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Xuejiping porphyry Cu deposit is located in the south of Yidun island arc, northwest Yunnan. Tectonically, it lies in the middle part of the “Sanjiang Rivers” metallogenic belts. There are ore‐barren quartz diorite, quartz diorite porphyrite (QDP), and ore‐bearing quartz monzonite porphyry (QMP) intruded into clastic‐volcanic rocks of the late Triassic Tumugou Formation. This study focused on determining why the ore‐bearing porphyry is the QMP. It analyzed the correlation between chronology, geochemistry, magmatic oxygen fugacity, and mineralization. Results show that the crystallization age of QDP is 228.7 Ma and that of QMP is 215.0 Ma, which is relatively younger. The molybdenite Re‐Os age is 219.9 Ma. The oxygen fugacity of QDP and QMP magma are higher than Fayalite‐Magnetite‐Quartz buffer, but the oxygen fugacity value of QDP magma is relatively dispersed, with high and low values. While the QMP falls in the high oxygen fugacity range. The δ 18 O value of QDP (from 5.66 to 6.27 with an average of 6.02) is higher than that of QMP (from 5.53 to 6.42 with average 5.76). It indicates that the QMP contains more components from the mantle than QDP. It is suggested in this paper that QMP have more potential of ore‐formation in the later stage. The QDP caused ore‐forming elements pre‐enrichment in the region. Millions of years later, the QMP intruded in QDP, the pre‐enriched elements were activated and superimposed with the ore‐forming elements of QMP, then the ore‐forming elements were carried and integrated into the ore deposit, and enriched once again.
Yang et al. (Fri,) studied this question.