Mafic-ultramafic Metallogenic Prognosis In Inner Mongolia, Northeast Erenhot, The Great Lakes Region

Through the study of characteristics of petrology, major elements, trace elements and rare earth element(REE) of mafic-ultramafic rocks samples retrieved from the DaHu area , Inner Mongolia together with the comparison between our results and previous published results of similar deposits in China suggest a metallogenic potential of the mafic-ultramafic rocks in the DaHu area. The achievements made in this paper are as fellows: (1)The main rock types are pyroxene peridotite and hornblende gabbros of which the pyroxene peridotite distribute in the South zone while the hornblende gabbros distribute in Middle zone. Theω(MgO) value of the hornblende gabbros range from 3.79% to 7.85% with the average value of 5.39%. Meanwhile, theω(Al2O3) value range from 16% to 19% with most data located between 15.44% and 18.34% on the chart. The value ofω(TiO2)range from 0.72% to 1.74% with an average value of 1.18%. The average value ofω(MnO)andω(CaO) is 0.5% and 10%, respectively. Datas above indicate that the rocks are tholeiitic series. Petrological Mix Indexδis less than 3 with an average value of 2.2. Theω(SiO2)value range from 49% to 51% whileω(Na2O)<ω(K2O).Theω(K2O+Na2O)value is <4%. All datas suggest that the rocks is calc-alkaline gabbro. Trace element analysis show high content of Ba, low content of Th, remarkable depletion of U,Nb and Ta. REE distribution pattern show a relatively smooth negative slope with no obvious Eu anomaly. The average REE value range from 1059.37ppm to 1743.75ppm with an average value of 1464.58ppm.The light REE have some enrichment comparing with heavy REE. (2) The metal mineral assemblage is chalcopyrite - pyrrhotite– pentlandite, and the Ni content of whole rock range from 15.6ppm to 136.8ppm, mainly 25ppm in Hornblende gabbro. Pyroxene peridotite Mineralized Zone have a metal mineral assemblage of pentlandite-pyrrhotite-chalcopyrite in depth of over 100m underground and violarite-millerite-polydymite near the surface. Metal elements exist in the Mineralized Zone include copper, nickel and cobalt, Ni content is up to 0.245%. Mafic-ultramafic rocks in Dahu area occur similar geochemical characteristics with that bearing Cu-Ni-(PGE) deposits in East Tianshan.and MgO-Ni diagram shows that there is a sulfide liquid segregation during the magma evolution in the research area. Geochemical analysis of mafic - ultramafic rocks in our research area show certain characteristics of plate tectonic rift enviroment. The mantle source of mafic-ultramafic rocks may be depleted mantle.