Sulfide mineralization in Oku-Aizu geothermal field, with the genetical relation to the epithermal gold deposits.

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Sulfide scales were collected from a test well of the Oku-Aizu geothermal field. Veinlets composed of similar sulfide minerals in the country rocks were also obtained from the drilled cores. Both were deposited from the compressed geothermal hot water in this area. These minerals are listed in Table 1. The analyses of the geothermal water are shown in Table 4. From the analytical data, the pH of the geothermal fluid is calculated to be 4.47 at 250°C in the case of activity coefficient equals to unity.<BR>Two kinds of rock alteration are recognized in this area. The one is alteration caused by the neutral to alkaline hot water. The other is due to the acid hot water. The minerals of both alterations are shown in Table 3. The rocks along fissures containing the present compressed geothermal fluid are subjected to neutral to alkaline alteration. From these facts, the relation log fo2 pH-log f<SUB> s2</SUB> of the environment of the geothermal fluid is shown in Fig.9. The acid alteration occurs sporadically in this area. The acid alteration would be referred to the vapor-dominated system, separated by boiling from the compressed hot geothermal fluid. It might condense to acid hot water during cooling.<BR>In Japan, the epithermal gold deposits are classified into two types. The one is gold (silver) -quartz (adularia) vein, surrounded by the rocks suffered neutral to alkaline alteration; adulariazation, sericitization and/or chloritization. The other is the massive silicified rock containing gold, which is surrounded by the rocks subjected to acid alteration; kaolinization and alunitization. The both types of gold deposits might be attributed to the same phenomena as the two kinds of geothermal fluids stated above.
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Sulfide mineralization in Oku-Aizu geothermal field, with the genetical relation to the epithermal gold deposits.

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