GEOCHEMICAL JOURNAL
Online ISSN : 1880-5973
Print ISSN : 0016-7002
ISSN-L : 0016-7002
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Displaying 1-2 of 2 articles from this issue
ARTICLE
  • Fujun Zhong, Jie Yan, Kaixing Wang, Jiayong Pan, Fei Xia, Guoqi Liu, W ...
    Article type: ARTICLE
    2022 Volume 56 Issue 3 Pages 74-95
    Published: 2022
    Released on J-STAGE: June 07, 2022
    Advance online publication: May 03, 2022
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    The Changjiang U ore field developed typical granite-related U mineralization in the Zhuguangshan complex, China. Pitchblende is the most important ore mineral in these mineralizations. In this study, the mineralogy and geochemistry of pitchblende were investigated by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) to identify the genesis of the Changjiang U ore field. Pitchblende exhibits colloidal, fragmented, spherulitic and fine-grained crystals in U ores. Its geochemical compositions are similar to those of other granite-related U deposits in South China, which have elevated contents of U, Sr, As and W; low contents of Pb, Th, Zr, Nb, Ta, Hf, Co, Ni and rare earth elements (REEs); and variable amounts of Ca, Si, Bi, Y, V and Zn. These geochemical signatures suggest that mineralization occurred through hydrothermal genesis and that the hosting Youdong and Changjiang granites acted as the dominant U sources. The uraninite in these granites might be the major U source mineral. Uranium mineralization occurred under the following conditions: low temperature (<250°C), low oxygen fugacity (log fO2 = –29.5 – –25.5), weakly acidic (pH = 5.3–5.9), high CO32– and F contents and a silicon-saturated solution. Rapid changes in the physicochemical conditions of the ore-forming fluid are responsible for the precipitation of pitchblende. Combined with previous studies, we propose that U-rich granites, Cretaceous-Tertiary crustal extensions, regional faults and hydrothermal alterations were the critical factors for U formation in the Changjiang ore field.

EXPRESS LETTER
  • Ai Genda, Minoru Ikehara, Atsushi Suzuki, Wahyoe S. Hantoro, Mayuri In ...
    Article type: EXPRESS LETTER
    2022 Volume 56 Issue 3 Pages e1-e7
    Published: 2022
    Released on J-STAGE: June 28, 2022
    Advance online publication: May 26, 2022
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    Supplementary material

    The Indonesian Throughflow (ITF) plays an important role in the heat flux and water budget between the Pacific and Indian oceans and may modulate climate variability. During the boreal winter monsoon, low-salinity, buoyant water carried from the Java Sea to the Southern Makassar Strait retards the sea-surface transport of the ITF, which may affect the Asian monsoon and climate change. However, observation records are inadequate to elucidate the marine environment around the Indonesian Seas. We analyzed coral Sr/Ca and δ18O from the Seribu Islands, Java Sea, and reconstructed sea-surface temperature (SST) and sea-surface salinity (SSS) for 1931–2002. The SST data indicate abrupt warming in the mid-1950s and, almost simultaneously, a rapid SSS shift to saline conditions. The relationships between SST around the Seribu Islands and climate variability in the Pacific and Indian oceans have changed after this abrupt warming events. Before the mid-1950s, during September to November, SST varied with the Indian Ocean Dipole, whereas El Niño–Southern Oscillation also affected SST variation after the mid-1950s. This abrupt change seems to be related to a regime shift in the tropical Pacific and Indian oceans, but there are no clear changes corresponding to other regime shifts such as that of 1970s. SSS variation exhibits no relationship with climatic factors, indicating that the dominant controlling factors of SST and SSS should be considered separately. Marine conditions in the Java Sea that affect the ITF show unique behavior, and further local studies in the Indonesian Seas are crucial to understand ITF behavior.

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