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@ Stratigraphy and hydrothermal alteration based on well data from the Oku-aizu geothermal system, Japan

Wadalite, rustumite, and spurrite from La Negra mine, Queretaro, Mexico

Abstracts for Geological Survey Seminar, no. 250

Stratigraphy and hydrothermal alteration based on well data from the Oku-aizu geothermal system, Japan

Yoji Seki and Masaho Adachi

1997

vol. 48 (7), p. 365-412, 15 figs.

Keywords: Oku-aizu geothermal system, stratigraphy, hydrothermal alteration, well logging

Abstact: The stratigraphy and hydrothermal alteration around the Okuaizu geothermal system were studied in core and cuttings from geothermal wells drilled by NEDO and Okuaizu GeothermalCo., Ltd. The basement is thought to be composed of a pre-Tertiary granodiorite and/or sedimentary rocks. Early to middle Miocene formations (Oh-hizawa, Takizawagawa, Ogino and Urushikubo Formation) consist mainly of rhyolitic to dacitic lavas, pyroclastic rocks intercalated with clastic rocks, and minor basaltic rocks, which are considered to unconformably overlie the basement. The late Miocene caldera-related formation (Iriyamazawa Formation) consisting of dacitic ash flow tuffs, debris avalanche deposits and lake sediments, unconformably overlies the early to middle Miocene formation. Pleistocene lacustrine sediments and ryolitic lava domes (Sunagohara Formation), formed in a caldera environment, unconformably cover the Miocene formations. Five alteration zones can be distinguished using clay minerals assemblages: chlorite+sericite zone (PR), chlorite/smectite or sericite/smectite mixed clay zone (ML), smectite+/-zeolite zone (SZ), kaolinite+/-sericite zone (K) and sericite zone (S). Based on the stratigraphy, present temperature and the spatial distributuin of each alteration zone, three stages of hydrothermal alteration can be recognized in the Okuaizu geothermal district: 1) middle Miocene submarine hydrothermal alteration, 2) late Miocene caldera related hydrothermal alteration, and 3) hydrothermal alteration associated with the pesent geothermal system. 1) is characterized by a widespread PR zone in the early Miocene formations. 2) is limited to the Iriyamazawa Formation deposited in a 7 Ma caldera. The PR and ML zones are seen in the lower part, and the SZ one in the upper part of the caldera, separated spatially from similar alteration zones in the early Miocene formations. 3) has a spatioally zoned distribution of the alteation zones depending on the present temperature in addition to some other alteration zones. A zonal distribution of the SZ zone in the upper low-temperature portion, the PR zone in the lower high-temperature portion and the ML zone in the intermediate portion is observed. In addition, the sporadic K zone, which was probaly formed by acidic fluids generated by the interaction of oxidized underground water with H2S and CO2 gases related from the geothermal fluid during boiling, and the S zone around a high-temperature reservoir and adjacent to the k zone are observed. Anhydrite is found in the high-temperature reservoir and adjacent to the K zone are observed. Anhydrite is found in the high-temperature portion (>100 to 150Ž) and various carbonate minerals are also common within the system.

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Wadalite, rustumite, and spurrite from La Negra mine, Queretaro, Mexico

Yasuo Kanazawa, Masahiro Aoki and Hideo Takeda

1997

vol. 48 (7), p. 413-420, 2 figs., 8 tables.

Keywords: wadalite, rustumite, spurrite, calc-silicate, skarn, Mexico

Abstract: Wadalite, rustumite, and spurrite-rare calc-silicates-occur in skarns formed by Tertiary diorite intrusion into Upper Jurrassic to Lower Cretaceous limestone at La Negra mine, Queretaro, Mexico. Wadalite and rustumite occur zonally in the rock mainly composed of spurrite. They are associated with hydrogrossular, andradite, calcite, and magnetite in main and occur as fine grains less than 200ƒสm in size. Spurrite occurs as an aggregate of comparatively large irregular crystals up to 3 mm.
@Wadalite is colorless and optically isotropic with the refractive index, n=1.708. Optical properties of wadalite is so similar to hydrogrossular that it is difficult to tell the former from the latter. Dcalc=3.066 g/cm. The electron microprobe analyses of wadalite yield an empirical chemical formula, Ca5.99(Al3.35Si2.70Mg0.63Fe3+0.25)ƒฐ6.93O16Cl2.93, or ideally Ca6(Al,Si,Mg,Fe3+)7O16Cl3 on the basis of O=16. The crystal is cubic with the space group I43d, the cell dimension a=12.014(1)๐ and V=1734(1)๐3, and Z=4. Wadalite in this study is richer in Mg and lower in (Al+Fe3++Mg)/Si ratio than that of the original description.
@Rustumite is translucent to white. It is optically biaxial negative with ƒฟ=1.641, ƒภ=1.646, ƒม=1.651, andƒม-ƒฟ=0.010. Dmeas=2.85 and Dcalc=3.017 g/cm3. The microprobe analyses of rustumite yield an empirical formula, Ca9.97Si5.00O18(Cl1.56(OH)2.44)ƒฐ4.00ฅ0.26H2O, or ideally Ca10Si5O18(Cl,OH)4 on the basis of O=18 and Cl+OH=4. The space group is C2/c and the cell dimensions are a=7.625(1), b=18.576(2), c=15.519(1)๐, ƒภ=103.75(1)฿, and V=2135(1)๐3. Z=4.
@Spurrite is bluish gray. It is optically biaxial negative withƒฟ=1.641, ƒภ=1.670, ƒม=1.678, andƒม-ƒฟ=0.037. Dmeas=2.94, Dcalc=2.882 g/cm3. The wet chemical analysis of spurrite yields an empirical formula Ca4.97(Si1.95Al0.05)ƒฐ2.00(C0.87B0.16)ƒฐ1.03O11, or ideally Ca5(SiO4)2(C,B)O3 on the basis of O=11. The space group is P21/a and cell dimensions are a=10.472(2), b=6.724(1), c=14.174(6)๐, ƒภ=101.31(3)฿, and V=978.7(3)๐3. This spurrite contains a small amount of boron which replaces the carbon in the CO3 equilateral triangles in the structure.

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Abstracts for Geological Survey Seminar, no. 250 -Geological information for advanced society-

1997

vol. 48 (7), p. 421-427.