Genesis of Cretaceous igneous rocks and its related large scale porphyry Cu-Au mineralization in Chating, the Middle-Lower Yangtze River Metallogenic Belt: The geochemical constrains

Highlights

• The ore-forming Qtz-diorite porphyry are earlier than the barren diorite porphyry in Chating.

• The Chating adakitic rocks are from the slab-derived with subducted sediments.

• The barren diorite porphyry generated from the partial melting of the enriched SCLM.

• High oxygen fugacities favorable for the Cu–Au mineralization in Chating.

Abstract

The newly discovered large Chating porphyry Cu–Au deposit is associated with early Cretaceous intrusive rocks in the Middle-Lower Yangtze River Belt (MLYRB), eastern China. Two types of intrusive rocks in the Chating area have been identified, i.e., the quartz-diorite porphyry, related to the Cu–Au mineralization, and the diorite porphyry that is ore-barren. Zircon LA–ICP–MS U–Pb dating yields 145–136 Ma for the Chating intrusions. They both belong to High-K calc-alkaline series, with enrichments of large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), depletions of high field-strength elements (HFSEs) and heavy rare earth elements (HREEs), and no or slightly negative Eu anomalies. The quartz-diorite porphyry shows high Sr contents and Sr/Y ratios; low MgO, Cr and Ni contents; medium (La/Yb)_N and Mg# (34.6–54.6) and low K₂O/Na₂O (<1) values; decoupling of Sr/Y and (La/Yb)_N; and low Th/U (3.49–6.08) values, thus exhibiting characteristics of slab-derived adakitic rocks. In addition, it has slightly enriched Sr–Nd–Hf isotopes ((⁸⁷Sr/⁸⁶Sr)_i = 0.70655–0.70717, ε_Nd(t) = −6.9 to −8.8, and ε_Hf(t) = −7.6 to −10.6) and high radiogenic Pb isotopes ((²⁰⁶Pb/²⁰⁴Pb)_i = 18.22–18.70, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.63–15.66 and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.49–38.97), reflecting the involvement of subducted sediments in the magma source. Therefore, the fertile quartz-diorite porphyry was most likely generated by the partial melting of the slab-derived, subducted sediments and a partial degree of metasomatized mantle-derived magma with crustal assimilation. As to the barren diorite porphyry, it has similar Sr–Nd–Pb isotope and trace element features as the fertile adakitic samples. However, it shows relatively lower SiO₂ contents; high contents of MgO (3.89–4.06 wt%), Cr and Ni (avg. 47.6 ppm and avg. 15.01 ppm); high Mg# (51.2–69.8); and more enriched Hf isotope characteristics (−8.5 to −12.9), which imply that this barren diorite porphyry could be generated from the partial melting of enriched metasomatised lithospheric mantle assimilated with old continental crust materials. The δ³⁴S values of the pyrite and chalcopyrite of the ores in Chating indicate a magmatic-derived source, resembling other porphyry-skarn Cu–Au deposits in the MLYRB. The fertile Qtz-diorite porphyry adakitic rocks are characterized by higher oxygen fugacities than the barren diorite porphyry, which are closely related with the partial melting of the subducted oceanic slab in an oxidizing environment, thus making them favorable for Cu–Au mineralization.

Introduction

The Middle-Lower Yangtze River Metallogenic Belt (MLYRB) is one of China’s most important polymetallic metallogenic belts. It is characterized by porphyry-skarn copper–gold deposits and magnetite-apatite iron deposits (Chang et al., 1991, Pan and Dong, 1999, Yang and Lee, 2011, Zhou et al., 2015). The MLYRB contains many porphyry Cu–Au deposits (e.g., Jiguanzui in Edong, Shaxi in Luzong, and Shujiadian in Tongling; Fig. 1). Most of the porphyry Cu–Au deposits are related to early Cretaceous calc-alkaline intrusive rocks with adakitic features (or adakite-like, e.g., high Sr/Y; Deng et al., 2016, Li et al., 2013, Wang et al., 2006, Wang et al., 2016, Xie et al., 2012, Xu et al., 2002, Yang and Lee, 2011), and the petrogenesis of these ore-bearing adakitic rocks is still controversial, including models such as 1) the fractional crystallization of basaltic magmas possibly coupled with crustal contamination (Li et al., 2009, Xie et al., 2011), 2) the partial melting of the subducted Pacific oceanic crust with the involvement of sediments (Ling et al., 2009, Wang et al., 2013, Xie et al., 2012) or ridge subduction (Sun et al., 2010), 3) the partial melting of the thickened or delaminated lower continental crust of the Yangtze block (Liu et al., 2010, Wang et al., 2006, Wang et al., 2007b, Yan et al., 2015), and 4) the mixing of mantle-derived and crust-derived magmas (Xie et al., 2008, Xie et al., 2011, Xu et al., 2004, Xu et al., 2014, Zhou et al., 2015, Chen et al., 2016).

The Xuancheng region is a new-discovered ore district in the MLYRB (Xu et al., 2019; Fig. 1), where several copper, molybdenum and iron deposits have been discovered, e.g., the Chating Cu–Au deposit (Hong et al., 2017, Jiang et al., 2017, Xiao et al., 2019), the Qiaomaishan Cu–W deposit (Qi et al., 2020), the Magushan Cu–Mo deposit (Qi et al., 2019) and the Shizishan Cu deposit (Huang et al., 2013, Qian et al., 2017). The newly explored Chating porphyry Cu–Au deposit is the largest deposit in the Xuancheng region (>1.65 Mt Cu at 0.54% and 248.15 t Au at 0.43 g/t). The previous studies carried on the deposit to date mainly address the geological, geochemical and mineral geochemistry features of the ore-bearing Qtz-porphyry (Jiang et al., 2017, Xiao et al., 2019, Xu et al., 2019). Jiang et al. (2017) proposed that the fertile magmas of the Chating Cu–Au deposit were most likely derived from the mixing of the melting of a thickened enriched lithospheric mantle and a lower mafic lower-crust. Xiao et al. (2019) concluded that the Chating deposit has affinities to a porphyry-type Cu-Au system via its magnetite and biotite geochemistry. However, Xu et al. (2019) suggested that the Chating deposit is a cryptoexplosive breccia type deposit. Therefore, the geological features, the diagenetic mechanisms, the ore-forming materials sources and the geodynamic processes of the Chating Cu–Au mineralization were not well constrained. In particular, the geochemical differences and origin between the ore-bearing Qtz-porphyry and barren diorite porphyry remain unclear.

In this paper, we investigate the ore-bearing quartz diorite porphyry and ore-barren diorite porphyry in the Chating deposit. New whole-rock major, trace element, zircon LA–ICP–MS U–Pb dating, Sr–Nd–Pb–Hf isotopic data and in-situ sulfur isotope analyses for these intrusions and ores are presented in this paper in order to better understand and document the potential differences in the geochemical conditions and origins of these rocks, the formation of the Chating Cu–Au deposit and the geodynamic processes.