Genesis of a very young gold mineralization in the Noya geothermal area, Kyushu, Japan. Y. Morishita1 andN. Takeno1, 1Geological Survey of Japan (No. 7 Higashi 1-1-1, Tsukuba 305-8567 Japan. yuichi@gsj.go.jp).

Introduction: We have recognized gold minerali-zation (gold bearing calcite-adularia-quartz veins) fromdrill cores in the Noya geothermal area, located in theENE-WSW trending Beppu-Shimabara graben, centralKyushu, Japan. Many hot springs and fumaroles aswell as zones of hydrothermal alteration are present inthe graben. The Noya area is located on the marginalzone of a cauldron. The area also falls on the shoulderof a positive Bouguer gravity anomaly which is cen-tered approximately 3 km northeast of the deposit. Gold mineralization and geochemistry: Manycalcite-adularia-quartz veins were found at a depth of160 to 220 m below the surface in drill hole 51-WT-1.The veins are white in color, and are virtually sulfide-free (Fig. 1). High calcite contents (70 wt.% on aver-age) are characteristic of the gold-bearing veins. Thehost rock is mainly composed of andesitic lapilli tuffwhich has been altered to propylite. Additional 24drills have revealed that the gold concentration in theveins ranges from 0.1 to 400 ppm and the spatial sizeof the gold mineralization zone expands 1500m by600m at 400 to 550m above sea level. A K-Ar age of0.37±0.01 Ma was obtained from an adularia-rich veinsample with a gold content of 6 ppm. The vein-typedeposit is blind because of its young formation age,which is younger than those of most epithermal depos-its. In the vicinity of the Noya area there are andesiticvolcanic rocks of Pleistocene age (0.5~0.4 Ma), whichis supposed to be an effusive phase of the sourcemagma for the ore-forming hydrothermal system. The down hole temperature of the hydrothermal fluidin drill hole 51-WT-1 is 150°C at a depth of 200 m.Homogenization temperatures of fluid inclusions invein calcite average ~190°C from drill core samplesfrom the same depth. The fluid inclusion homogeniza-tion temperatures are higher than fluid temperaturestoday. This early-stage hydrothermal activity of theNoya geothermal system might responsible for the goldmineralization. The δ18O values of vein calcite andquartz decrease with increasing depth, suggesting thatthe formation temperatures of calcite and quartz in-crease with increasing depth. The relationship betweenthe δ13C and the δ18O values of vein calcite and thehomogenization temperatures of fluid inclusions gavethe calculated δ13C value of the ore-forming fluid. Thecalculated δ13C value of -6‰ for the Noya ore-formingfluid (Basement rocks are granitic rocks) is higher thanthat of -11‰ from the Kushikino mining area, southernKyushu, where the basement is consisted of organicmatter containing sedimentary rocks.

We have estimated the physicochemical condition ofthe ore-forming fluid using the geochemical data of thepresent hydrothermal fluid. Mineral stability in thecalculated fluid is shown by a plot of saturation indexvs. temperature. Calcite, quartz and muscovite are inequilibrium with the fluid, however, K-feldspar is un-dersaturated. The calculated fluid is in the muscovitestability field on the activity diagram for sodium andpotassium minerals at an ore-forming temperature.This fluid moves to the K-feldspar stability field withincreasing pH when a few percent of water is vaporizedby boiling. Gold mineralization in the Noya area was found byapplying a hypothetical ore-forming model [1] to thecentral Kyushu region. Volcanic activity in the Noyaarea might be related to localized sub-surface domingof magma, which is thought to account for the positiveBouguer gravity anomaly over the area, resulting in theformation of vein fractures. Although there is no directisotopic evidence for the magmatic contribution to theore formation, it is assumed that an intrusive magmahas uplifted the basement rocks, which most likely pro-vided with the heat source needed for driving the Noyaore-forming hydrothermal system. Reference: [1] Morishita Y. and Kodama K. (1986)Mining Geology, 36, 475~485.

Fig. 1 Gold-bearing calcite-adularia-quartz vein withthe host andesitic lapilli tuff (Width: 47mm).

Eleventh Annual V. M. Goldschmidt Conference (2001) 3331.pdf