The Arinem deposit is an epithermal–type, gold–silver–base metal mineralization, hosted by the Arinem vein system which cross–cut volcanic rocks of Oligocene to Miocene age. There are three main stages (I to III) of ore mineralization in quartz–illite–calcite veining. The physico–chemical conditions and the origin of the hydrothermal fluids in the Arinem deposit are examined. We find the fS2 to have decreased from ~ 10−8 to 10−16 atm and the fTe2 to have increased from ~ 10−15 to 10−8 atm with decreasing temperature of ore fluids during mineralization. Oxygen (δ18O), hydrogen (δD), and sulfur (δ34S) isotopic values as derived from quartz, fluid inclusions, and sulfides in the mineralized veins range −6.0 to −1.7‰, −66 to −34‰, and −3.6 to −1.8‰, respectively. The sulfur isotope values indicate that sulfur was mostly derived from a magmatic source, whereas the oxygen and hydrogen isotope values suggest an influx of meteoric water into the hydrothermal system. As a consequence of the mixing of magmatic and meteoric waters and changes in the physico–chemical conditions of the ore fluids (i.e., ore fluid temperature and chemistry), higher temperature mineralization was triggered during stage I, followed by lower temperature mineralization during stage II. Continued influx of meteoric water may have caused the ore fluids to become oversaturated, resulting in the precipitation of unmineralized stage III veins.
The constituent and formation process of the Martian surface soil are fundamental problems to understand the evolution and current environment of the Mars. At the Martian volcanoes, iron–rich basaltic rocks should be subject to the hydrothermal alteration by sulfuric acid–bearing solutions. In this work, we carried out alteration experiments of the synthetic iron–rich basaltic material simulated the typical Martian basalt to elucidate the soil formation processes on the Martian surface. In the run products at 100 °C with initial concentration of 0.1N sulfuric acid solution, quite characteristic, snowflake–like hematite fine particles of submicrometers in diameter were produced continuously from 7 days to 112 days. In the run products at longer than 28 days at 100 °C and all run durations at 150 °C, initial precipitation of hydrous and low–crystalline iron oxide minerals form nuclei for crystalline aggregates of hematite with several micrometers in diameter. The ferric iron oxide particles are not produced with 0.001N sulfuric acid solutions and distilled water. Ferrous iron in olivine and glass phase can be oxidized to ferric iron by reaction with sulfuric acid in the low temperature hydrothermal conditions in anoxic conditions. Hematite particles produced in this study are small enough to be blown up by the Martian winds to disperse from alteration sites in volcanic regions to the whole planet. The iron oxide minerals which brought reddish color of the Martian dust may be products of sulfuric acid–bearing hydrothermal alteration with volcanic activities rather than a low temperature weathering related to the ancient Martian climate.
Adachiite, CaFe3Al6(Si5AlO18)(BO3)3(OH)3(OH), a Si–poor member of the tourmaline supergroup, is found in the hydrothermal vein cutting emery from Nabagasako of the Kiura mine, Saiki City, Oita Prefecture, Japan. Adachiite occurs as a constituent (several to 300 µm thickness) of hexagonal prismatic crystals, and forms a zoned structure closely associated with schorl. It is transparent with brownish–purple to bluish–purple color, while the massive aggregate shows black in color. Adachiite is trigonal, R3m, a = 15.9290(2), c = 7.1830(1) Å, V = 1578.39(4) Å3, and Z = 3, determined via single crystal XRD refinement (R1 = 0.038). Adachiite is the first member of the tourmaline group formed via Tschermak–like substitution.
Rocks of the dunite–clinopyroxenite series (dunite, wehrlite, olivine clinopyroxenite, and clinopyroxenite) are common as cumulates formed around Moho in arc–related environments, but their formation processes remain unclear. They are common as xenoliths of Group I from Takashima in the Southwest Japan arc, and a description of their formation process is provided here. The rocks vary from dunite to clinopyroxenite via wehrlite and olivine clinopyroxenite, and all showing mosaic equigranular to weakly porphyroclastic textures. The rocks are completely free from plagioclase, and they contain <3 vol% chromian spinel. Some of them contain up to 3 vol% orthopyroxene; these are approximate mixtures of olivine and clinopyroxene. As the dunites change to clinopyroxenites, the Mg# [= Mg/(Mg + total Fe) atomic ratio] varies from 0.93 to 0.84 in olivine, and from 0.92 to 0.87 in clinopyroxene. The Cr# [= Cr/(Cr + Al) atomic ratio] of the chromian spinel varies from 0.8 to 0.2 with decreases in the Mg# of olivine and clinopyroxene. The Mg–Fe distribution relation between olivine and clinopyroxene suggests their subsolidus equilibration is around 800–900 °C. Initial Mg#s expected at a magmatic temperature indicate that their formation proceeded from magma in the order of Mg–rich dunites followed by clinopyroxenites and then less Mg–rich dunite–wehrlite–olivine clinopyroxenite. This suggests a zigzag liquid path, starting from a mantle–derived olivine–oversaturated magma, around the olivine–clinopyroxene cotectic boundary. Continuous crystallization of olivine or clinopyroxene due to supersaturation could have enabled the magma to straddle the cotectic boundary to form alternately clinopyroxene– and olivine–oversaturated magmas.
Gabbroic xenoliths from Ichinomegata crater, Northeast Japan arc, contain a large amount of amphiboles. Some of them are obviously of secondary origin in hornblende–pyroxene gabbros; clinopyroxene in the pyroxene–spinel symplectite, which is subsolidus reaction products of olivine and plagioclase by cooling, was replaced in part with amphibole. Minerals (amphibole, clinopyroxene and spinel) in the symplectite contain appreciable TiO2 (up to 1.6 wt%), although olivine and plagioclase are free from TiO2. The amphibole in the symplectite is significantly enriched in Na, K, Rb and Ba relative to the other symplectite minerals that contain very low amounts of these elements. The metasomatic formation of amphibole in the symplectite was accompanied with addition of, at least, Ti, Na, K, Rb and Ba. The metasomatism is characterized by enrichment of incompatible elements, especially LILE but not HFSE (Nb, Ta, Zr and Hf). This was caused by infiltration of fluid possibly released from hydrous arc magmas. Metasomatic processes observed in the lower crustal gabbros are also recorded in peridotite and websterite xenoliths of upper mantle origin from Ichinomegata. This means widespread modification of mineral chemistry and mineral assemblage from the upper mantle to the lower crust beneath the arc.
We observed the breakdown of natural garnet in a high–pressure experiment (0.85 GPa and 1100 °C for 2 days) using a piston–cylinder apparatus. Two different types of kelyphites were formed: (1) isochemical and (2) metasomatic. The isochemical kelyphite was composed of orthopyroxene + spinel + plagioclase, whereas the metasomatic one, having a composition altered significantly from the original garnet, was composed of olivine + spinel + plagioclase. These synthetic kelyphites were studied and characterized using EPMA, SEM, EBSD, and TEM and were shown to have many microstructural features as well as their mineral assemblages in common with observed natural kelyphites. The kelyphitization was very heterogeneous and local and was apparently controlled by grain surfaces and intra–grain cracks in garnet. The metasomatic kelyphite apparently formed by a reaction between garnet and aqueous fluids that may have been introduced from outside the platinum capsule through holes accidentally made in the capsule.
The Maizuru Belt in southwest Japan and the Khanka Massif in Far East Russia include both Siluro–Devonian and Permo–Triassic granitoids. In order to elucidate the simultaneity of granitoid magmatism in the Maizuru Belt and the Khanka Massif, we investigated zircon U–Pb ages using LA–ICP–MS for granitoid samples from the Maizuru area in the northern zone of the Maizuru Belt and from the Vladivostok area in the southernmost part of the Khanka Massif. Five granitoid samples from the Vladivostok area yielded ages of 422.2 ± 2.5 Ma, 260.7 ± 3.1 Ma, 301.7 ± 2.4 Ma, 249.7 ± 3.5 Ma, and 431.9 ± 2.7 Ma. Additionally, a porphyry sample yielded an age of 423.7 ± 3.2 Ma. Four granitoid samples from the Maizuru area yielded ages of 291.6 ± 4.3 Ma, 443.8 ± 4.1 Ma, 279.7 ± 2.4 Ma, and 259.0 ± 3.0 Ma. In reference to the coexistence of the Triassic sedimentary sequence and Siluro–Devonian and Permo–Triassic granitoids across the Sea of Japan, it seems that there were strong relationships between the northern zone of the Maizuru Belt and the southernmost part of the Khanka Massif. Our data provide additional evidence to support a geological connection between southwest Japan and the Vladivostok area before the Miocene opening of the Sea of Japan.
Montebrasite and amblygonite in an Li–Cs–Ta enriched (LCT) pegmatite from Nagatare, Fukuoka Prefecture, Japan, contain various alteration minerals: fluorapatite, crandallite, goyazite, waylandite, wardite, viitaniemiite, morinite, muscovite, lepidolite, and cookeite. They are associated with lacroixite, quartz, and topaz. Among these minerals, wardite, viitaniemiite, morinite, and lacroixite are newly discovered in Japan. The secondary phosphates and fine–grained mica form fine veins along cleavages and composition planes of polysynthetic twins in the montebrasite and amblygonite. Lacroixite has a different texture from other phosphates, which suggests a possibility of exsolution within montebrasite–amblygonite series. Various secondary phosphates show Ca–, Na– and Sr–metasomatism with leaching of Li, and the formation of low–F montebrasite from montebrasite–amblygonite series indicates an F–OH exchange. However, fluorapatite, morinite, and viitaniemiite crystallized in an F–rich environment. Montebrasite–amblygonite series minerals undergo an acidic alteration to muscovite in the last stage, which is the same process that other Li minerals undergo, such as tourmaline and spodumene.
Vol.31 (1944) No. 5 and No.6 in the predecessor journal ″The Journal of the Japanese Association of
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