In Situ Mineralogical Constraints on Magmatic Process for Porphyry Deposits in the Upper Crust: A Case from Tongchang-Chang'anchong Porphyry Deposits, SW China

The magmatic process within upper crust encompasses various contents such as the transition between magmatic and hydrothermal systems and changes in oxygen fugacity (fO(2)), which ultimately play key roles in the formation of porphyry Cu deposits (PCDs). However, tracing these magmatic processes, especially in porphyry systems, is not an easy task. This study reported the detailed process of magmatic fluid exsolution and systematical variation of magmatic fO(2) within the upper crust of a Tongchang-Chang'anchong porphyry Cu deposit, based on detailed investigations of mineral crystallization sequences and compositional features of the minerals in the fertile porphyries. Results indicate that the fertile porphyries show a high initial fO(2), with Delta FMQ >= +3.0 (?FMQ is the deviation of logfO(2) from the fayalite-magnetite-quartz (FMQ) buffer). The magmatic fO(2) (Delta FMQ) continued to decrease to similar to+2 until fluid exsolution occurred at similar to 790 ? due to wall-rock contamination. The magmatic fluid exsolution process caused a temporary increase in the fO(2) (to Delta FMQ = similar to+3.4). The high magmatic fO(2) during this process (790-750 degrees C) resulted in a higher content of ore-forming materials in the exsolved magmatic fluid. When the temperature dropped below 750 degrees C, the magmatic fO(2) began to continuously decrease and eventually reached Delta FMQ = similar to+0.6. The lower magmatic fO(2) hindered the further migration of ore-forming materials through the exsolved fluid during this process (< 750 degrees C). Results of this study indicate that the initial magma during the upper crustal magmatic process of PCDs generally has a high fO(2), and the contamination of reduced components can significantly decrease the magmatic fO(2). The early magmatic fluid exsolution process can maintain a high magmatic fO(2) condition, thereby efficiently extracting ore-forming minerals and producing ore-forming fluids, which is the key to the formation of PCDs. The latter continuous decrease in magmatic fO(2) during the fluid exsolution process may be the reason preventing the Tongchang-Chang'anchong porphyry Cu deposit to form a giant PCD.