Bitumen Sm-Nd, pyrite Rb-Sr and zircon U-Pb isotopes constrain timing of ore formation and hydrocarbon deposition in the Erdaokan Ag-Pb-Zn deposit, NE China

Highlights

• Bitumen Sm-Nd age of 234.6 Ma is interpreted as the hydrocarbon deposition timing.

• The ore-forming age of Erdaokan is constrained at middle Triassic.

• Bitumen Sm-Nd can be used to constrain the hydrothermal mineralization age.

Abstract

Dating of bitumen can potentially constrain both the timing of mineralization and hydrocarbon deposition in hydrocarbon-bearing hydrothermal deposits. Here we report for the first time direct Sm-Nd dating on bitumen from the Erdaokan Ag-Pb-Zn deposit, a newly discovered, large, hydrocarbon-bearing hydrothermal deposit with 2964 kt of ore and 1535 t Ag with an average grade of 517 g/t Ag, 0.6 wt% Pb and 0.41 wt% Zn in the Back-Arc basin of NE Great Xing’an Range, NE China. Dating results show that Sm-Nd isochron age of four bitumen samples is 234.6 ± 1.2 Ma and an isochron age that includes three galena and four bitumen samples is 234.9 ± 1.4 Ma. In order to verify these ages, we performed Rb-Sr dating on pyrite and galena from the same deposit and U-Pb dating of zircon from the adjacent diorite porphyrite dike, which yielded ages of 232.9 ± 2.3 Ma and 234.2 ± 2.8 Ma, respectively. The highly concordant ages from multiple techniques confirms that bitumen Sm-Nd isotopic dating is reliable, and that the ages obtained in our study represent the hydrocarbon deposition as well as mineralization timing in the deposit. We conclude that, in the Duobaoshan metallogenic belt, the Erdaokan Ag-Pb-Zn deposit is the first identified large, Middle Triassic, hydrocarbon-bearing silver-lead–zinc deposit and is of great significance to the regional metallogenesis research and prospecting work in the Great Xing’an Range.

Introduction

Although due to low thermal stability, the amount of organic matter is generally low in hydrothermal deposits, their role in metal transportation, diagenesis and mineralization processes is more significant than commonly recognized (Saxby, 1976, Proskurowski et al., 2008, Gu et al., 2012). Hydrocarbons from the organic matter play an important role in the formation of different sulfide-bearing deposits (Parnell, 1988). Bitumen has been reported in association with many different metal deposits around the globe, such as lead–zinc deposits in the Mississippi Valley (Marikos et al., 1983), strata-bound copper deposits in North Chile (Cisternas and Hermosilla, 2006, Rieger et al., 2008) and Pine Point lead–zinc field in Canada (Powell and Macqueen, 1984). However, bitumen associated with metallic mineral deposits has been investigated for many decades, but constraining the timing of bitumen formation remained elusive (Selby et al., 2005). Previous attempts, such as Pb-Pb (Parnell and Swainbank, 1990), U-Pb (Parnell and Swainbank, 1990) and in-situ SIMS U-Pb (Decrée et al., 2011, Luo et al., 2015, Lesbros et al., 2017) dating on uraniferous bitumen from various metal deposits yielded inconsistent results because of element migration. Until now, only the Re-Os system has been successfully applied on bitumen from the MVT deposits, black shales, or maltenes and asphaltene in crude oil reservoirs to constrain the timing of hydrocarbon deposition (Cohen et al., 1999, Creaser et al., 2002, Kendall et al., 2004, Selby and Creaser, 2005a, Ge et al., 2016, Georgiev et al., 2016). However, researchers stressed that further studies on Re-Os isotope systematics in source rocks and hydrocarbons are required to effectively apply the Re-Os technique on mineral and petroleum systems (Selby et al., 2005, Selby and Creaser, 2005b).

The Sm-Nd dating technique has been successfully applied on fluorite, carbonate, tourmaline and sulfide minerals in hydrothermal deposits, which provided the isotopic make-up of the mineral, or rock and constrained reliable mineralization ages (Chesley et al., 1991, Chesley et al., 1994, Jiang et al., 2000, Peng et al., 2003, Oberthür et al., 2009, Su et al., 2009, Henjes et al., 2014). A comprehensive study of Pb, Sr and Nd isotopes in the bitumen, crude oil and kerogen from the Karamay, Liaohe and Tarim areas proved that Sm-Nd isotopes in bitumen can serve as a good tracer of the processes which led to the hydrocarbon deposition and evolution (Zhu et al., 2001). Therefore, Sm-Nd isotope systematic in bitumen is measurable and potentially constrains mineralization age in hydrothermal deposits.

In this study, we report ages of bitumen from the Erdaokan Ag-Pb-Zn deposit located in the Back-Arc basin of NE Great Xing’an Range, NE China using Sm-Nd isotopic technique for the first time. Rb-Sr dating of pyrite and galena from the main metallogenic stage and U-Pb dating of the adjacent diorite porphyrite were also carried out to verify the application of the Sm-Nd technique on bitumen and to provide constraints on the timing of hydrocarbon deposition and ore mineralization.