Mineralogical Characteristics of Magnetite in the Duobuza Porphyry Copper (Gold) Deposit and Their Geological Implications

Magnetite is extensively developed within various alteration zones of the mining district. Some magnetite is closely associated with copper mineralization, possessing significant research value. The Duobuza Cu (Au) deposit is a typical porphyry-type deposit within the Bangong Co-Nujiang metallogenic belt and was the first porphyry Cu-Au deposit discovered in the Duolong copper–gold ore district. Currently, this deposit contains copper resources exceeding 3 million tons @0.46%, with associated gold resources exceeding 80 tons @0.19 g/t. This study focuses on magnetite from the Duobuza deposit. Through field geological logging and microscopic identification combined with electron microprobe analysis (EMPA) and in situ LA-ICP-MS testing, mineralogical and mineral chemical research on magnetite is conducted. This research aims to elucidate the genesis of magnetite in the Duobuza deposit and its implications for mineral exploration. Five magnetite types with different occurrences can be distinguished in the Duobuza deposit: Mt1 is magmatic magnetite; Mt2, Mt3, Mt4, and Mt5 are hydrothermal magnetite, with Mt5 being closely associated with copper mineralization. Mt1 is relatively enriched in Ti, V, Al, and Cr but depleted in Mn and Si; Mt2 is relatively enriched in Ti and Al but depleted in Si and Cr; Mt3 is relatively enriched in Al but depleted in Mg; Mt4 is relatively enriched in Ti, Al, V, Zn, and Mn; and Mt5 is relatively enriched in Mg, Si, Ti, Al, Mn, and Zn but depleted in Cr. Based on the Al + Mn vs. Ti + V discrimination diagram, magnetite formed in a medium- to high-temperature environment, with hydrothermal magnetite Mt4 forming at the lowest temperature. Vanadium (V) content can be used to estimate the oxygen fugacity (fO2) during mineralization. Mt1 exhibits the highest V content, indicating relatively low oxygen fugacity, whereas Mt4 shows the lowest V content, suggesting relatively high oxygen fugacity. Mt5 has a higher V content compared to other early-stage hydrothermal magnetites, suggesting that a lower fO2 formation environment favors the precipitation of metal sulfides in the mining district. Trace element analysis of magnetite from the Duobuza, Bolong, and Naruo mining districts reveals that magnetite from all three deposits is enriched in Si and Al and depleted in Ca and Ni. Magmatic magnetite from the Naruo and Duobuza deposits exhibits similar elemental distribution patterns. Hydrothermal magnetite from the Duobuza deposit shows significantly higher Ti and V contents compared to magnetite from the Bolong and Naruo deposits.