Anoxic Ocean inhibited the formation of porphyry copper deposits

Abundant porphyry copper deposits (PCDs) are distributed in the Central Asian Orogenic Belt (CAOB), but the majority are concentrated in the Circum-Balkhash and Tuva-Mongolia micro-continents (BM), limited deposits have been identified in the Circum Junggar Collage (CJC), the mechanisms that drive uneven distributions of PCDs remain controversial. We provide a compilation of published elemental and isotopic data of magmatic rocks from the CAOB, which show that the CJC arc rocks possess lower Cu concentrations, V/Sc and Sr/Y ratios, and zircon ∆FMQ values, but higher zircon δ¹⁸O values compared to the BM rocks, demonstrating the CJC rocks has lower porphyry ore potential and magmatic oxidation states that were triggered by the addition of reduced sediments. Substantial biogenic sediments within the accretionary complex belts in the CJC imply widespread anoxia of the Junggar ocean due to isolation from wider Paleo-Asian Ocean, which promoted the generation of reduced arc magmas and further inhibited the formation of PCDs. The arc magmatic oxygen fugacity shift indicates that the ocean redox state was recorded in the deep Earth and significantly control the generation of PCDs in arcs.

Acknowledgements: The present study was financially supported by Hong Kong Research Grant Council Joint Laboratory Funding Scheme (JLFS/P-701/18), NSFC Major Project (41890831) and Hong Kong RGC grants (17307918 and 17308023), and HKU Internal Grants for Member of Chinese Academy of Sciences (102009906) and Distinguished Research Achievement Award (102010100). This work is a contribution of the Joint Laboratory of Chemical Geodynamics between the University of Hong Kong and Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.