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 and hydrothermal systems changes in oxygen fugacity (ƒO2), which ultimately play key roles formation of porphyry Cu deposits (PCDs). However, tracing these processes, especially systems, is not an easy task. This study reported detailed fluid exsolution systematical variation ƒO2 a Tongchang–Chang’anchong deposit, based on investigations mineral crystallization sequences compositional features minerals fertile porphyries. Results indicate that porphyries show high initial ƒO2, with ΔFMQ ≥ +3.0 (ΔFMQ deviation logƒO2 from fayalite–magnetite–quartz (FMQ) buffer). (ΔFMQ) continued to decrease ~+2 until occurred at ~790 °C due wall-rock contamination. caused temporary increase (to = ~+3.4). during this (790–750 °C) resulted higher content ore-forming materials exsolved fluid. When temperature dropped below 750 °C, began continuously eventually reached ~+0.6. lower hindered further migration through (< °C). magma crustal PCDs generally has contamination reduced components can significantly ƒO2. early maintain condition, thereby efficiently extracting producing fluids, PCDs. latter continuous may be reason preventing deposit form giant PCD.