Литосфера (Oct 2016)
Sr-Nd Systematics and REE distribution in the type magnesite deposits in Lower Riphean of South Urals province
Abstract
The geochemistry and REE of magnesite and host carbonate rocks of two Lower Riphean deposits of the South Urals province (Satka and Ismakaevo ore fields) confirm the metasomatic nature of the magnesite. The magnesites of both deposits are enriched in heavy lanthanides, in contrast to the host carbonate rocks. Ore dolomites have positive Eu anomaly, which is higher in Ismakaevo ore field. According to the Sr-Nd isotope data, formation of these magnesite types in the South Urals province has been related to different stages of regional tectonic activation and thermal fluid migration: the beginning of the Middle Riphean (Mashak rifting event) for Satka and the middle-end of the Middle Riphean for Ismakaevo. The Sr and Nd isotopes indicate on prevailing crustal fluid source. However, the fluid into Satka ore field contained additional admixture of mantle component. It is confirmed by the high average value of εNd (-5.0) in the magnesite in comparison to the εNd of host carbonate rocks (-7.0). In contrast, crustal fluid dominated in Ismakaevo ore field, where εNd in magnesite ranges from -11.3 to -9.0, and it lower than in limestone (in average -6.4). Moreover, the Ismakaevo metasomatic dolomites and magnesites are enriched in radiogenic87Sr and Fe in compared to the Satka magnesite. These features indicate more intensive interaction of ore-bearing fluids with the host alumosilicate rocks. The reason of difference was the geological position of objects in regard to location to Mashak rift. The Satka ore field was located within the area of development of Mashak rift magmatic rocks at the same stage. Ismakaevo magnesite was formed to the west of Mashak rift zone at later tectonic stage.
