This article reports geochemical behavior and enrichment of Al, Ga and Sc by laterization of basalts and sandstone in the Bolaven Plateau, southern Laos. The Bolaven Plateau consists of Neogene - Quaternary basalts and underlying Cretaceous sedimentary rocks. Laterites derived from the basalts and sedimentary rocks are developed on these parent rocks. With increasing the degree of laterization, loss of mobile elements such as Si, alkali elements and alkaline earth elements lead to the enrichment of immobile elements of Al, Ga and Sc relative to the parent rocks. Accordingly, Al₂O₃ contents range from 10 to 15 % in the basalts, from 16 to 30 % in the saprolites, from 17 to 48 % in the basaltic laterites and from 15 to 47 % in the sedimentary laterite. Ga contents range from 6 to 20 ppm in the basalts, from 9 to 46 ppm in the saprolites, from 24 to 83 ppm in the basaltic laterites and from 22 to 68 ppm in the sedimentary laterites. Sc contents range from 14 to 23 ppm in the basalts, from 14 to 45 ppm in the saprolites, from 14 to 69 ppm in the basaltic laterites and from 13 to 84 ppm in the sedimentary laterites. No significant difference is recognized in the enrichment of these elements between the basaltic laterites and sedimentary laterites. A similar geochemical behavior and positive linear correlation are recognized between Ga and Al and between Sc and Fe, suggesting that Ga and Sc exist by replacing Al and Fe in the laterites, respectively. Ga/Al ratios range widely but Ga is almost no depleted or slightly enriched relative to Al with increasing the degree of laterization. Sc/Fe ratios indicate that Sc may be slightly depleted relative to Fe by strong laterization.

View full abstract

    High-grade indium-polymetallic ores of the Toyoha deposits, Hokkaido, which mainly belong to the stage IV mineralization, were studied chemically at E-W trending Shinano-Izumo-Iwami Veins, WNW-ESE trending Soya Vein , and N-S-trending Sorachi-Nemuro Veins. The indium contents go up to 1.0 wt % in the ores (Sorachi, -430 mL). Averaged indium contents and 1000 In/Zn ratio are obtained as follows: Shinano Vein(n=29): 568 ppm and 9.0; Izumo Vein (n=7): 582 ppm and 1.9; Iwami Vein (n=17): 371 ppm and 6.4 (n=16, excluding the highest value of 444.6); Soya Vein (n=4) 1,467 ppm and 3.1; and Nemuro-Sorachi Vein (n=5): 4,050 ppm and 9.1. The whole average is 854 ppm (n=62) and 7.1 (n=61). These indium-rich ores occur in the southeastern part of the Toyoha deposit, where the hydrothermal ore solutions were considered flown out from the depth.
    Indium contents of the ores are positively correlated with zinc contents on the Shinano Vein (correlation coefficient of 0.65), but unclear on the whole veins (correlation coefficient of 0.51). Positive correlation between indium and tin is only seen locally (e.g., Iwami Vein). Within available level of 500 meters, zinc content decreases but tin and arsenic contents increase with the depth. Distribution of indium has some similarity with that of tin and arsenic vertically. These chemical characteristics suggest that indium was closely associated with tin and arsenic, besides zinc and cadmium in the hydrothermal fluids. Compared with similar indium-rich ore deposits in sedimentary terrains in Bolivia, indium-contents are similar in the two regions. Mafic components such as iron, copper, nickel, cobalt, arsenic, and silver are, however, richer in the Toyoha deposits than in the Bolivian deposits. Manganese, antimony, bismuth and tin are predominant in the Bolivian ore deposits. These chemical characteristics reflect general difference of the host rocks, juvenile mafic volcanics vs. sedimentary and felsic volcanics, of the two regions.

View full abstract

    At the Shraoi area located along Pacific Ocean between the west and central parts of Hokkaido, geothermal water of ca. 40-60 ^°C has been regionally produced in a large amount by 400 to 1,600 m deep drilling since 1960's. A study of geochemistry and isotope geochemistry for characterizing the hydrothermal systems distributed at ca. 1,500 m depth and down to ca. 3,000 m depth, and understanding their origins has been conducted not only at the Shiraoi area, but also in the three regional areas surrounding it (the Quaternary volcanic area to the west, Ishikari Lowland and the Paleogene area to the east) using surface, ground and geothermal waters. The results could be summarized, on the bases of the results of previously-reported various researches for these areas, as following (1) to (4):

View full abstract

    We report nine new datings of the three major pyroclastic eruptions of the younger Haruna volcano, at the central part of Japan. Two ¹⁴C ages were obtained from the Shirakawa pyroclastic flow deposit, three ages from the Futatsudake Shibukawa tephra, and four ages from the Futatsudake Ikaho tephra, erupted from the younger Haruna volcano. The ¹⁴C ages, 44740±450 yr BP and 45030±620 yr BP obtained from the Shirakawa pyroclastic flow deposit, indicate the age of the Hassaki eruption, oldest and largest eruption of the Younger Haruna volcano. Three ¹⁴C ages ranging from 1540 to 1640 yr BP are obtained from the Haruna Futatsudake Shibukawa tephra. The four samples obtained from the Haruna Futatsudake Ikaho tephra shows same ¹⁴C ages of 1480 yr BP. These ¹⁴C ages confirm the age of three major eruptions of the Younger Haruna volcano. The eruption age of the Futatsudake Shibukawa tephra is the late 5^th century - early 6^th century. The eruption age of the Ikaho tephra is considered as between the late 6^th century and the beginning of the 7^th century.

View full abstract

Geological Survey of Japan, AIST has been carrying out the cooperative research entitled "Hydrological and geochemical research for earthquake prediction in Taiwan" with Disaster Prevention Research Center, National Cheng-Kung University, Taiwan since 2002. We made much contribution to clarifying the mechanism of groundwater changes and their recoveries related to the 1999 Chichi earthquake, constructing a groundwater observation network composed of 16 wells in Taiwan and understanding the earthquake-related groundwater changes observed by the new groundwater observation network through this cooperative research. In Taiwan seismicity is more active and crustal deformation is more rapid than in Japan. Therefore observation and analysis of groundwater changes related to earthquake and crustal deformation in Taiwan will enable us to make rapid progress in hydrological and geochemical research for earthquake prediction. This cooperative research will also give important information for evaluation of long-term groundwater changes in tectonically active areas like Japan and Taiwan.

View full abstract

    Late Triassic radiolarian fossils were extracted from siliceous mudstone in an accretionary complex of the Ashio Terrane in the Kamo district (Quadrangle series 1:50,000 of the Geological Survey of Japan), Niigata Prefecture, Japan. The siliceous mudstone occurs as a block within a muddy matrix. The siliceous mudstone also belongs to the upper unit of the Senmi Complex, which is distributed widely in the Kanbara Mountains within the Kamo district. The fauna in the siliceous mudstone consists mainly of species of the genera Canoptum, Betraccium, Cantalum and Fontinella, including Canoptum sp. cf. C. rhaeticum, Fontinella sp. cf. F. clara, Fontinella sp. cf. F. primitiva etc. Based on radiolarian biostratigraphy of these taxa previously documented elsewhere, the radiolarian fauna studied was dated to the Late Triassic (Rhaetian). This is a first report of Triassic radiolarian fossils from clastic rocks of the Ashio Terrane in the Kanbara Mountains; thus, this find contributes to reconstructions of oceanic plate stratigraphy of the Ashio Terrane in this region.

View full abstract