Mineralization Of Pb-Zn In Dongmozhazhua And Mohailaheng Pb-Zn Deposits, Zaduo Area, Qinghai Province

Dongmozhazhua and Mohailaheng Pb-Zn deposits locate in Yushu area on the eastern and northern margins of Tibetan orogenic belt and are controlled by thrust-nappe structures.They are typical Pb-Zn deposits which occured in thrust belt.Based on detailed research of the two deposits,the writer gives the her understanding of the deposits' geology,characters and sources of the ore fluid,and ages of the two deposits in this paper.And finally give the type of the two deposits and the metallogenic model of Dongmozhazhua and Mohailaheng Pb-Zn deposits.The host rocks of Dongmozhazhua deposit are limestones in Bolila Formation,late Triasssic and in Gadikao Formation,early-middle Permian.The host rocks of Mohailaheng deposit are limestones in carbonate Formation,Zaduo Group,early Carboniferous.Ore bodies in the two deposits are both stratiform or strata-bound and controlled by thrust faults.Sphalerite,galena and pyrite are the major sulfide minerals and marcasite,chalcopyrite and tennantite are the minor sulfide minerals in the two deposits.The gangue minerals are dolomite,calcite,barite,quartz,fluorite and sericite.Based on analyses of sulfide minerals by EPA,it is found that the strongest activities of ore fluid occurred near NO.1101 exploratory trench in Mâ… mineralization belt in Dongmozhazhua deposit and near NO.33 exploratory trench in Mâ… mineralization belt in Mohailaheng deposit.The ore fluid in Dongmozhazhua deposit was hign salinity(22～28 wt%NaCl) and low temperature 100.3～140.8℃) and the ore fluid in Mohailaheng deposit was middle to high salinity(19～27w%tNaCl) and middle to high temperature(200～380℃).One of the sources of the ore fluid for the two deposits was evaporated seawater which were kept in the carbonate stratums.The system of ore fluid in the two deposits were Ca²⁺～Mg²⁺～Na⁺～Cl^-～SO₄^2-～F^- brine system and some reductive gases,like CH₄,are found by gases analysis for group fluid inclusions.Dongmozhazhua and Mohailahen Pb-Zn deposits have similar C,O isotopic compositions,and the C of the ore fluids mainly came from the dissolution of carbonate rocks.The Ore fluid of the two deposits has similar H,O isotopic compositions,too,the water of the Ore fluid came from the hot brine in the basin and mixed with some meteoric water.The sulfides of the two deposits also have similar S isotopic compositions,and have the characteristic of light sulfur isotope concentration.The S mainly came from biogenic sulfate reduction or some organic substance contained S.The results of analysis of REE in calcites in the two deposits indicate the Ore fluid of the two deposits had the same source,and were reductive.The age of Dongmozhazhua deposit is 35.015±0.034Ma and it is older than Mohailaheng deposit which age is 32.22±0.36 Ma or 31.75±0.28 Ma.Compared Dongmozhazhua and Mohailaheng deposits with Pb-Zn deposits in Tuotuo river basin,which are on the west of the two deposits,and Pb-Zn deposits in Lanpin Basin,which are on the east of the two deposits,it is found that all Pb-Zn deposits have the same geological backgrounds,regional ore controlled structures and similar ore-forming ages,so it is may be a huge Pb-Zn ore belt,which is nearly 1000km and controlled by thrust-nappe system,on the eastern and northern margins of Tibetan plateau.Combined with all the understanding above,it is found that it was the same ore-forming progress in the two deposits,and the depth of Mohailaheng deposit were larger than the depth of Dongmozhazhua deposit when they were formed.Compared the two deposits with the similar Pb-Zn deposits in the world, the two deposits could be typed in like-MVT Pb-Zn deposit.In the end the metallogenic model of Dongmozhazhua and Mohailaheng Pb-Zn deposits is summarized.