothermal field is a high temperature liquid dominated geothermal field situated in a volcanic complex. The present production is 60 MWe supplied from the production zone of Sileri area. The geothermal fluid from Sileri is quite friendly having moderate salinity, neutral pH and sodium-chloride water. However, sulfide minerals deposit in the production well of the Dieng geothermal field. This paper reports the paragenesis of sulfide minerals deposited in the geothermal well in order to understand the sulfide mineral deposition in respond to boiling of hydrothermal fluid. Microscope observation of the polished section of the sulfides deposit revealed that the deposition occurred in four stages with a distinct mineral assemblage. In the first stage the mineral assemblage consists of dominantly chalcopyrite, while the second stage consists of chalcopyrite, galena and sphalerite. The next stage consists of a bit galena, sphalerite, chalcopyrite and pyrrhotite. During the third stage the pyrrhotite well developed with the grain size much bigger than other minerals. Finally in the last stage the assemblage consists of pyrrhotite with the smaller size. This implies that the chemical condition of the geothermal fluid in term of sulfidation state is getting lower that is possibly caused by further boiling enable more H2S gas escape.