New evidence from a calcite-dolomite carbonatite dyke for the magmatic origin of the massive Bayan Obo ore-bearing dolomite marble, Inner Mongolia, China |
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| Authors: | M J Le Bas Yang Xueming Rex N Taylor B Spiro J A Milton Zhang Peishan |
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| Institution: | (1) Department of Geology, University of Leicester, Leicester, UK;(2) INCO Exploration, Copper Cliff, Ontario, Canada;(3) National Oceanography Centre, University of Southampton, Southampton, UK;(4) Department of Mineralogy, Natural History Museum, London, UK;(5) Institute of Geology, Chinese Academy of Sciences, Beijing, China |
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| Abstract: | Summary New data on Sr and Nd isotope composition and major and trace element distribution in dolomite-calcite carbonatite dykes at
Bayan Obo are provided, and a Mid-Proterozoic age is deduced. The dykes and the neighbouring massive dolomite (H8) body have
similar geochemical characteristics, interpreted to indicate a carbonatitic magmatic origin. The occurrence of riebeckite-bearing
fenitized quartzites marginal to both dykes and H8 dolomite body, and the presence of xenoliths in the latter, supports this
conclusion. Taken together with previously published stable isotope data, these data confirm a mantle-derived origin for the
H8 body.
The oxygen isotope composition of the dolomite and magnetite in the dykes is lower than that in the fine-grained dolomite.
Oxygen data from samples of the coarse-grained dolomite host are either similar to the dykes or to the fine-grained type in
agreement with their other geochemical characteristics. The carbonate-magnetite thermometric pairs of the fine-grained dolomite
indicate a range of 350–540 °C, which is probably lower than that of the original main magmatic emplacement. This supports
the distinction made between the original coarse-grained dolomite marble and dyke composition from the later fine-grained
dolomite.
Thus the large H8 dolomite is interpreted as a carbonatite intrusion that contains wall-rock xenoliths and caused fenitization
of the hanging wall, foot wall and the xenoliths, and that the coarse-grained portions of the H8 marble are those portions
that, in the Late Proterozoic to Palaeozoic, escaped recrystallization to fine-grained dolomite and subsequent REE-Fe mineralization.
Currently at National Oceanography Centre, University of Southampton, Southampton, UK
An erratum to this article is available at . |
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