Magma mixing affected the Late Triassic porphyry mineralization in the Yidun arc in SW China

Abstract

The Yidun arc was formed by the westwards subduction of the Garze-Litang Ocean in the Late Triassic. It trends from near north to south. Many intermediate-acid intrusions became widely distributed in the Yidun arc during this period. Large to super-large porphyry-skarn deposits developed only in the southern Yidun arc, with no corresponding deposit in the northern part. It has been found that some mafic microgranular enclaves (MMEs) are exposed, and there is certain evidence of magma mixing in the Yidun arc in recent years. To clarify the relationship between widely developed magma mixing and metallogenic differences in the Yidun arc, the zircon U-Pb dating and geochemistry of quartz monzonite porphyry (QMP) and its MMEs in the Pulang super-large porphyry Cu deposit in southern Yidun have been carried out. Then, the granodiorite (GD, ore-barren) and MMEs of the Daocheng batholith in the middle and the Cuojiaoma batholith (ore-barren) in the north of the Yidun arc were compared. The results show that the oxygen fugacities of the Pulang QMP and MMEs are higher, and the MMEs are slightly higher than those of the QMP overall. The oxygen fugacity of MME in the Daocheng GD is slightly higher than that of FMQ, but it is still lower than that of QMP and MME in Pulang. That is, the magmatic oxygen fugacity of the Daocheng GD is lower than that of the Pulang QMP and MME. There are more magmas fractionated in the south than in the north of Yidun due to the difference in geological structure between the north and south. Therefore, the degree of magma mixing is gradually reduced in the Yidun arc from south to north (Pulang, Daocheng to Cuojiaoma), and the metallogenic potential is also higher in the south and lower in the north.