Mineralization associated with multiphase porphyritic dikes in the Yuanmenlou Cu-polymetallic Deposit, South Qinling: insights from U–Pb dating of zircon and garnet and zircon trace element geochemistry

 Objective The Zhashui-Shanyang ore-concentrated area is a key area within the South Qinling orogenic belt, known for its porphyry-skarn Cu-Au polymetallic deposits. However, most of the discovered deposits are of medium to small size, highlighting the urgent need for exploration breakthroughs. The Yuanmenlou Cu-polymetallic deposit, a newly discovered skarn-type deposit, is currently under exploration in this district. The ore body is mainly hosted within a thick (vertically extending over 500 meters) skarn zone, located at the exocontact of multiphase porphyritic dikes. This study aims to constrain the diagenetic and metallogenic ages, elucidate the genetic relationship between magmatism and mineralization, and discuss the implications for regional-scale mineralization. Methods Based on detailed studies of deposit geology and petrographic characteristics, LA–ICP–MS U–Pb dating of zircon and garnet, as well as trace element analysis of zircon, were carried out, and the results were subjected to an integrated comparative analysis. Results The multiphase porphyritic intrusions in the Yuanmenlou deposit occur as small stocks (controlled by the intersection of N–S-trending fault zones and NW–trending fold belts) and dikes (controlled by NW-trending interlayered structural zones). The main rock types are granodiorite porphyry, granite porphyry, and quartz diorite porphyrite. Zircon LA-ICP-MS U-Pb ages of five different dike samples range from 147.2~148.0 Ma, and the garnet from the skarn yielded a U–Pb age of 151.5 ± 3.4 Ma. Based on rare earth element (REE) compositions in zircon, the calculated magmatic oxygen fugacity values range from FMQ+0.49~FMQ+1.01, with an average of FMQ +0.84 (n=91). The REEs show strong positive Ce anomaly (Ce/Ce*=152~251), weak negative Eu anomaly (Eu/Eu*=0.68~0.76), and relatively high (Ce/Nd)/Y ratios. Conclusions The porphyritic intrusions with varying lithofacies in the Yuanmenlou area originated from a single magmatic event, with differences attributed to magma differentiation and wall-rock assimilation. The timing of diagenesis and mineralization is Late Jurassic, and the ore-forming magma was moderately oxidized and water-rich. Multiphase dike emplacement was closely associated with the formation of thick skarn zones and multilayered copper orebodies. These findings expand the exploration potential of the Zhashui–Shanyang ore cluster and offer valuable guidance for future prospecting.