Characteristics Of Shell Dunite Of The Luobusha Deposit And Their Implications For Fluid Processes,Tibet

Luobusha deposit is a typical podiform deposit in ophiolite belt,which is the largest chromite deposit in China.Its metallogenic mechanism is still a question.The magma channel model and melt-rock model cannot explain the observed geological phenomena.A new model of ophiolite podiform deposit must be set up.One of the typical characteristics of podiform deposit the shell dunite,which existing only at the edge of the ore body.The shell dunite is closely related to metallogenic mechanism,but the research of the shell dunite has not been paid much attention.The key to set up the new metallogenic model of ophiolite podiform deposit is the origin of the shell dunite.The field observation showed a boundary between the ore body and the shell dunite,but the boundary between shell dunite and harzburgite was not clear.The serpentine veins were widely developed,and there were variations of pyroxene content longitudinally.We divided the section into 6 belts to sample closely,carried out a detailed geochemical and petrographic study.Geochemical characteristics of whole rock indicated mantle peridotite characteristics.On the basis of detailed lithofacies observation under the microscope,the electronic probe composition profiles of typical minerals in different facies belts were analyzed,and the original minerals and reactive minerals were identified.The composition profiles showed there were Mg-Fe exchange reactions among minerals,and the solution-precipitation reaction equation was established.According to the habit of crystallization,the original crystal group,the ore forming crystal group and the solution-precipitation crystal group were identified.The mineral thermometer showed that the consolidation temperature of the harzburgite is 990 centigrade.The non-ore belts and ore body belts had different temperature ranges,and the latter had larger span.Therefore,the formation of the shell dunite is considered as a result of melt-rock reaction.The pyroxene in primitive harzburgite reacts with fluid to form clinopyroxene and olivine.The clinopyroxene is taken away by the fluid,while the olivine produced by reaction compact with primitive olivine to form dunite.The degree of the reaction is higher near the ore body to form the shell dunite.