| 马坑铁矿床稀土元素地球化学研究 |
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| 引用本文：.1984.Geochemistry of rare earth elements in the making iron deposit[J].Mineral Deposits,3(1):1~11 |
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| Geochemistry of rare earth elements in the making iron deposit |
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| Abstract:Geochemical studies of REE furnish further evidence for the submarine hydrothermal-sedimentary origin of Makeng iron deposit proposed previously by the authors. Basalt from ore-bearing volcanic-carbonate rock formation is very similar to oceanic ridge basalt in major elements and REE pattern. It has whole rock Rb-Sr isochron ages of 364±46 m.y., Sr87/Sr86 ratios of 0.7097±0.0017 and Sm/Nd ratios of 0.32-0.42. The slightly heightened La and Ce contents and Sr87Sr86 ratios may be attributed to alteration. The K- and Si- rich metasomatic rocks underlying the main orebody have extremely variable major elements but comparatively stable REE pattern. They are apparently enriched, in light REE as compared with the original basalt, with La/Yb increased from 2.98 to 17.64 and Sm/Nd from 0.37 to 0.19, suggesting the derivation of metasomatic fluids from the crust. During the K-metasomatism, the total REE abundances and La/Yb ratios were evidently getting higher with the increase of K2O and Al2O3, accompanied, however, by only slight variation in heavy REE, in the process of Si-metasomatism, a sharp decrease in REE abundances took place with the increasing SiO2 content. The matasomatic rocks obviously differ from Lower Carboniferous sandstone in REE composition, implying diverse origin for them. Investigation reveals that REE patterns of metasomatic rocks depend predominantly on such factors as REE patterns of original rocks and metasomatic fluids, and mineral composition of metasomatic rocks. The lower part of the deposit or the major ore body of quartz-magnetite occurring in basalt is noticeably different in REE pattern from the upper part minor ore bodies of calcite-magnetite present in carbonate rocks. The major ore body quite coincides with basalt in REE pattern except for its distinct negative Eu anomaly governed chiefly by physico-chemical conditions of sedimentary media; light and heavy REE it contained underwent no apparent fractionation with ∑La-Eu/∑Gd-Lu being 2.86 and Sm/Nd 0.32; its Rb-Sr whole rock isochron age is 346±68 m.y. and Sr87Sr86 0.7060±0.003. All this points to its intimate genetic relationship with the basalt. The calcite-magnetite minor ore bodies have extraordinarily low total REE contents but remarkably enriched light REE (∑La-Eu/∑Gd-Lu=6.72); they bear striking similarity with some iron formations of carbonate facies or even limestones in REE pattern. In contrast with the recent seawater and sea floor sediments, no marked negative Ce anomalies are observed in the Makeng iron oies. Studies show that, rather than being governed by contemporaneous sea waters, the REE pattern of the submarine hydrothermal-sedimentary iron formations are principally controlled by REE composition of ore-forming fluids, physico-chemical conditions' of sedimentary environment, sedimentary facies and some other factors. |
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