Mechanism of Opal-CT deposit from the Lam Narai Volcanics in Lop Buri, Thailand by using Petrography

Common opals were discovered in the rhyolite host rock, where various structural features such as cavities, fractures, and veins are observed. These voids served as pathways for the infiltration of silica-rich fluids originating from meteoric water, establishing a hydrothermal connection. The aqueous solution derived from meteoric water actively dissolves silica from the rhyolite host rock, thereby increasing the silica saturation within the fluid. Subsequently, the silica-saturated hydrothermal fluid fills the cavities and veins within the host rock. The margin of the host rock underwent alteration as it interacted with infiltrating hydrothermal fluid in a phenomenon known as hydrothermal fluid-rock interaction. At the periphery of the cavities, quartz crystals begin to form, effectively reducing the concentration of dissolved silica in the solution. As a result, the subsequent precipitation of amorphous opals occurs, predominantly within the inner part or central region of the vein or cavity. The formation of opals associated with Lam Narai volcanic activity occurred at a relatively high temperature of about 105-170 °C through a hydrothermal process.