Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 102 , Issue 3
Showing 1-5 articles out of 5 articles from the selected issue
  • Sereenen JARGALAN, Hirokazu FUJIMAKI, Tsukasa OHBA
    2007 Volume 102 Issue 3 Pages 163-173
    Published: 2007
    Released: July 03, 2007
    [Advance publication] Released: February 17, 2007
    Geochemical, mineralogical and geochronological investigations have been carried out on the lamprophyre dike in the Tsagaan Tsahir Uul gold deposit area, Bayankhongor, Central Mongolia. The lamprophyre is composed of olivine, amphibole, and clinopyroxene as phenocrysts with plagioclase, potassium feldspar, biotite, and amphibole as the groundmass. The amphibole is mostly pargasite; some grains are rich in titanium and constitute kaersutite. Pyroxene is slightly enriched in titanium. Although the olivine ranges from Fo92.9 to Fo84.6, a distinct compositional variation between the core and rim in a single grain is lacking. Biotite and phlogopite are dominant. Plagioclase is mainly andesine (An45Ab49Or6) and oligoclase (An12Ab74Or14), and potassium feldspar is orthoclase.
         The major oxides content of the lamprophyre has a good correlation with SiO2. TiO2, FeOt, MgO, CaO, and MnO decrease, and Al2O3 and K2O increase with increased SiO2. Trace element composition is characterized by high values of Large Ion Lithophile Elements (LILE) (Ba, Rb, Sr) and also some compatible elements (Cr and Ni). The MORB normalized spider diagram shows slight depletion of Nb in contrast to Ba and Ce. The REE pattern shows LREE enrichment, and rapid decrease in HREE practically without an Eu anomaly. The Rb-Sr internal isochron yields an age of 248.61 ± 18.5 Ma (Zechstein, Late Permian) with 87Sr/86Sr initial ratio of 0.706096 ± 0.00002.
         Mineral assemblage and chemical characteristics are close to those of calc-alkaline lamprophyres (CALs). An Al2O3 versus TiO2 tectonic discrimination diagram of the shoshonitic and potassic igneous rock shows that the Tsagaan Tsahir Uul lamprophyre might have originated within an intra-plate tectonic setting. The high-Ti content of the lamprophyre also suggests that the magma could have originated in an intra-continental tectonic setting. Small but similar intrusions emplaced during the Permian period are spatially and temporally associated with the main gold and copper mineralization in the Bayankhongor metallogenic belt. The obtained age of the lamprophyre dike (248.61 ± 18.5 Ma) demonstrates that its intrusion was a contemporary event. This was the youngest plutonic body in the Tsagaan Tsahir Uul area, which is close in age to the gold-bearing quartz veins. Although there is no direct genetic evidence associating the auriferous veins and the lamprophyre in the field, they occur together and possibly share a similar tectonic setting.
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  • Ichiro SUNAGAWA, Yasushi TAKAHASHI, Hiroyuki IMAI
    2007 Volume 102 Issue 3 Pages 174-181
    Published: 2007
    Released: July 03, 2007
    [Advance publication] Released: January 25, 2007
    Alternating layers of calcite and aragonite, precipitated from mineral spring in fracture zones of serpentine mass, occur at Kashio, Oosika mura, Nagano Prefecture, Japan. Element mapping by means of Micro-area X-ray Fluorescence (MXRF) conclusively demonstrated that strontium concentrates more in aragonite layer, and not in calcite layer. Other elements including magnesium show no positive correlation with aragonite precipitation. MXRF analyses of aragonite samples from other localities and origins also indicated concentration of Sr as major impurity component, demonstrating that Sr plays essential role in metastable nucleation of aragonite in the precipitation of CaCO3 polymorphs from aqueous solution. This can be understood on the basis of modifications of surface energy term in CaCO3 nucleation in the presence of Sr. Temperature changes trigger to increase the Sr concentration in mineral spring, leading to metastable nucleation of aragonite.
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  • Makio OHKAWA, Masaaki MIYAHARA, Eri OHTA, Kenichi HOSHINO
    2007 Volume 102 Issue 3 Pages 182-193
    Published: 2007
    Released: July 03, 2007
    [Advance publication] Released: February 09, 2007
    Iron oxides and hydroxides found in the so-called maghemite-bearing iron oxide ores from the Kumano mine, Yamaguchi Prefecture, Japan have been investigated using polarizing microscopy, reflecting microscopy, powder X-ray diffractometry (XRD), and electron probe microanalysis (EPMA) to reevaluate the previous study carried out by Shibuya (1958).
         The results show that the iron oxide described as maghemite by Shibuya is actually magnetite, showing a compositional zoning of Si with trace amounts of Al and Ca, and that only hematite and goethite are found as alteration products. The compositional zoning of the magnetite may indicate that the silicon-bearing magnetite was formed during reducing conditions.
         The a lattice parameter of a = 8.376(1) Å for the silicon-bearing magnetite is shorter than that of normal magnetite. This contraction in the crystal lattice is probably caused by a coupled substitution: Si4+ for Fe3+ on the tetrahedral sites and a divalent cation (mainly Fe2+) for Fe3+ on the octahedral sites, with a simple substitution of Al3+ for Fe3+ on the tetrahedral sites.
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  • Yasuo ISHIZAKI
    2007 Volume 102 Issue 3 Pages 194-210
    Published: 2007
    Released: July 03, 2007
    [Advance publication] Released: February 17, 2007
    The 2300 years B.P. eruption of the Yakedake volcano in central Japan consisted primarily of lava extrusion and dome growth in the summit area, and a repetitive gravitational collapse of the dome produced a series of block-and-ash flows known as the Nakao pyroclastic flow deposit (NPFD). Based on the geochemistry and mineralogy, the juvenile materials in the NPFD can be assigned to five groups: light-colored, porphyritic dacite (white dacite) showing little or no petrographic evidence of magma mixing, dark-colored hybrid andesite (black andesite) with disequilibrium phenocryst assemblages and textures, banded lava with streaks of white dacite and hybrid andesite, basaltic andesitic enclaves (hybrid enclave) having the same disequilibrium phenocryst assemblage as the hybrid andesite, and basaltic enclave (primitive enclave) lacking any evidence of magma mixing. Compositional data from the phenocrysts and whole rocks demonstrate that the juvenile materials of the NPFD preserve a magma mixing/mingling event between a basaltic magma (49.5 wt% SiO2, T ∼ 1075 °C from olivine-melt geothermometry), which is compositionally similar to the primitive enclave, and a dacitic magma (64.6 wt% SiO2, T = 790-800 °C from Fe-Ti oxide geothermometry), which is compositionally similar to the white dacite. The NPFD eruption was caused by an invasion of the basaltic magma into the preexisting, highly crystalline dacitic magma chamber. The composition of the dacitic magma remained constant throughout the eruption. However, heating by the basalt magma increased its temperature locally up to T ∼ 950 °C. Minor disruption and consecutive quenching of the replenishing basaltic magma may have formed the primitive enclave. Simultaneously, the replenishing basaltic magma entrained small amounts of the dacitic magma, producing a hybrid basaltic andesitic magma layer at the base of the chamber. The quenched part of the hybrid layer was preserved as the hybrid enclave. Finally, the simultaneous ascent of the dacitic magma and liquid interior of the hybrid layer through a common conduit promoted the mixing/mingling of the two magmas and caused the coeruption of diverse lava types.
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