The Geological And Geochemical Characteristics Of The Bainaimiao Copper Deposit Of Inner Mongolia And Its Geological Significance

The Bainaimiao copper deposit of Inner Mongolia is located in the copper polymetallic belt of the Ondor Sum Caledonian accretion zone in Xing'an-Mongolia orogenic belt. The deposit is comprised by the northern and the southern ore belt. The host rock of the southern ore belt is greenschist while the orebodies in the northern belt mainly occurred in the Caledonian granodiorite porphyry. Because of its complicated process of mineralization, the metallogenesis of this deposit is rather controversal and the study of this deposit is of great importance.The metallogenic process can be divided into early, middle and late stages, which respectively characterised by quartz-pyrite, quartz-chalcopyrite, molybdenite and quartz,calcite, galena, sphalerite. The ?18OH2O and ?D ratios of fluid inclusions within quartz separates range-3.2‰~5.5‰ and-94.2‰~-69‰. Along with mineralization carrying on,?D ratios were almost invariant, while ?18OH2O were gradually decreased. Similarly, The?13CPDB and ?18OSMOW of hydrothermal carbonate minerals range-5.4‰~-2.4‰ and-3.1‰~10.9‰. The ?13CPDB kept steady and the ?18OSMOW gradually reduced during the process of mineralization. The ?34S values of the sulfide range from-0.6‰ to-6‰. The C-H-O-S isotopic compositions from the south and the north ore belt show that the fluid at different ore forming stages mainly come from magma and mantle system and there is no obvious difference between them. However, the fluid from the south ore belt has more close connection with meteoric water and crust substances.The content and ratio of Co and Ni in this deposit are relatively high, which indicate that the Bainaimiao copper deposit has the property of magmatic hydrothermal deposit and VHMS transformed and enriched by metamorphism. The result of EPMA study show that there is no obvious difference in the mineralization process of the south and the north ore belt. However, the ore-forming process in north ore belt is more complicated, which is consistent with the C-H-O-S isotopic compositions.The present paper is the first time to report accurate Re-Os isotopic data acquired from 7 molybdenite separates in this deposit. The Re-Os isotopic model age ranges from(439.6±6.4) Ma to(450.0±6.6) Ma and weighted average age is(446.5±3.5) Ma. The Re-Os isotope analysis also gives an isochronal age of(451.2±3.5) Ma and the MSWD is1.3, which shows that large-scale mineralization occurred in Late Ordovician. The Bainaimiao copper deposit is the product of Caledonian magmatic and tectonic evolution along with its fluid activity.Combined with geological features of the deposit, the authors conclude that the deposit, based on primitive exhalative sedimentary mineralization, is a porphyry copper deposit engendered by island arc granodioritic porphyry under subduction environment.Then the Bainaimiao copper deposit was formed.