Magmatic constraints on the Ermi porphyry copper mineralization, Northeast China: Evidence from zircon U-Pb geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic geochemistry

Abstract

The Ermi porphyry Cu deposit (PCD), located on the northeastern margin of the North China Craton (NCC), is a medium-sized PCD with a reserve of > 0.11 Mt and an average grade of 0.6% Cu. Copper mineralization occurs mainly within the intrusions of quartz diorite, granodiorite, granodiorite porphyry and granite porphyry. To reveal the magmatic contributions to mineralization, we conducted zircon U-Pb dating and element geochemical and Sr-Nd-Hf isotope analyses of mineralized intrusions at Ermi on the basis of field and petrographic observations. The zircon LA-ICP-MS U-Pb ages of quartz diorite, granodiorite, granodiorite porphyry and granite porphyry are 95.0 ± 1.0 Ma (n = 16, MSWD = 0.97), 94.2 ± 1.4 Ma (n = 15, MSWD = 0.93), 93.9 ± 1.2 Ma (n = 19, MSWD = 0.22) and 92.0 ± 1.2 Ma (n = 18, MSWD = 0.41), respectively. All of these intrusive rocks belong to metaluminous, calc-alkaline I-type granites. They exhibit high 87Sr/86Sr(t) values (0.7075 to 0.7084), low negative εNd(t) values (-16.7 to −12.6), and variable but mainly negative εHf(t) values (-12.4 to −6.4), with corresponding TDM2(Hf) ages of 2.1 ~ 2.7 Ga. These findings indicate that the initial magma originated from the partial melting of the Archean lower crust of the NCC. Combined with the dynamic background of the study area, we believe that they formed in an active continental margin environment related to the rollback of the subducting Paleo-Pacific slab in the Cretaceous. The oxygen fugacity (fO2) values estimated from rare earth element (REE) and Ti concentrations in zircons show that: The fayalite-magnetite-quartz buffer (ΔFMQ) values of the quartz diorite and granodiorite are −2.6 and 0.87, respectively. The ΔFMQ values of the granodiorite porphyry and granite porphyry are 2.68 and 2.13, respectively. The high oxygen fugacity promoted the enrichment of Cu and other ore-forming elements in the magma. However, the results of estimating crustal thickness from the (La/Yb)N ratio of magma show that the Ermi PCD formed under the condition of a thinned or shallow lower crust (<25 km). Thinned continental crust may lack massive sulfide accumulation and is not conducive to the large-scale storage and accumulation of fertile magmas in the middle or upper crust, which may limit the mineralization scale of the Ermi PCD.