Geochronology and geochemistry of the Fe ore-bearing Zhonggu intrusions of the Ningwu Basin: Implications for tectonic setting and contemporaneous Cu-Au mineralization in the Middle–Lower Yangzte Metallogenic Belt

Abstract

The Zhonggu ore field is located in the southern Ningwu volcanic basin, the largest iron ore district within the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) of eastern China. This area has produced more than 2000Mt of iron ore although comparatively little research has been done on the ore-bearing intrusions in this region despite the significance of the mineralization in this area. This in turn means that the petrogenesis and the links between these intrusions and the metallogenesis in this area remain unclear. This study focuses on the six intrusions that are associated with almost all of the iron deposits within the Zhonggu ore field, namely the Gushan porphyritic gabbrodiorite, the Longshan gabbrodiorite, the Hemushan diorite, the Baixingshan diorite, the Zhongjiu diorite and the Taipingshan monzonite. All of these rocks are either high- to medium-K and calc-alkaline or are shoshonitic, although the former dominates over the latter. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb dating yielded ages of 132.6±1.6 to 131.6±1.6, 130.7±2.2 to 129.4±1.9, 131±2.0, and 132.3±2.1Ma for intrusions associated with the Longshan, Taipingshan, Baixiangshan and Zhongjiu deposits, respectively, indicating that all of these intrusions were emplaced in the Early Cretaceous (132.6–129.4Ma). This magmatism and mineralization was contemporaneous with the second stage (135–127Ma) of magmatic activity that has been documented elsewhere within the MLYRMB. The mineralized intrusions associated with the Zhonggu iron ore field have similar chondrite-normalized rare earth element (REE) and primitive mantle-normalized multi-element variation patterns that are characterized by light REE (LREE) and light ion lithophile element (LILE; Rb, Ba, K, and Sr) enrichments and heavy REE (HREE) and high field strength element (HFSE; Nb, Ta, Zr, and Hf) depletions. These intrusions also have total REE (ΣREE) concentrations that are consistent with the evolution of the magma from mafic-intermediate to felsic. The magmas that formed these intrusions were derived from an enriched region of the mantle and underwent contamination by the assimilation of continental crustal or lithospheric material. The magmatism within the Zhonggu iron ore field formed during an Early Cretaceous period of lithospheric thinning and extension. In addition, the arc affinities of the intrusions in this region indicative of a relationship to the subduction of the Paleo-Pacific Plate, a process that was most likely the main factor in the generation of the mineralization within the MLYRMB. The Fe mineralization-related intrusions in the study area are also distinctly different from intrusions in this region associated with Cu–Au mineralization, most likely as a result of the heterogeneous distribution of slab-derived fluids in this period over time. This research also highlights the exploration potential of this region, indicating that the 132–129Ma monzonites and diorites in this area and beyond should be considered prime targets for Fe exploration but are also unlikely to be related to Cu–Au mineralization.