Anomalous Fluoride and Warm Water Wells May Indicate Blind Geothermal Systems in Cenozoic Basins of Northern Thailand

Abstract

Anomalous temperatures (37°C) and high fluoride in water wells occur in the middle of some Cenozoic basins in Northern Thailand. An arcuate belt of high fluoride (up to 8 mg/l) occurs in an arcuate belt 20-km long and 6-km wide in the middle of Chiang Mai basin, a structurally complex rift basin, with sediments up to 2-km thick. Local geothermal gradients from shallow wells near Lamphun along this belt are 30 to 48°C/km (Thienprasert and Rakaskulwong, 1984). Hot Springs associated with arcuate faults that parallel this belt occur in crystalline rocks 20 km to the east (i.e. Mae Tha fault), and fluoride contents are up to 17 mg/l, associated with hot springs water ~100°C, with geochemically estimated reservoir temperatures of 133°C (CM_01). Along the active strike-slip Mae Chan fault in northernmost Chiang Rai Province is a prospective hot springs system (CR_04) with 99.5°C surface flow and 127°C geochemically estimated reservoir within granitic rocks. Along this fault 20-km to the east 41°C water flows from a swampy basin, with springs aligned along the fault. We suggest that these emanations of warm or high fluoride waters may be indicators of prospective blind geothermal systems associated with faults in the basin. The faults may capture deeply-circulating waters – driven by the hydraulic head of bounding mountains with 1000 m of relief. The faults or deep basin sands may be significant geothermal reservoirs at depth of 1 to 2 km