A Petrologic Evaluation of the Laguna Sector of the Southern Negros Geothermal Project

Abstract

Petrologic studies of Lagunao wells show that hydrothermal alteration in the sector is mainly produced b,v recent neutral-pH fluids. The characteristic alteration assemblage consists of sniectite, illitic cla-vs, epidotes associated with chlorite, calcite and quartz. The sulfide mineral phases, on the other hand* are mainly pyrite and chalcopyrite with minor covellite/chalcocite, bornite, mackinawite (?), sphalerite and galena.

Based on the existence of stable h-vdrofhermal alteration, the upflow of hot, neritral-chloride brine in Lagunao is postulated to occur beneath the area bounded by Ticala Fault in the northwest and Lagunao Fault in the northeast. The ourfow regions, on the other.hand are the areas northeast of Lagunao Fault and northwest of Ticala Fault where,f%cificlw is mainly hwizontal and deep such that relict alteration at shallow levels remains extant.. The southwestern bounk;, of the tipjiÆow is infen?:! Lo be the Kojpohm Zanq Splay C based on the dmeasing trend of imthernts towards LG-2D. Towar& the southeast, fne Odlirmon Fault likely acts as th; margin of the neutral upflow since all the aquifers of wells drilled southeast of this structure prowd to 5c acidic.

Acid fluids in Lagunao tapped b.v wells BL-1D and LG-ID are believed to have deep-seated origins based on a comparison with the results of the petrologic evaluation of Cmayan sector of the Bacon-Manito Geothermal Project, and on the results of suvur isotope analysis in Palinpinon. ?.!-e acidity of the fluids can be attributed to the presence of acidic gases such as SO2 and H S exsolved from thc ,-qoling magmatic source of the Lagunao dome.