Rb-Sr and Sm-Nd Isochron Ages of the Dongmozhazhua and Mohailaheng Pb-Zn Ore Deposits in the Yushu area,southern Qinghai and Their Geological Implications

Located on the northeast margin of the Qiangtang terrane between the Jinshajiang suture zone and Bangonghu‐Nujiang suture zone, the Dongmozhazhua and Mohailaheng Pb‐Zn deposits in the Yushu area of Qinghai Province are representative Pb‐Zn deposits of the Pb‐Zn‐Cu polymetallic mineralization belt in the northern part of the Nujiang‐Lancangjiang‐Jinshajiang area, which are in the front belt of the Yushu thrust nappe system. The formed environments of these two deposits are different from those of sediment‐hosted base metal deposits elsewhere in the world. The authors hold that they were formed during the Indian‐Asian continental collision and developed within the fold‐thrust belt combined with thrust and strike‐slip‐related Cenozoic basins in the interior of the collisional zone. Studying on the metallogenic epochs of these two deposits is helpful to the understanding of ore‐forming regularity of the regional Pb‐Zn‐Cu mineralization belt and also to the search for new deposits in this region. The age of the Dongmozhazhua deposit has been determined by the Rb‐Sr isochron method for sphalerite residues, whereas the age of the Mohailaheng deposit has been determined by the Rb‐Sr isochron method for sphalerite residues and the Sm‐Nd isochron method for fluorite. The age of the Dongmozhazhua deposit is 35.0±0.0 Ma ((⁸⁷Sr/⁸⁶Sr)₀=0.708807) for sphalerite residues. The age of the Mohailaheng deposit is 32.2±0.4 Ma ((⁸⁷Sr/⁸⁶Sr)₀=0.708514) for sphalerite residues and 31.8±0.3 Ma ((¹⁴³Nd/¹⁴⁴Nd)₀=0.512362) for fluorite with an average of 32.0 Ma. Together with the regional geological setting during mineralization, a possible tectonic model for metallogeny of the Dongmozhazhua and Mohailaheng Pb‐Zn deposits has been established. These two ages are close to the ages of the Pb‐Zn deposits in the Lanping and Tuotuohe basins, indicating that it is possible that the narrow 1000‐kilometer‐long belt controlled by a thrust nappe system on the eastern and northern margins of the Tibetan plateau could be a giant Pb‐Zn mineralized belt.