大分県野矢地熱地域における金鉱化作用

抄録

In this study we report the occurrence of gold mineralization in the Noya geothermal area, located in the ENE-WSW trending Beppu-Shimabara graben, central Kyushu, Japan. The Noya geothermal area is located on the marginal zone of the Shishimuta caldera. The area also falls on the shoulder of a positive Bouguer gravity anomaly which is centered approximately 3 km northeast of drill hole 51-WT-1. In the vicinity of the Noya area there are several lava flows of Pleistocene age which erupted during the mineralizing stage of the Noya geothermal system. Basement rocks in the study area are not exposed. However, they are thought to be composed of Cretaceous metamorphic rocks and granitic rocks, overlain by volcanic rocks and lacustrine deposits of Pliocene to Pleistocene age.
Many hot springs and fumaroles as well as zones of hydrothermal alteration are present in the graben. Gold bearing quartz-calcite-adularia veins were found in drill cores 51-WT-1 and 51-WT-2 from the Noya area. The host rock is mainly composed of andesitic lapilli tuff which has been altered to propylite. The main alteration minerals are kaolinite and smectite which occur from surface down to 125m below ground level in the 51-WT-1 core. These minerals are replaced by chlorite, a chlorite/smectite mixed-layer mineral, and an illite/smectite mixed-layer mineral below the 125m level. Silica minerals include cristobalite and tridymite from surface to the 100m level, however, a phase change to quartz occurs at deeper levels. A K-Ar age of 0.37±0.01 Ma was obtained from an early-stage adularia-rich vein sample with a gold content of 5.5ppm. This age is relatively young compared with the formation ages of most epithermal deposits in Japan.
In drill hole 51-WT-1, several quartz-calcite-adularia veins occur at a depth of 164 to 215m below ground surface. The veins are white in color, and are virtually sulfide-free. High calcite contents (70 wt.% on average) are characteristic of the gold-bearing veins (up to 20ppm Au) in drill hole 51-WT-1. The average Ag/Au ratio is 0.8 in the veins.
In contrast to drill hole 51-WT-1, drill hole 51-WT-2 contains adularia-bearing silicified rock with 0.1 to 0.4 ppm of gold. Many quartz-calcite-adularia veinlets, which contain a few ppm of gold, cut the silicified body at a depth of 25 to 61m. Veins in drill hole 51-WT-2 can be divided into two types based on their color; gray-colored veins and white-colored veins. The gray-colored veins consist of quartz, adularia, calcite and sulfides with associated high concentrations of As, Sb, Hg and S. The white-colored veins consist of quartz, adularia and calcite, and cut the gray-colored veins. Quartz is the dominant vein mineral followed by lesser amounts of adularia and calcite, respectively. The average Ag/Au ratio is 4.9 in the veins, which is significantly greater than that in the veins of 51-WT-1.
The present down hole temperature of the hydrothermal fluid in drill hole 51-WT-1 is -150°C at a depth of 200m. Homogenization temperatures of fluid inclusions in early-stage vein calcite average -190°C from drill core samples of the same depth. The fluid inclusion homogenization temperatures are considered to be related to early hydrothermal activity in the Noya geothermal system with fluid temperatures at least 40°C higher than today. This early-stage of hydrothermal activity is coincident with gold mineralization in the Noya geothermal system.