Geology and genesis of auriferous porphyritic monzogranite and its correlation with the Qiyugou porphyry-breccia system in East Qinling, Central China

Abstract

The Qiyugou deposit in the Qinling Orogenic Belt includes breccia pipe hosted gold mineralization and newly discovered porphyry gold mineralization, whereas the genetic relationship between porphyry stocks, stockwork-disseminated mineralization and breccia pipe-hosted mineralization remains obscure. Here we present the results from an integrated field investigation, petrography, whole-rock geochemistry, zircon U-Pb geochronology, trace elements and Hf-O isotopes on a suite of porphyritic monzogranite from the newly discovered auriferous ‘191 pluton’, to elucidate the petrogenesis, magma source, timing of magmatic-hydrothermal event and the possible genetic link with the Qiyugou porphyry-breccia system. The porphyritic monzogranite shows varying degree of hydrothermal alterations including potassic alteration, phyllic alteration, silicification, sericitization and chloritization, resulting in the high loss on ignition values (LOI = 2.88–6.64 wt%) and linear relationships between LOI and large ion lithophile elements (LILEs) which suggest element mobility in fluids. The fluid immobile high field strength elements (HFSEs) and zircon trace elements indicate that the porphyritic monzogranite is typical I-type granite. Zircon U-Pb dating of the porphyritic monzogranites yield weighted mean ages of 136.4 ± 1.1 Ma, 134.6 ± 1.8 Ma and 135.2 ± 1.1 Ma, representing the emplacement age of the auriferous ‘191 pluton’. Zircon Hf-O isotopes show εHf(t) values from −25.6 to −20.0 with crustal model ages ranging from 2807 to 2458 Ma, and δ18O values from 4.75‰ to 5.66‰, indicating that the parental magma involved mixing between the partially melted Taihua Group lithologies and mantle-derived component. The available geochronological data constrain the timing and duration of magmatic-hydrothermal activities in the Qiyugou deposit, including pre-mineralization 158.7–150.1 Ma granitic magmatism, syn-mineralization 136.4–128.6 Ma porphyry magmatism and Au mineralization, and post-ore 128.9–124.7 Ma granitic dyke intrusion. The duration of the magmatic-hydrothermal activity in the Qiyugou deposit ranges between 136.4 Ma and 128.6 Ma, when the breccia pipes and granitoid plutons formed through multiple pulses of magma emplacement and fluid exsolution from a fertile magma chamber. The 136.4–131.3 Ma porphyry magmatism and gold mineralization in the ‘191 pluton’ represent an early stage magmatic-hydrothermal activity in the Qiyugou porphyry-breccia system. We predict that the deeper levels of the breccia pipes could be potential target for porphyry gold ore bodies in the Qiyugou deposit.