Mineralogy and fluid composition at Dieng geothermal field, Indonesia

Abstract

The Dieng Geothermal Field, Central Java, Indonesia is located in an area composed of andesite lavas, tuffs and volcanogenic sediments. Sixty five specimens of core samples from three wells (Nos.1, 3, and 5) were examined by petrographic microscope, and supplemented by X-Ray Diffractometer (XRD), X-Ray Fluorescene (XRF), Electron Probe MicroAnalyser (EPMA), and Scanning Electron Microscope (SEM) techniques. The important hydrothermal minerals present in the cores are quartz, anhydrite, calcite, kaolinite, smectite, illite, mixed-layer clays, epidote, adularia, albite, actinolite, biotite, chlorite, scolecite, pyrite, pyrrhotite, andradite, and sphene. Kaolinite, smectite, mixed-layer clays, anhydrite, and scolecite are commonly present at shallow levels and low temperatures. Epidote, albite, adularia, actinolite, biotite, and andradite were formed at higher temperatures and deeper levels in the field. The other minerals calcite, illite, quartz, chlorite, and pyrite occur over a wide range of temperature and depth within the boreholes. Mineral distribution indicates changes in hydrothermal activity in terms of fluid composition, carbon dioxide concentration, and patterns of fluid flow within the rocks. Deep alkaline-chloride waters have flowed upwards and sidewards through fissures or connected pore spaces prior to boiling. Minor boiling has taken place immediately above this alkalinechloride water level, and is followed by major boiling at around 1200 m depth. This boiling has produced the acid - sulphate steam condensate within the rocks above 1200 m depth in the are a peripheral to borehole No.1. This acid - sulphate water has leached out most of the constituents (e.g. Mg, Fe, Na, Ca, and K) from rocks above this levels. It suggests a shift of activity vertically from acid - sulphate at shallow levels to alkaline - chloride type waters at deeper depth. Infiltration of cold groundwater is thought to have prevented further boiling and caused slight cooling of the system in the areas of boreholes No.3 and 5. This cooling process has been assessed using chlorite geothermometry based on the Fe/Fe+Mg ratios of chlorite in borehole No.3. This suggests that there is a shifting of activity laterally from higher temperatures in the area of borehole No.1 to lower in the areas of boreholes No.3 and 5. Surface fluid chemistry also indicates the pattern of fluid flow in the system, which is flowing laterally and appear in the south (Group 4 hot springs) and western part (Group 5 hot springs) of the Dieng area. The discharge history of well No.4, wide ranges of estimated subsurface temperatures,and measured temperatures in the boreholes indicate that the Dieng system (at deeper than 1200 m depth) consists of more than one aquifer. Mn and Zn distributions in the three boreholes suggests the presence of self-sealed zones at 600 - 650 m, 1050 - 1150 m, and 1400 - 1600 m depths in borehole No.3; immediately above 1700 m depth in borehole No.5; and possibly above 1200 m up to 1050 m depth in borehole No.1. Based on Rb distribution, a regime of higher permeability may exist at around 1250 m depth in borehole No.1; at 950 and 1150 - 1350 m in borehole No.3; and at around 1200 - 1350 m depth in borehole No.5. Such permeable zones have been cased in boreholes 3 and 5. Fluid - mineral equilibria and the presence of groundmass and vein epidote coexisting with calcite reveals that in the early stage of activity the fluid may have been poor in CO2, and was later enriched due to diorite intrusion. Subsequently, this intrusive body underwent contraction due to cooling and create the fluid path along the contact zones. Mineral stability diagrams indicate that the Dieng fluid is in equilibrium or nearly so with chlorite, epidote, K-mica, K-feldspar, and albite.