Chemical Reaction Path Modelling of Vein Mineralization in the Tongonan Geothermal Field, Leyte

Abstract

Using chemical speciation and reaction path calculations, boiling and mixing of the recalculated ìprimaryî geothermal water of the Tongonan geothermal system (Leyte, Philippines) were simulated. The main purpose of the computer simulation was to ascertain which of these two processes would be the better precipitating mechanism for ore minerals. Primary water was assumed to be represented by the least diluted well discharge (Well 410) as suggested by chloride-enthalpy diagram. Adiabatic boiling done from 300 to 100?C resulted in the precipitation of a sequence containing paragenetically early traces of gold with quartz; followed by quartz, acanthite, chalcocite and late quartz, sphalerite, galena, acanthite, bornite. Interestingly, the pH of the residual boiled solution decreased instead of increased contrary to the commonly accepted increase with the release of volatiles in the steam phase. Steam-heated end-member used for mixing was generated by titrating the vapor phase at 100?C which resulted from boiling of primary water and mixing with a sample of the groundwater. Mixing of this steam-heated water with the boiled primary water at 300?C resulted to the formation of the ore minerals: acanthite, bornite, chalcocite, covellite, galena, pyrite, and sphalerite. Gangue minerals include anhydrite, Mg-chlorite, alunite, kaolinite, muscovite, and quartz. Observed vein minerals in cores and well cuttings include base metals (sphalerite, galena, chalcopyrite), pyrite, anhydrite, quartz, calcite, epidote-clinozoisite, chlorite with minute specks of Au-Ag electrum, The results of the chemical modelling suggest that both boiling and mixing processes or a combination of these processes could have precipitated the ore minerals in the Tongonan geothermal system. However, the absence of some minerals predicted by the simulation (e.g. acanthite), likewise, the presence of other minerals in the veins (e.g., chalcopyrite) suggest that physico-chemical conditions other than those assumed in the simulations are prevailing in the system.