Application of Cathodoluminescence to Geothermal Exploration

Preliminary Case Studies of the Kakkonda and Mori Geothermal Fields

Abstract

The purpose of the paper is to clarify the application of cathodoluminescence (CL) to geothermal exploration such as detection of a productive fracture. The CL of anhydrites and calcites in the different origins or stages in the Kakkonda and Mori geothermal fields has been studied using the Luminoscope (ELM-3R) and the CL mode of a scanning electron microscope (CL-SEM). The CL images of natural hydrothermal anhydrite have brown, pale green colors and colorless. The CL images with brown and pale green colors result from both Sm³⁺ and Dy³⁺ emissions, and Mn²⁺ emission, respectively. Calcite in the three different origins or stages such as natural hydrothermal calcite, calcite scale and limestone shows CL images with red color, resulting from Mn²⁺ emission. Intensity of the CL spectrum of Mn²⁺ in the calcites suggests that Mn content in limestone is lower than that in other calcites. Based on these investigations, it is concluded that the CL of anhydrite and calcite is a useful method to discern the different origins or stages of these minerals, and to detect productive fractures in geothermal fields. Moreover, this method provides us important information for mixing degree of magmatic fluid with geothermal fluid in reservoir.