Abstract:The origin of mineralizing fluids responsible for the Hishikari vein-type epithermal Au deposits was studied on the basis of the hydrogen isotopic ratio (δD) of the inclusion fluid from vein quartz and adularia. The origin of hydrothermal fluids was estimated by combination of the present δD values and the oxygen isotopic ratios (δ 18 O) previously reported by Shikazono and Nagayama (1993).The water in the fluid inclusions was extracted by means of decrepitation of quartz at 500°C. Hydrogen was obtained by reduction of the collected water with Zn shot at 450°C. The δD values were determined by mass spectrometer.The δD values of inclusion fluid obtained from quartz range from -61 to -114 ‰. These are significantly lower than the δD value of the thermal water presently venting from the Hishikari deposits and that of local meteoric water.Hydrogen isotopic fractionation between water and amorphous silica, which might have initially precipitated from the hydrothermal fluids at least partly, is not a probable cause of this isotopic depletion, while some water might have been released from the initial hydrous amorphous silica during recrystallization to quartz observed presently. Thus, a part of ore fluids for the Hishikari deposits is supposed to have been originated from the water having anomalous δD values of lower than -100 ‰.Such D depletion cannot be caused by simple oxygen-shift of meteoric water or by contribution of magmatic volatiles. The δD values of water released from the shale samples of the Shimanto-Supergroup, a major host to the Hishikari veins range from -132 to -148 ‰. Therefore, the anomalous δD values of inclusion fluids from some vein quartz and adularia suggest that the water released from hydrous minerals of the sedimentary basement rocks by dehydration or the groundwater isotopically exchanged with sedimentary rocks at elevated temperatures during circulation, partly contributed to the hydrothermal fluids responsible for the Hishikari deposits.
The Kamioka Zn-Pb deposits consist of clinopyroxene-rich skarns replacing limestone lenses in Hida gneissose rocks. Paleozoic metabasite, early Jurassic Funatsu granitic rocks and late Cretaceous porphyritic dikes and stocks, are the known igneous rocks in the mining area. Oxygen isotopic compositions of eight separates each of clinopyroxene and quartz from the deposits (Table 1), and some minerals and rocks from the major rock units around the deposits (Table 2), have been analyzed. The estimated oxygen isotopic ratios of the fluids responsible for the formation of the deposits, range as low as -4%0 to + 3%0 (SMOW). These values are considerably lower than expected from magmatic waters, and demonstrate that none of the igneous rocks in the area could be a direct source of the skarn-and ore-forming fluids. The data instead show that the deposits are formed by a huge convective circulation of solutions of meteoric water origin, promoted by a hidden batholithic intrusion at the time of the late Cretaceous.
About 120 specimens of magnetite from various localities are examined by an electron microprobe analyzer. Magnetites containing more than one weight percent of silica but lack of any other components than ferrous and ferric iron, called silician magnetites in the present paper, are recognized in 23 skarn, one vein and one thermally metamorphosed massive sulfide deposits. Thus it is confirmed that this mineral occurs in nature much more frequently than so far expected. Besides silician magnetites, magnetites with appreciable amounts of Al2O3, CaO, MgO and other components along with silica, are also recognized in some skarn deposits. Magnetites with such unusual compositions are found only in hydrothermal environments, and it is suggested that precipitation mechanisms seem to be responsible for their formation.In silician magnetites, excess electric charge brought by the replacement of ferric iron in tetrahedral site by silicon, could be compensated by the replacement of ferric iron in octahedral site by ferrous iron, known as γ-Fe2SiO4 component. The natural occurrence of silician magnetites, however, gives no positive support to the existence of this component at crustal pressures. Instead a preliminary Mössbauer experiment demonstrates that one silician magnetite has a maghemite -like structure by the omission of ferrous iron from octahedral site.
Many skarn deposits of various ore types occur in the Japanese island arc with genetical relation to Cretaceous and Miocene felsic igneous activities. Carbon and oxygen isotope ratios were determined for interstitial carbonates, mostly calcites, in skarn silicates from 28 representative deposits. The data obtained are compared with the isotopic composition of calcites expected to precipitate from magmatic fluids, on the assumption of isotopic equilibrium at 300 400°C between the carbonate mineral and fluid. The fluids for tungsten, tin and some copper (-iron) deposits in the regions of ilmenite-series granitic activity are characterized by 13C-depleted carbon, probably derived from organic materials in sedimentary rocks. In the fluids for most deposits of other types, such as zinc-lead, zinc-copper (-iron) and copper (-iron), mag matic carbon is predominant. Carbon and sulfur isotope values for each deposit show a positive correlation among the studied deposits, suggesting that both carbon and sulfur came from common sources. For many deposits, calculated 6180 values of the fluids are in the range from +5 to 10% (SMOW). Such fluids are either magmatic in origin, or are isotopically equilibrated with felsic igneous rocks. Fluids with 180 depleted nature are recognized in some, mainly zinc-lead and zinc-copper (-iron), deposits, suggesting a contribution of surface-derived water to the formation of these deposits.
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