GEOLOGY, GEOCHEMISTRY, AND ORIGIN OF HIGH SULFIDATION CU-AU MINERALIZATION IN THE NANSATSU DISTRICT, JAPAN

The Nansatsu district of southern Kyushu has been the site of calc-alk aline volcanism for the last 10 m.y., shifting eastward with time. Ass ociated hydrothermal activity followed deposition of the volcanic host rocks by about 0.5 m.y. and was characterized by interaction of magma tic 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 f rom leaching of the original andesite lava and pyroclastic flows by ac id chloride-sulfate waters. These are inferred to have formed when mag matic vapors containing HCl and SO2 condensed into meteoric water. The residual silica (now quartz) orebodies are best developed where the h ost was initially permeable. The margins of the quartz bodies are abru pt, with narrow (1-2 m) halos representing the reaction front of acid waters isochemically dissolving the host rock. The halo comprises alun ite (strongly zoned in Na and K, with P-rich cores), dickite, and/or k aolinite +/- pyrophyllite, grading out into illite and interlayered il lite-smectite clays, and finally, propylitic alteration. This pattern is characteristic of deposits of this type throughout the world, for e xample, at Summitville, Colorado, and Lepanto, Philippines.Mineralizat ion occurred after initial leaching by the vapor condensates, with met als 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 restrict ed to the quartz bodies. Gold is most closely associated with enargite and pyrite; later minerals include covellite and then sulfur. The las t stage of activity was steam-heated, with descending waters oxidizing sulfides to geothite 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 remobili zation. Stable isotope results indicate that hypogene alunite formed f rom a mixture of magmatic fluid (deltaO-18 = 7 +/- 2 parts per thousan d, deltaD = -25 +/- 5 part per thousand, similar to nearby active volc anic discharges) with local meteoric water. In contrast, the clays in the marginal halo have isotopic compositions indicating a deltaO-18 sh ift of 6 to 8 per mil from local meteoric water values, probably due t o water-rock interaction, and the deltaO-18 values of residual silica quartz may also be due to meteoric water domination. Fluid inclusion s tudy of postmineralization quartz crystals indicates that the fluids h ad a salinity of about 1 wt percent NaCl equiv during late quartz grow th, though there is evidence in one sample for higher salinity fluid h aving been present, up to 30 wt percent NaCl equiv (some inclusions co ntain daughter minerals of halite and sulfur). The T(h) values of over 1,000 measurements on late quartz from the ore zones indicate that th e mean temperature during that stage ranged from <200-degrees-C at Ake shi to about 200-degrees-C at Iwato and 230-degrees-C at Kasuga. The p resence of vapor-rich inclusions, some with T(hv) similar to T(hl), in dicate 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 tab le. The mineralizing fluid was relatively oxidized (sulfide/sulfate ra tio about 3:1), close to pyrite-alunite coexistence. Under these redox conditions, a pH of 3 and over a temperature range of 200-degrees to 300-degrees-C, AuCl2- complexing may dominate over HAu(SH)2 at salinit ies above about 2 wt percent NaCl. Several conditions are conducive fo r high sulfidation mineralization to occur: (1) a crystallizing magma exsolves a fluid, with lower pressure conditions favoring metal fracti onation from melt to fluid, (2) the exsolved fluid separates into vapo r and saline liquid phases due to immiscibility, with the latter being metal rich, (3) the gas-rich (HCl and SO2 + H2S) vapor ascends to the surface, with at least a portion condensed into meteoric water, formi ng an acid fluid which leaches the host rock to create permeable zones for later mixing, and (4) the dense, metal-bearing fluid also ascends into this leached zone and precipitates Cu sulfosalts, sulfides, and Au upon mixing with meteoric water. If the saline liquid is not releas ed from its source adjacent to the magma, due to lack of fracturing, o r if there is a strong hydraulic gradient caused by high relief, only the vapor-related stage may occur. This will leave leached, barren roc k which is characteristic of many eroded volcanic terranes.