Control of magmatic oxidation state in intracontinental porphyry mineralization: A case from Cu (Mo-Au) deposits in the Jinshajiang-Red River metallogenic belt, SW China

The Jinshajiang-Red River porphyry Cu (Mo-Au) metallogenic belt (JRMB) is the most important intra-continental porphyry Cu (Mo-Au) mineralizing zone in the Sanjiang region, southwest China. The belt contains a number of giant deposits, including Yulong (6.50 Mt Cu) and Beiya (315 t Au) in the northern and center parts, and several small deposits in the southern part (e.g., Tongchang, 0.03 Mt Cu + Mo; Chang'anchong, 0.04 Mt Cu + Mo; Habo, 0.57 Mt Cu + Mo; and Chang'an 31 t Au). In order to investigate the mechanisms controlling the variation in size of these deposits, the LA-ICP-MS zircon U-Pb dating, bulk-rock geochemistry, and zircon trace-element analyses have been performed on the mineralization-related porphyries from the Tongchang district. Zircon U-Pb dating yielded concordant ages of 34.2 +/- 0.6 Ma (Tongchang), 33.7 +/- 0.8 Ma (Chang'anchong), 35.7 +/- 0.5 Ma (Habo) and 34.6 +/- 1.2 Ma (Chang'an). These porphyries are peraluminous with relatively high potassium contents (K2O; 4.2-5.7 wt%), and show shoshonitic affinities. Bulk rock Fe2O3/FeO ratios vary from 0.51 to 0.97, typical of moderately oxidized to strongly oxidized magmas. Zircon Ce4+/Ce3+ values vary between 25.9 and 371.8 with a mean of 129.3. The log(fo(2)) values vary from -20.7 to -9.6, and plot within the range of FMQ (fayalite-magnetite-quartz oxygen buffer) to MH (magnetite-hematite oxygen buffer), indicating an oxidizing parental magma. The mineralized porphyries from the Yulong and Beiya deposits, which were previous considered to have formed under the same tectonic conditions as those in the Tongchang district, have higher mean zircon Ce4+/Ce3+ values of 249.4 and 399.5, suggesting that the oxygen fugacities of the porphyries in the Tongchang district is relatively lower. This might imply that oxygen fugacity is an important factor that led to the differentiation of deposit size in the JRMB, and that larger porphyry deposits are associated with more oxidized magmas. (C) 2016 Elsevier B.V. All rights reserved.