Magmatic-hydrothermal Redox State and Ore-Controlling Factors for the Yangla Skarn Cu Deposit, Sanjiang Region, SW China

In the Paleo-Tethyan orogenic belt, reduced mineralization systems result in extensive saturation and segregation of magmatic sulfide phases, which in turn leads to the loss of chalcophile elements and hinders the precipitation of large amounts of copper. As a result, porphyry Cu deposits are rare within the belt. In this contribution, we investigate the composition of zircons from the granodiorite, fluid inclusions and hydrothermal scheelite from orebodies of the Yangla Skarn Cu deposit, and assess the magmatic-hydrothermal redox state of the system. The igneous zircons from ore-causative granodiorite have negative Eu anomalies (EuN/EuN* = 0.27 f 0.03, n = 11) and the calculated Delta FMQ values are -0.23 f 0.31 (n = 11). The ore-forming fluids in the pre-ore stage was rich in CH4, and precipitating hydrothermal scheelite in the syn-ore stage with low Mo contents (Mo =17.4 to 149 ppm, mean 57.7 ppm) and positive Ce anomalies (CeN/CeN* = 1.14 f 0.06, n = 29). The geochemistry suggests that the magmatic-hydrothermal system at Yangla is relatively reduced, consistent with the abundant development of pyrrhotite in the ore body. In addition, we have carried out zircon SIMS U-Pb dating and trace analysis of the barren quartz porphyry in the Yangla ore field, which gives an emplacement age of 227 Ma and is also derived from reduced magmas (Delta FMQ = -1.27 f 0.55; EuN/EuN* = 0.32 f 0.10, n = 7) that were less fertile for porphyry Cu formation. By comparing with the barren quartz porphyry, we suggest that the key control of the skarn ore body may have been that the low-permeability of carbonate units seal the mineralizing fluids, enhancing fluid-carbonate reactions and concentrating the sulfides in a relatively small area. Without these processes, it can only form barren porphyry system. We highlight that, unlike typical porphyry Cu deposits, intense fluidcarbonate rocks interaction and fluid focusing could result in skarn Cu formation, even though in relatively reduced magmatic-hydrothermal conditions.