Zircon and apatite fingerprints on the magma fertility for the Jurassic igneous rocks in the southern Lhasa subterrane, Tibet

Abstract

The products of long-lasting magmatism (ca. 170 Myr) related to the northward subduction of the Neo-Tethys Ocean are preserved in the southern Lhasa subterrane (SLT). However, only a few large porphyry Cu–Au deposits related to these igneous rocks have been discovered in the Xiongcun ore concentration area. Here, we present zircon and apatite data on Jurassic fertile suites (FS), barren suites (BS), and recently proposed prospecting target suites (PTS) in the SLT, intending to present a case study of the regional-scale porphyry metallogenic potential. The zircon geochemical data with Rayleigh fractionation modeling show that the zircon geochemical characteristics of the FS and PTS were affected by high proportions of amphibole and titanite crystallization, whereas the zircon geochemistry of the BS displays the effects of plagioclase fractionation or mixing with the least evolved melts. The zircon Ce4+/Ce3+ and Eu/Eu* values cannot be used to trace the oxygen fugacity of the magma due to prior or simultaneous mineral crystallization. However, the zircon Ce/Ce* and Ce/Nd ratios may be used as semiquantitative measures of the magma redox conditions. This result is similar to that calculated by the new magmatic oxybarometer using trace elements in zircon, showing that the FS (ΔFMQ = 1.21 ± 0.42) and PTS (ΔFMQ = 1.29 ± 0.51) have higher oxygen fugacity levels than the BS (ΔFMQ = 0.40 ± 0.88). The melt S contents estimated from magmatic apatites are lower in the FS and PTS than in the BS, which is attributed to prior anhydrite crystallization under high oxygen fugacity and low temperature conditions. The higher Cl in the melt of the FS (average 0.098 wt%) than in the melts of the PTS (average 0.045 wt%) and BS (average 0.04 wt%) shows that elevated Cl contents in melts are likely important for forming porphyry Cu–Au deposits. The combined geochemical characteristics of zircon and apatite suggest that a high oxygen fugacity (ΔFMQ > 1.2), prior or concurrent anhydrite crystallization, and high zircon Eu/Eu* (>0.6), Ce4+/Ce3+, and (Yb/Dy)N values can be used to refine potential porphyry Cu–Au targets on a regional scale in the SLT. High melt Cl contents and apatite MnO and Pb contents can effectively indicate fertile suites at the deposit scale.