Day 4 Route Maps
Published:January 01, 2001
The Nansatsu district of southern Kyushu has been the site of calc-alkaline volcanism for the last 10 m.y., shifting eastward with time. Associated hydrothermal activity followed deposition of the volcanic host rocks by about 0.5 m.y. and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions, resulting in the formation of high sulfidation Cu-Au deposits at Kasuga, Iwato, and Akeshi. The orebodies consist of >95 wt percent SiO2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride-sulfate waters. These are inferred to have formed when magmatic vapors containing HCl and SO2 condensed into meteoric water. The residual silica (now quartz) orebodies are best developed where the host was initially permeable. The margins of the quartz bodies are abrupt, with narrow (1-2 m) halos representing the reaction front of acid waters isochemically dissolving the host rock. The halo comprises alunite (strongly zoned in Na and K, with P-rich cores), dickite, and/or kaolinite ± pyrophyllite, grading out into illite and interlayered illite-smectite clays, and finally, propylitic alteration. This pattern is characteristic of deposits of this type throughout the world, for example, at Summitville, Colorado, and Lepanto, Philippines.
Mineralization occurred after initial leaching by the vapor condensates, with metals transported by a dense magmatic fluid. Mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies; ore grades are restricted to the quartz bodies. Gold is most closely associated with enargite and pyrite; later minerals include covellite and then sulfur. The last stage of activity was steam-heated, with descending waters oxidizing sulfides to goethite and locally remobilizing Au into fractures (this varies in degree between deposits). Erosion exposed the orebodies to supergene weathering, continuing the sulfide oxidation and Au remobilization.
Stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid (δ18O = 7 ± 2‰, δD = −25 ± 5‰, similar to nearby active volcanic discharges) with local meteoric water. In contrast, the clays in the marginal halo have isotopic compositions indicating a δ18O shift of 6 to 8 per mil from local meteoric water values, probably due to water-rock interaction, and the δ18O values of residual silica quartz may also be due to meteoric water domination. Fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent NaCl equiv during late quartz growth, though there is evidence in one sample for higher salinity fluid having been present, up to 30 wt percent NaCI equiv (some inclusions contain daughter minerals of halite and sulfur). The Th values of over 1,000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from <200°C at Akeshi to about 200°C at Iwato and 230°C at Kasuga. The presence of vapor-rich inclusions, some with Thv similar to Thl, indicate the presence at times of a two-phase fluid in the center of the ore zones, with depths of about 150 to 300 m below the paleowater table. The mineralizing fluid was relatively oxidized (sulfide/ sulfate ratio about 3:1), close to pyrite-alunite coexistence. Under these redox conditions, a pH of 3 and over a temperature range of 200° to 300°C, AuCl2− complexing may dominate over HAu(SH)2 at salinities above about 2 wt percent NaCl.
Several conditions are conducive for high sulfidation mineralization to occur:
a crystallizing magma exsolves a fluid, with lower pressure conditions favoring metal fractionation from melt to fluid,
the exsolved fluid separates into vapor and saline liquid phases due to immiscibility, with the latter being metal rich,
the gas-rich (HCl and SO2 + H2S) vapor ascends to the surface, with at least a portion condensed into meteoric water, forming an acid fluid which leaches the host rock to create permeable zones for later mixing, and
the dense, metal-bearing fluid also ascends into this leached zone and precipitates Cu sulfosalts, sulfides, and Au upon mixing with meteoric water
Figures & Tables
Epithermal Gold Mineralization and Modern Analogues, Kyushu, Japan
The late Cenozoic volcanic activity in Kyushu is characterized by large-scale volcano-tectonic depressions. The sites of these depressions together with associated volcanism and gold mineralization migrated southeastward in northern Kyushu and eastward in southern Kyushu. Thus, Quaternary gold deposits in Kyushu occur within 30 km west from the present volcanic front; the Pliocene gold deposits occur farther away from the active volcanoes.
Typical Quaternary gold mineralization in Kyushu is the quartz vein type with associated adularia and minor calcite. Although host rocks are predominantly andesitic volcanic rocks and sometimes rhyolite and lacustrine sediments, the major portion of high-grade quartz veins of the Hishikari deposit discussed here is hosted in basement sedimentary rocks close to the unconformity between the basement and overlying Quaternary andesites. Five distinct alteration types are recognizable on the basis of mineral assemblages for Quaternary gold deposits. Two are the deeper propylitic alteration and the shallower smectite-zeolite alteration of the widespread and temperature-controlled type; the rest are mica-chlorite alteration, argillic alteration, and silicification of the fracture-controlled type.
Most Quaternary and some Pliocene gold deposits in Kyushu are located near small Bouguer anomaly highs in areas of regional gravity anomaly lows. In the case of the youngest deposits the gravity anomaly highs are overlapped by low electrical resistivities. The small gravity anomaly highs have been ascribed to underlying uplifted blocks of basement. The low resistivity anomalies are caused by the presence of argillic alteration of the shallow portion of the mineralized systems.
The importance of circum-Pacific volcanic belts as a source of rich gold deposits has been highlighted by recent discoveries of deposits such as McLaughlin, California; El Indio, Chile; Lihir, Papua New Guinea; and Hishikari, Japan. In particular, epithermal gold deposits related to Quaternary volcanoes have become the focus of exploration in island-arc systems of the western Pacific rim, in which Kyushu lies. Kyushu is the principal gold-producing area in Japan. There are several important gold deposits which are related to waning or extinct geothermal systems of Quaternary age. The erosion level in the Quaternary volcanic areas is shallow so that some of the surface expressions are different from those of older deposits. The purpose of this paper is to outline the characteristics of gold mineralization of Quaternary volcanic areas in Kyushu on the basis of detailed geologic and geophysical investigations undertaken over the last ten years. Major epithermal gold deposits in Kyushu occur in late Cenozoic volcanic areas which were formed by the subduction of the Philippine Sea plate beneath the Eurasian plate.