A porphyry-skarn metallogenic system in the Lesser Xing’an Range, NE China: Implications from U–Pb and Re–Os geochronology and Sr–Nd–Hf isotopes of the Luming Mo and Xulaojiugou Pb–Zn deposits
Introduction
The Luming and Xulaojiugou deposits are located in the southeastern segment of the Lesser Xing’an Range, northeast China (NE China). The Luming porphyry Mo deposit is one of the largest Mo deposits in NE China and was discovered in 2003 (Han et al., 2010, Shao et al., 2012, Tan et al., 2012). The proven industrial Mo resource reaches 890,300 t with an average grade of 0.084% (Shao et al., 2012). While Xulaojiugou is a medium-sized skarn Pb–Zn deposit about 15 km away from the Luming deposit (Han et al., 2009). This deposit was discovered in 1985, and the explored Pb and Zn resources are 54,707 t and 107,259 t, with average grades of 2.18% and 2.06%, respectively. The mineralization of these two deposits are both associated with monzogranite.
A number of geological and geochronological studies have been carried out on the Luming deposit (Han et al., 2009, Han et al., 2010, Ma and Chen, 2012, Shao et al., 2012, Tan et al., 2012, Yang et al., 2012). Ma and Chen (2012) and Yang et al. (2012) reported LA-ICP-MS zircon U–Pb ages of 195.4 ± 1.4 Ma and 176 ± 2.2 Ma, respectively, for the monzogranite in Luming district. While Shao et al. (2012) obtained two different SHRIMP zircon U–Pb ages of 201.1 ± 3.9 Ma and 176.2 ± 4.3 Ma for a single monzogranite sample. Tan et al. (2012) presented a LA-ICP-MS zircon U–Pb age of 187.1 ± 1.2 Ma for the monzogranite and a Re–Os isochron age of 177.4 ± 3.5 Ma for molybdenite in the Luming deposit. In summary, the precise mineralization age of the Luming deposit is still under contention, and no work has been done on the petrogenesis and tectonic setting of the ore-related monzogranite. As for the Xulaojiugou deposit, the current research is only limited to descriptions of the geological features (Han et al., 2009).
In this study, we present LA-ICP-MS zircon U–Pb dating and Sr–Nd–Hf isotopic analyses for the ore-related monzogranites in the Luming and Xulaojiugou deposits, together with molybdenite Re–Os dating for the Luming deposit, in order to characterize the Luming–Xulaojiugou ore-forming system and the tectonic setting.
Section snippets
Regional geology
The Lesser Xing’an Range is located in the eastern segment of the Central Asian Orogenic Belt (CAOB), which evolved from amalgamation of multiple microcontinental blocks between the Siberia and North China Cratons (Wu et al., 2000, Wu et al., 2001, Wu et al., 2011, Jahn, 2004, Windley et al., 2007, Jahn et al., 2009, Glorie et al., 2011). The eastern CAOB experienced a complicated geodynamic evolution involving the closure of the Paleo-Asian and Mongol-Okhotsk Oceans in Paleozoic–Mesozoic, and
Deposit geology
The Luming porphyry Mo deposit and the Xulaojiugou skarn Pb–Zn deposit are located in the Lesser Xing’an Range, Heilongjiang Province of China (Fig. 1). They are spatially associated with coordinates of 47°22′08″–48°23′12″N, 128°31′54″–128°33′22″E (Luming), and 47°17′14″–47°17′46″N, 128°25′37″–128°26′19″E (Xulaojiugou), respectively (Fig. 2, Fig. 3).
Major and trace elements
Samples for major and trace element analyses include the monzogranite from the Luming deposit (Fig. 4f), and the medium-grained and porphyritic monzogranites from the Xulaojiugou deposit (Fig. 4g and h). The samples were crushed in a steel jaw crusher and then powdered to 200 mesh in an agate mill. Major element compositions were analyzed by X-ray fluorescence spectroscope (XRF) (Magix_pro2440) techniques at Hubei Geological Research Laboratory, with analytical uncertainties of 5%. Trace
Major and trace elements
Major and trace element compositions of the monzogranites in the Luming and Xulaojiugou deposits are listed in Table 1. These rocks show comparatively high SiO2 contents of 69.08–73.07 wt.%, with Na2O + K2O contents of 7.13–8.63 wt.% and Al2O3 contents of 13.29–15.53 wt.%. They all have low MgO (0.56–1.12 wt.%), TiO2 (0.33–0.46 wt.%) and P2O5 (0.06–0.14 wt.%) contents. In the K2O versus SiO2 diagram (Fig. 5a), the monzogranite in the Luming deposit mainly plot into the shoshonitic field, while
Timing of magmatism and mineralization
Zircon U–Pb dating of the monzogranite in the Luming deposit suggests that it was emplaced at 180.7 ± 1.6 Ma, while the emplacement ages of the medium-grained and porphyritic monzogranites in the Xulaojiugou deposit are 181.2 ± 1.1 Ma and 179.9 ± 1.0 Ma, respectively. According to their field occurrences, the medium-grained monzogranite in the Xulaojiugou deposit is cut through by the porphyritic monzogranite, indicating that the porphyritic monzogranite formed slightly later than the medium-grained
Conclusions
The Luming porphyry Mo and Xulaojiugou skarn Pb–Zn deposits are temporally and spatially associated, forming a porphyry-skarn metallogenic system. The Mo mineralization in the Luming deposit occurred at about 177.9 ± 2.6 Ma, slightly after the crystallization of the ore-bearing monzogranite (180.7 ± 1.6 Ma). The medium-grained and porphyritic monzogranites from the Xulaojiugou deposit yield crystallization ages of 181.2 ± 1.1 Ma and 179.9 ± 1.0 Ma, respectively, constraining the age of Pb–Zn mineralization
Acknowledgements
This study was funded by the Special Scientific Research Fund of Public Welfare Profession of China (Grant No. 201211008) and the resource compensation of Heilongjiang Province (Grant No. SDK2010-25). The constructive and detailed reviews by Dr. Nejib Jemmali have helped us improve our paper and are gratefully acknowledged. We thank Han Chengman and Li Zongming from the Heilongjiang Institute of Geological Sciences for their help in field investigations. Zeng Guoping is thanked for his
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