Abstract

The Kushikino mine is located in the Tertiary gold mining areas of southern Kyushu, Japan. The ore deposits occur in andesitic volcanics as fissure-filling epithermal veins, consisting of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite, and sulfides. Quartz and calcite are dominant throughout the mineralization. Three major stages of mineral deposition are recognized as follows; (I) first opening of fractures and deposition of translucent quartz, (II) repeated fracturing and the main stage of the Au-Ag mineralization with deposition of milky quartz and calcite, and (III) deposition of barren calcite.Oxygen and carbon isotope compositions were determined for quartz and calcite from the Kushikino 1 (the Champion vein) and the Arakawa 4 veins. The ranges of the isotopic compositions of the minerals are as follows (numbers of determination are given in parentheses):Vein Mineral delta 18 O SMOW (ppm) delta 13 C PDB (ppm)Kushikino 1 Quartz 5.5 to 9.7 (29) Calcite 2.3 to 8.5 (13) -11.2 to -8.3 (13)Arakawa 4 Quartz 8.4 to 10.7 (8) Calcite 5.6 to 9.9 (5) -9.7 to -8.9 (5)No distinct difference in oxygen isotope compositions is observed between the stage I translucent quartz and the stage II milky quartz. In stage II both quartz and calcite have wide ranges in delta 18 O values, extending for the entire compositional ranges of the veins mentioned above. The stage III calcite has higher delta 18 O values than does the stage II calcite.The wide range in the delta 18 O values of the stage II minerals corresponds to a temperature decrease from 220 degrees to 140 degrees C. The isotopic data can be interpreted to indicate a mixing of deep hydrothermal fluid and low-temperature meteoric ground water, if the deep hydrothermal water was unexchanged or partially exchanged meteoric water with a limited range of delta 18 O values (lower than - 3ppm at 300 degrees C). Incorporation of a large quantity of high delta 18 O thermal waters is not plausible. The delta 13 C-delta 18 O trend of calcite is interpreted to show a temperature decrease and a change in the carbon species of the fluid. Calcite crystallized in equilibrium with a fluid in which H 2 CO 3 was dominant at temperatures higher than 140 degrees or 150 degrees C, whereas HCO (super -) 3 was dominant at lower temperatures. This accords with inferences based on phase equilibria. The stage III calcite crystallized in a lower temperature range of the HCO (super -) 3 -dominant field.Judging from fluid inclusion data and mineral assemblages, changes in pressure or pH of the fluid were not so important in stage II. The quartz-calcite deposition and Au-Ag mineralization in Stage II may have taken place as a result of rapid cooling of the fluid due to mixing. Mixing of two fluids, both of which were saturated with calcite but which had different temperatures, caused supersaturation of the calcite and, thus, deposition of it along with a temperature decrease. Deposition of a large quantity of calcite in stage III would be due to a pressure decrease caused by the opening of fractures at shallow depths. A subsequent decrease in pH of the fluid may have caused the kaolinite alteration near the surface.

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