| Title | Systematics of Rare Alkalis and Halogens in the High-Temperature Mahanagdong Geothermal Field, Leyte, Philippines |
|---|---|
| Authors | Erlindo ANGCOY, Stefan ARNORSSON |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | rare alkalis, halogens, Mahanagdong |
| Abstract | The analyses of rare alkalis and halogens in water samples obtained from production wellheads were studied to further understand the characteristics of the high-temperature geothermal field of Mahanagdong, Leyte, Philippines. The fluids are saline and mineralized that typically characterize fields associated with subduction zones and hosted by andesitic rocks. The rare alkali metals Li, Rb and Cs (which are reactive in the deep parts of the reservoir beyond drilling depths) have highly positive correlations with Cl, suggesting that they are considerably mobile and are least affected by relatively shallow processes. Ternary diagram following the methods of Giggenbach (1991) shows data points departing from the composition of the rocks in the crust but closely retain the rocks’ original Li/Cs ratio. The fluids discharged from wells drilled in the northeastern part of the field show upflow characteristics (i.e., most depleted Rb and least loss of Cs). Cross plots of Na/Li and Rb/Cs were also done taking advantage that the levels of very mobile alkalis Li and Rb are elevated in fluids from higher temperatures but decrease at lower temperatures as they tend to be fixed by secondary minerals. Consistently, fluids from two of the northeastern wells are inclined towards the “upflow corner”. The updated data of northern well postulated to be nearest the upflow area prior to the drilling of the northeastern wells have shifted towards higher Na/Li and lower Rb/Cs indicating probable loss of Li and Rb in the fluid of the producing area. Fluoride levels in Mahanagdong range 1-5 mg/kg and are generally undersaturated with fluorite. A significant positive correlation between the activities of F- and OH- in the modeled aquifer fluids is observed. Br concentrations are on the average one order of magnitude higher than those of F at ~18 mg/kg. Br has strong positive correlation with the mobile Cl. The Cl/Br ratios (383-557) plot within the seawater and evaporite region but are lower than seawater value (650). The Cl/Br ratios suggest that the magmatic-related components in Mahanagdong geothermal fluids are likely derived from seawater or crustal fluids recycled in subduction zones. |