The Origin of Chlorine in the Fluids of the Tongonan Geothermal System, Philippines

Abstract

Two extreme arguments have been used in the past to explain the origin of chlorine in high temperature geothermal fluids, namely, chlorine is continuously leached from the surrounding country rock, or it is of magmatic origin. Two further variations are that chlorine can originatefrom both sources with one or the other predominating. Assuming a constant concentration of 10,000 and minimum natural upflow rate of 1.6 x for the Tongonan system fluids (the present system is at least 5 x and the rocks still contain up to chlorine could have been totally leached from 10 km3 volume of reservoir rocks in the centre of the field during the initial years of geothermal activity at Tongonan. The most likely option for the origin ofchlorine in the present-day Tongonan fluids is that HCI has reacted at magmatic temperatures with the granite heatsource rocks to form a water phase and a saline vapour. The vapour condenses (NaCl dissociates) in the overlyingmeteoric water system at temperaturesand sets up convective circulation typical of water-dominated geothermal systems. A minor contribution could still come from the reservoir rock.