From magma replenishment to epithermal mineralization: Combined evidence from zircon U-Pb, (U-Th)/He and trace element data

Abstract

The Dahalajunshan Formation volcano-volcanoclastic rocks in Chinese Western Tianshan host specific Paleozoic epithermal gold deposits as well as volcanogenic Fe-Cu mineralization. Although numerous studies have been taken to constraint its geological, geochemical, geochronological features as well as petrogenesis, the link between volcanism and mineralization is not well constrained. In this contribution, accessory zircon was collected from the intermediate to felsic volcanic rocks outcropped at the Jingxi-Yelmand gold deposit. Detailed cathodoluminescence imaging illustrates ubiquitous resorption surfaces and embayments, and in situ LA-ICPMS trace element analysis further indicates periodic recharge of magma chamber by mafic magma input from depths. U-Pb geochronology constrains two main pulses of magmatism at ~370 Ma and 365–362 Ma, respectively. The duration of magmatic activity in the magma chamber is estimated to 52 ± 6 Myrs, using the U-Pb age of the oldest zircon antecryst (406.9 ± 5.4 Ma) and the youngest zircon autocryst (354.6 ± 3.0 Ma) as cutting edge and taking the analytical uncertainty into consideration. (U-Th)/He dating of zircon from the first pulse yields an age of 354 ± 15 Ma, which is identical to U-Pb age within analytical uncertainty. However, zircons from the second pulse of volcanic rocks yield much younger (U-Th)/He centered at 292.6 ± 7.6 Ma due to significant post-eruption reheating events. We propose that the intermediate to acid volcanic rocks are constructed by incremental addition of small magma batches. The periodic injection of more mafic magma and subsequent mixing may act as important trigger of not only volcanic eruption but also epithermal mineralization. Gold mineralization at the deposit should be no later than 292.6 Ma.