The timing, origin and T-fO2 crystallization conditions of long-lived magmatism at the Yangla copper deposit, Sanjiang Tethyan orogenic belt: Implications for post-collisional magmatic-hydrothermal ore formation

Abstract

An integrated study involving zircon geochronology, Hf isotopic characteristics and trace elements combined with detailed field investigation was carried out for the felsic magmatic system in the Yangla skarn copper deposit area in the Sanjiang orogenic belt, southwestern China. The intrusive units are composed of syn-mineralized dioritic enclaves, granodiorite, quartz monzonite porphyry (238–230Ma), and granite stocks and monzogranite dikes (~223Ma), bracketing a time span of ca. 15Myr. The uniform Hf isotope characteristics during an ~15Myr period suggest a primary control of the isotopic signature by a stable, long-lived, hot reservoir in the deep lithosphere. The occurrence of mafic enclaves and the identical intermediate initial εHf values (−5.9 to 1.7) of granodiorite, granite, and monzogranite, suggest that the felsic magmatic system was produced by remelting of Neoproterozoic lower crustal rocks that mixed with minor amounts of mantle-derived melts. Application of comprehensive indices of zircons, such as Th/U, Zr/Hf, T(Ti in zircon), and Ce/Nd, implies that (1) mafic melt injected into felsic magma during the early stage, from which the granodiorite and dioritic enclaves formed; (2) progressive evolution from granodiorite to quartz monzonite porphyry with addition of crustal components, as revealed by the positive correlation between εHf and Th/U; (3) the incorporation of the crustal component into the residual magma from which quartz monzonite porphyry formed, reduced the fO2 of the mineralized and barren quartz monzonite porphyry and settled much of dense sulfides out at depth; and (4) another discrete reduced magma pulse, from which the granite and monzogranite dikes crystallized at slightly higher Ti-in-zircon model temperatures, suggesting a slightly more reduced condition of the hidden intrusive portion of magma chamber. In general, the magmatic system is interpreted to be related to post-collisional lithospheric extension after slab break-off. The Cu, S, and water of the felsic magmas were mainly derived from the recycling of Neoproterozoic hydrous arc lower crustal rocks triggered by asthenospheric upwelling.