Late Triassic Magma Mixing and Fractional Crystallization in the Qingchengzi Orefield, Eastern Liaoning Province: Regional Petrogenetic and Metallogenic Implications

Abstract

The Qingchengzi orefield is an important Pb-Zn-Au-Ag polymetallic orefield in NE China. The Indosinian magmatism has formed the Shuangdinggou batholith and the Xinling stock, as well as dikes of quartz monzonite porphyry and lamprophyre. According to petrographic characteristics, the Shuangdinggou intrusion can be divided into the main suite and the central suite. Zircon U-Pb dating yielded crystallization ages of 215.0–220.9 Ma for these various Qingchengzi magmatic units, which are within analytical error and represent coeval magmatism. The Shuangdinggou main suite contains abundant mafic microgranular enclaves (MMEs) and shows features of magma mixing. Geochemically, the major oxide contents of the MMEs and their quartz monzonite host show well-defined linear fractionation trends. The REE and trace element patterns of the MMEs and their host are similar, which demonstrates certain degree of geochemical homogenization between the two during magma mixing. The Shuangdinggou main suite shares similar geochemical characteristics with typical high Ba-Sr granites (Ba=1 082 ppm–2 051 ppm, Sr=803 ppm-886 ppm), and was likely originated from the mixing between a melt derived from partial melting of the thickened lower crust and the enriched mantle. The central suite was likely formed by fractional crystallization of the main-suite magma. The Xinling intrusion may represent a branch of the Shuangdinggou intrusion and has the same genesis as the central suite. The quartz monzonite porphyries geochemically mimic the Shuangdinggou main suite, and may also be an epioic facies of the Shuangdinggou intrusion. The MMEs and lamprophyres may have been derived from incomplete magma mixing. Formation of the Pb-Zn and Au-Ag deposits in the Qingchengzi orefield may have been related to the granite/quartz monzonite porphyries near the Shuangdinggou intrusion, which were formed by magma mixing and fractional crystallization.