Gold Mineralization and Occurrence of Sinter in the Hoshino Area, Fukuoka Prefecture, Japan

摘要

Several epithermal gold deposits occur in the Hoshino area, which is located in the western end of the late Cenozoic Hohi volcanic zone, north-central Kyushu, Japan. The area is characterized by intermediate to felsic extrusive rocks of Pliocene age. In the Hoshino area, the shallow manifestation of the hydrothermal activity is exposed on the surface. Several outcrops of sinter are still preserved on the top of hydrothermally altered volcanic rocks, and high-grade gold-bearing quartz veins occur on the surface at lower levels. The hydrothermal alteration resulted into well-developed alteration zones. The zonal alteration pattern, primarily of near-neutral pH type, is characterized by the outer smectite zone of a lower temperature, and the inner mixed layer minerals zone of a higher temperature. Quartz vein-related or fracture-controlled alteration, is represented by the occurrence of interstratified illite/smectite and K-feldspar, suggesting a potassium-enriched alteration. The mineralization in the Hoshino area is recognized on the surface by the occurrence of gold-bearing quartz veins distributed mainly in the mixed layer minerals zone. The fracture system related to the gold mineralization is mainly characterized by NW-SE trend. The alteration pattern and the mineralogical composition of the veins suggest that the mineralizing fluids had near-neutral pH and the mineralization is of low-sulfidation-type. Fluid inclusion data and textures observed in quartz veins indicate that gold precipitated during boiling. The chemical composition of quartz veins shows that high-grade gold-bearing quartz veins are characterized by higher content of Al_2O_3, K_2O and Rb. Several outcrops of silica-sinters are distributed on the top of the mixed layer minerals zone. Although their structures are not very well preserved, because of later silicification, the Hoshino sinters still show characteristic textures identical to those observed in modern sinters, such as the presence of plant fossil incorporated into the sinters, the strongly developed depositional laminations and the columnar structures perpendicular to the depositional surfaces. Quartz is the only silica mineral constituting the Hoshino sinters presently. The conversion of amorphous silica into quartz was probably governed by higher temperatures resulting from later hydrothermal activity. This later hydrothermal activity is reflected by the intense silicification affecting mainly the lower parts of the sinters and also by the presence of quartz veins cutting the sinters. The distribution of sinters in the Hoshino area is very significant. The presence beneath the sinter of concealed high-grade gold-bearing quartz veins should be highly considered and exploration work is strongly suggested.