資源地質
Online ISSN : 2185-4033
Print ISSN : 0918-2454
ISSN-L : 0918-2454
54 巻 , 2 号
選択された号の論文の6件中1~6を表示しています
  • 淡路 俊作, 中村 謙太郎, 森口 恵美, 加藤 泰浩, 石井 輝秋
    2004 年 54 巻 2 号 p. 125-138
    発行日: 2004/12/10
    公開日: 2009/06/12
    ジャーナル フリー
    Mineralogical and geochemical features of low-temperature hydrothermally altered pillow basalts dredged from the western Pacific seamount (Uyeda Ridge) are documented. The altered basalts consist mainly of secondary clay minerals and primary Ca-rich plagioclase with minor K-feldspar and phosphate mineral and rare clinopyroxene relicts. Clay minerals are composed of a mixture of Fe-oxyhydroxide, smectite, and celadonite. Smectite in non-porous samples is Mg-rich, whereas smectite in porous samples is Mg-poor.
    Whole-rock chemical compositions reveal that the Uyeda Ridge basalts are oceanic island tholeiite. Compared to average chemical compositions of typical oceanic island tholeiitic basalts, the altered Uyeda Ridge basalts are enriched in K2O and P2O5, and depleted in MnO, MgO, and CaO. Notable enrichment of K is due to the presence of K-rich celadonite and K-feldspar. Phosphorus enrichment is attributed to the presence of phosphate mineral. The altered basalts are mostly depleted in MgO, although the degree of depletion in several non-porous samples is less striking than that in porous samples, probably reflecting the difference of Mg content of smectite. Considerable depletion of Ca is due to the breakdown of clinopyroxene, glass, and plagioclase. The Ca- and Mg-depletion suggest that low-temperature hydrothermal alteration of seamounts is effective for providing Ca and Mg into the ocean.
    The minimum flux of Ca leached from the altered Uyeda Ridge basalts through the low-temperature hydrothermal alteration is estimated to be 1.8×10-11 mol/g/y. It is very likely that the Ca was released into the ocean, and then precipitated as carbonate sediments (CaCO3) on seafloor. This means that the altered basalts of seamounts play a significant role as a potential sink of CO2 in the atmosphere-ocean system.
  • 薬師 大五郎, 円城 寺守
    2004 年 54 巻 2 号 p. 139-144
    発行日: 2004/12/10
    公開日: 2009/06/12
    ジャーナル フリー
    The Takara ore deposit is located in the Misaka Mountain, Yamanashi Prefecture in central Japan and formed at Miocene as volcanogenic massive sulfide ore deposit. In the Takara ore deposit, gold exists in the pyrite type ore that is mainly composed of pyrite. The Au-bearing pyrite type ore shows a massive structure, and pyrite has the zoning structure by alternating As or Sb contents. Especially, the As-zoning structure is predominant in pyrite from the Au-bearing pyrite type ore. On the other hand, the pyrite type ore without gold shows a disseminated texture with quartz, and pyrite has uniform chemical composition. Gold is not observed under the microscope and by the back-scattered microprobe image. Gold could exist as invisible Au. As (and probably Sb) contents in pyrite show negative correlation with S and Fe contents. As is considered to replace both S and Fe. It seems to be that the zoning structure by alternating As or Sb contents in pyrite from the Aubearing pyrite type ore was formed at low temperature.
  • 東原 雅実, 栗原 雅臣, 山田 毅, Iván Guerrero MEJIA, Raúl Felipe C&A ...
    2004 年 54 巻 2 号 p. 145-157
    発行日: 2004/12/10
    公開日: 2009/06/12
    ジャーナル フリー
    We studied the geology of 50 deposits out of 103 metal ore deposits distributed in central and southwestern Honduras of Central America. Based on geological characteristics, these 50 deposits were classified into the following five types: 1) orogenic gold, 2) sedex, 3) hydrothermal (low-sulfidation epithermal, high-sulfidation epithermal, mesothermal, hypothermal), 4) porphyry copper, and 5) skarn.
    K-Ar dating of K-bearing hydrothermal-alteration minerals from 18 deposits out of the 50 deposits yielded 206 Ma for one orogenic gold deposit, 86-67 Ma for four hydrothermal deposits, 40-2.5 Ma for 12 hydrothermal deposits and 13 Ma for one porphyry copper deposit. Moreover, formation ages of sedex and skarn deposits, which yielded no mineral proper for K-Ar dating, were inferred from related geological data. Taking the K-Ar ages and the inferred formation ages into consideration, we concluded that metallogenic evolution of central and southwestern Honduras were as follows: 1) "in Early Jurassic" orogenic gold deposits were formed, 2) "from the latter half of Jurassic to the middle of Early Cretaceous (?)" sedex deposits were formed, 3) "from the middle of Late Cretaceous to Oligocene" hydrothermal and skam deposits were formed, 4) "in Miocene" hydrothermal, porphyry copper, and skam deposits were formed, 5) "in Pliocene" hydrothermal deposits were formed.
    Comparison of the above-mentioned metallogenic evolution to the plate-tectonic evolution revealed by an existing study reached an interpretation that each type of mineralization might have taken place under the control of plate-tectonic setting of the Chortis block, in which Honduras is located, as follow: 1) orogenic gold deposits were formed under the condition that a part of the Honduras Group accreted to the Chortis block in Early Jurassic, 2) sedex deposits were formed under tension stress condition of the Chortis block, which moved eastward due to seafloor spreading from the latter half of Jurassic to the middle of Late Cretaceous, 3) hydrothermal, porphyry copper, and skarn deposits were formed under active igneous condition caused by subduction of oceanic plates after the middle of Late Cretaceous.
  • 小川 泰正, 鹿園 直建, 石山 大三, 佐藤 比奈子, 水田 敏夫, 中野 孝教
    2004 年 54 巻 2 号 p. 159-166
    発行日: 2004/12/10
    公開日: 2009/06/12
    ジャーナル フリー
    Contents of rare earth elements (REEs) and Sr isotopic composition of anhydrite sampled from Matsumine, Shakanai and Hanaoka Kuroko deposits are analyzed. Sr isotopic values of anhydrite samples (0.70772-0.708661) have the intermediate values between the values of Miocene seawater (0.7088) and the country rocks (0.7035-0.7050). Anhydrite in Kuroko deposits exhibits two chondrite-normalized REE patterns; a pattern containing a marked peak in Sm, and another exhibiting linear-pattern which decreases from light to heavy rare earth elements (REE).
    Anhydrites from Kuroko deposits are similar to those from recent submarine hydrothermal systems (TAG and Grimisey Fields) in chondrite-normalized REE patterns and S and Sr isotope ratios. That indicates that these anhydrites would have similar formation mechanism, namely anhydrites from Kuroko deposits would have formed by the mixing of seawater and hydrothermal solution, similarly to those from TAG and Grimsey Fields. Further, variation of anhydrites from Kuroko deposits would be caused from the variation of seawater / hydrothermal solution ratios.
  • 薬師 大五郎, 円城寺 守
    2004 年 54 巻 2 号 p. 167-174
    発行日: 2004/12/10
    公開日: 2009/06/12
    ジャーナル フリー
  • 2004 年 54 巻 2 号 p. 175-189
    発行日: 2004/12/10
    公開日: 2009/06/12
    ジャーナル フリー
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