Geochronological and geochemical study on the Yulong porphyry copper ore belt in eastern Tibet, China

Abstract

The Yulong copper belt, along part of the Red River-Ailao Shan fault system and its northwestern extension in eastern Tibet, consists of five porphyry pipes that contain a total copper resource of over 8 million tons. The porphyries are characterized by high alkali content (K2O+Na2O > 6%), K2O/Na2O > 1, and marked negative Ti, Ta and Nb anomalies on mantle-normalized incompatible element diagrams. U-Th-Pb laser ICP-MS dating of zircons from the Yulong porphyries showed that they were emplaced over a 4.3 Ma period and that they Young systematically from northwest to southeast as follows: Yulong, 41.2 +/- 0.2Ma; Zalaga, 38.5 +/- 0.2Ma; Mangzong, 37.6 +/- 0.2 Ma; Duoxiasongduo, 37.5 +/- 0.2Ma; and Malasongduo, 36.9 +/- 0.4 Ma. We suggest that the source of the shoshonites was lower crust that was pushed into the mantle by the compressive component of transpressional movement on the adjacent Tuoba-Mangkang fault and that the compositional variation in the porphyries is due to mixing between magmas of different composition, generated by different degrees of partial melting of a heterogeneous source region.