Genesis of the Dachang Sn-polymetallic and Baoshan Cu ore deposits, and formation of a Cretaceous Sn-Cu ore belt from southwest China to western Myanmar

Large scale Cretaceous magmatism and SnCu mineralization occurred in an E-W-trending ore belt from southwest China to western Myanmar. We investigated the genesis of the world-class Dachang Sn-polymetallic and newly discovered Baoshan Cu deposit in Guangxi Province, southwest China, and the tectonic setting of this belt. The Dachang ore field contains a giant vein ore body (No. 100) and stratiform ore bodies (No. 91 and 92). The cassiterite U-Pb age (92.5 +/- 4.4 Ma) of the No. 100 ore body is similar to the zircon. U-Pb age of the Longxianggai biotite granite (90.8 +/- 1.0 Ma), suggesting the, vein ore body was related to Cretaceous magmatism. The fluid inclusions in the No. 92 ore body are similar to those in the No. 100 ore body, and contain CO2-H2O, liquid-rich and halite-bearing fluid inclusions, with homotenizaiion temperattires of 300-410 degrees C. These data further suggest that mineralization was related to Cretaceous magmatism. The mineralization in the Baoshan Cu deposit occurs as veins in the Baoshan porphyry and adjacent country rocks. The Baoshan porphyry yielded a zircon U-Pb age of 90.6 +/- 1.3 Ma, Eu-N/Eu* = 0.05, Ce-N/Ce* = 12.2 and epsilon(Hf)(t) = -11.39 to -7.37. These data indicate that the porphyry magma was reduced and crustally derived. Low Sr/Y ratios (< 0.1) and large negative Eu anomalies (Eu-N/Eu* = 0.03-0.06), along with rare earth element tetrad effects further imply the porphyry magma was highly fractionated. The Baoshan porphyry was genetically associated with Cu mineralization, but is distinct from the typical Cu-mineralizing porphyry magmas. This Cretaceous Sn-Cu ore belt was likely related to subduction and roll-back of the_Neo-Tethys Plate rather than northwestward subduction of the Paleo-Pacific Plate.