Volatite Composition And Its Constraints On The Genesis Of The Yushigou Podiform Deposition, Qilian Orgenic Belt

The podiform chromite deposit is one of the two major primary chromite deposits in the world.It is also the main type of chromite deposit in China,and its formation has always been a concern for geologists.The results of previous studies have basically reached a consensus on the genetic structure of the deposits,but the sources of ore-forming materials and enrichment mechanisms of chromite deposits have not been well resolved.The fluids captured in minerals indicate a different environments.There are a large number of ophiolites in the northern Qilian orogenic belt,of which the Yushigou ophiolites is the most complete and has a podiform chromite deposit.The original work focused on mineral ore,solid elements and mineral inclusion.In this paper,the fluid volatiles of peridotite and chromite is used to studied,then we discussed the constraints of fluid volatiles on ore formation,Thus we obtained the following recognition:1.The results show that the fluid volatiles in the Yushigou chromite and peridotite have a different characteristics.The fluid volatiles in the peridotite have two peaks:between 400⁸00°C and 800¹200°C.However,the chromite has no obvious phase,mainly released in the interval of 400¹000°C.The chemical composition of the fluid volatiles is mainly H₂O?the content is 2221.95mm³/g?,following CO₂ and H₂.It maybe indicated that chromite and peridotite have different evolution processes.2.?¹³C_CO2 in chromite ranges from-26.39 to-7.49‰,and?¹³Cc_H4 ranges from-44.23 to-3.47‰.The?¹³C which has a large variation indicates that the fluid volatiles not only from mantle carbonaceous but also the shell source carbon.3.The results by the method of vacuuming crush show that the noble gas have a large varition.It is shown that the isotope of them has the characterics of atmospheric saturated fluid?ASF?and continental crustal?CC?and upper mantle?UM?.Besides,the²¹Ne/²²Ne-²⁰Ne/²²Ne is distributed between the upper mantle?UM?and the continental crust evolution line?CC?,indicating that the ore-forming fluid is not only controlled by the mantle and crust,but also the atmospheric saturated fluid.4.The increasment of H₂O-CO₂ fluid component may be related to fluid displacement in the subduction zone.The precipitation of ore-forming Cr elements,accumulation of chromium droplets and the formation of large ore pulp may be controlled by the high content of water in the podiform chromite ore.The fluid?H₂O?not only promotes the melting of the mantle peridotite,but also participates in the mineralization,and may also act as a transport carrier for the aggregation of the chromium droplets.