<報文>中国金川鉱床産ニッケル-銅鉱石の構成鉱物とその組織

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  • チュウゴクキンセン コウショウサン ニッケル ドウ コウセキ ノ コウセイ コ
  • <Original Papers>Constituent Minerals and Textures of Nickel-Copper Sulfide Ores from the Jinchuan Deposit, China

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Magmatic nickel-copper ores from the Jinchuan deposit, Gansu Province, China, were analyzed mineralogically with the aid of optical microscope, EPMA, X-ray diffractometer and AAS. The deposit occurs in the Jinchuan ultramafic intrusive body, one of mafic-ultramafic rock bodies which intruded into the pre-Cambrian metamorphic rocks in the Caledonian, along the second order fractures derived from a deep fracture. This developed in the north of the west edge of the Alashan uplifting within the North China Metaplatform. Ores are classified, on megascopic textural characteristics, into three types, sideronitic, spotted and disseminated ores. They almost always contain pyrrhotite (Fe<1-x>S), pentlandite ((Fe, Ni)_9S_8), chalcopyrite (CuFeS_2) and serpentine (Mg_6Si_4O<10>(OH)_8) as common major ore constituting minerals. Mackinawite (FeS) and cubanite (CuFe_2S_3) often appear as exsolution product from earlier sulfide minerals. Violarite (FeNi_2S_4) and smythite (Fe_9S_<11>) occur as alteration product from pentlandite. A small amount of bornite (Cu_5FeS_4) and pyrite (FeS_2) can be found in some of disseminated ores. Sideronitic ore, composing the main parts of the rich ore bodies, shows textures typical for magmatic nickel-copper sulfide deposits. Sulfides are occupying and cementing grain boundaries of such gangue minerals as olivine, pyroxene, etc. or their altered minerals. Occasionally, gangue minerals maintain their primary crystal form completely. In sideronitic ore, sulfides usually contain many contraction cracks which are filled with serpentine, suggesting that the cooling of the sulfide melt proceeded very rapidly. Contraction cracks become more narrow in spotted ore than in sideronitic ore and they are rarely found in disseminated ore. Pyrrhotite group minerals in sideronitic ores include troilite (FeS), hexagonal (Fe_<1-x>S) and monoclinic (Fe_7S_8) pyrrhotites, but those in spotted and disseminated ores are dominated overwhelmingly by monoclinic pyrrhotite. Troilite and hexagonal pyrrhotite rich in iron often contain "flame" pentlandite exsolved from high-temperature pyrrhotite phase with decrese in temperature. The "flame" penlandite cannot be found in monoclinic pyrrhotite. In contrast to this, monoclinic pyrrhotite contains more nickel in its structure than troilite or hexagonal pyrrhotite. The nickel content in monoclinic pyrrhotite ranges from 0.05 to 0.20 weight percent, while usually, that in troilite or hexagonal pyrrhotite is less than 0.0n weight percent and hardly exceeds 0.03 weight percent. Ordinary pentlandite contains 25 to 27 atomic percent of nickel regardless of ore type, but nickel content in "flame" pentlandite is around 18 atomic percent. A kind of sideronitic ore unusually rich in chalcopyrite includes pentlandite containing 0.5 to 1.3 weight percent of cobalt, but coexisting pyrrhotite or chalcopyrite does not contain cobalt in amounts detectable with EPMA. An earthy mineral found in spotted ore contains more than 10 weight percent copper and about 0. n weight percent nickel. It shows a very clear anisotropic property, suggesting it is a mineral belonging to the valleriite ([(Cu, Fe)S]_4 [(Mg, Al)(OH)_2]_3)-haapalaite ([(Fe, Ni)S]_4 [(Mg, Fe)(OH)_2]_3) family. This mineral must be "bokudokou", ([CuFeS_2][(Mg, Al, Fe)(OH)_2]), so called in China. Some disseminated ores contain garnierite ((Mg, Ni)_6Si_4O_<10>(OH)_8) whose nickel-content ranges from 0.5 to 1.0 weight percent, although usual nickel-content in serpentine does not exceed 0.0n weight percent.

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