Geochemical studies of thermal waters from Kanlaon Volcano, Negros Island, Philippines

Abstract

Abstract Kanlaon is a stratovolcano situated in central Negros Island, Philippines. It has a record of 30 eruptions since 1866 ranging from phreatic to phreatomagmatic (VEI\u202f=\u202f1–2) events. Geothermal manifestations in Kanlaon Volcano are evidenced by the presence of hot springs, boiling mud pools, and warm rivers with temperatures up to 95\u202f°C. It has developed a significant hydrothermal system within the volcanic edifice. Through time, it evolved into two distinct hydrothermal systems independent of each other. A mature hydrothermal system, represented by Pataan thermal area is characterized by neutral Na\u202f+\u202fK chloride (bicarbonate) fluids and an immature system, represented by Hagdan spring is characterized by the presence of acid-sulfate waters. Representative water samples were collected between 2014 and 2017 to further characterize the two systems. Chemical and sulfur isotopic analyses were performed on thermal waters to classify the samples that are linked to the co-existence of these two hydrothermal systems. Results show that thermal spring s pH ranges between 2.5 and 8.1 and conductivity ranges between 88\u202fμS/cm to 3930\u202fμS/cm. Based on their Cl-SO4-HCO3 relative abundances, the thermal waters are classified as acid sulfate, acid-sulfate chloride, neutral chloride and neutral bicarbonate waters. Temperatures of deep-water hydrothermal reservoirs were calculated using fluid-mineral geothermometers and correlated with available borehole temperatures. The Na-K-Mg geothermometer was used to evaluate equilibrium between water and reservoir rocks. Results show that the samples are not in equilibrium with the minerals in the reservoir. Temperature of equilibrium for the mature hydrothermal system was extrapolated to 270\u202f°C based on available borehole temperatures. The linear trend towards the equilibrium temperature in the Na-K-Mg geothermometer can be explained by either groundwater/meteoric dilution. This is consistent with the light sulfur isotopic signatures between δ34S\u202f=\u202f−3.4‰ and\u202f+\u202f1.2‰ of the mature hydrothermal system (Mambucal), which imply that the origin of sulfates is linked to surficial oxidation of H2S or dilution and mixing of meteoric water. On the other hand, immature hydrothermal system (Hagdan) shows a significantly higher sulfur isotopic composition (δ34S\u202f=\u202f+8.2‰), which could either result from the disproportionation of magmatic SO2 or from the hydrolysis of elemental sulfur at high temperature between 100 and 350\u202f°C.