The role of mercury, arsenic and boron as pathfinder elements in geochemical exploration for geothermal energy

In Japan there are about one hundred geothermal areas which occur in the following geological units: green-tuff areas including sedimentary basins, Neogene plutonic rock areas, Pliocene—Quaternary sedimentary basins and Quaternary volcanic areas.Geochemical exploration for geothermal energy in the Onikobe basin in the vicinity of the Narugo geothermal area, Miyagi Prefecture, was carried out by the Geochemical Exploration Research Group of MMIJ.The Onikobe basin is regarded as a caldera and is covered with lake deposits composed of conglomerate, sandstone, mudstone, dacitic tuff and andesitic tuff breccia and lava flow.In the southern part of the basin a Quaternary dacitic volcano and two fumaroles occur. Hotsprings also issue from the lake deposits in the southwestern part of the basin.The tectonic line, including Narugo and Onikobe geothermal areas, extends into the geothermal areas in Ogachi region, Akita Prefecture. Geothermal manifestations in this area can be divided into three groups: acid type hot springs, named Kawarage, containing hydrochloric acid and neutral to alkaline type hot springs called Oyasu, Ohyu and Takanoyu. Their host rocks are green-tuff and lake deposits of the basin in the green-tuff areas. These geothermal areas were also surveyed by the research group of MMIJ.