The geochronology of the Haobugao skarn Zn-Pb deposit (NE China) using garnet LA-ICP-MS U-Pb dating

Highlights

• Garnet LA-ICP-MS U-Pb dating yielded ages of 139.10 ± 5.40 and 140.70 ± 1.89 Ma, which firstly identified the timing of the Haobugao skarns.

• Garnet trace element compositions can reflect physicochemical conditions of hydrothermal fluids.

• Combined garnet U-Pb, zircon U-Pb and molybdenite Re-Os geochronological data could precisely date the magmatism, skarn formation and mineralization.

Abstract

The Haobugao skarn Zn-Pb ore deposit (reserves of 0.29 Mt @ 4.24% Zn, 0.15 Mt @ 2.25% Pb) is located in the southern Great Xing’an Range (SXGR) Cu-Mo-Ag-Au-Pb-Zn-Fe metallogenic province in northeastern China. The ore bodies mainly occur near the contact zones between the early Cretaceous granite and the lower Permian carbonates and sometimes in distal locations. Two types of garnet are identified, i.e., green and fine-grained garnet (Grt A), and brown and coarse-grained garnet (Grt B). The Grt A contains a wider compositional range (Adr_71.46-95.07Grs_0.16-24.10), whereas the Grt B contains a relatively narrow compositional range (Adr_87.65-97.52Grs_0.07-10.11). The Grt A displays chondrite-normalized HREE-enriched and LREE-depleted patterns with negative Eu anomalies, suggesting that the Grt A was likely crystallized from a nearly neutral fluid. In contrast, the Grt B is LREE-enriched, HREE-depleted with pronounced positive Eu anomalies, indicating that the Grt B was probably formed from a mildly acidic fluid. The Grt A contains lower U contents than the Grt B, implying that the Grt A was likely formed under a relatively oxidized condition, while the Grt B crystallized from a more reduced condition.

Skarn mineralization at Haobugao (139.10 ± 5.40 and 140.70 ± 1.89 Ma, garnet LA-ICP-MS U-Pb ages) and Zn-Pb mineralization (138.27 ± 0.14/0.69/0.81 and 138.82 ± 0.07/0.68/0.80 Ma, molybdenite ID-N-TIMS Re-Os ages) were associated with the emplacement of the granitoids dated from 143.49 ± 0.76 to 140.85 ± 0.75 Ma (zircon LA-ICP-MS U-Pb ages). The integrated geochronological data suggest that the ore-related granitoids, skarn, and Zn-Pb mineralization in the Haobugao deposit all formed in the early Cretaceous, which occurred in an extensional tectonic setting associated with regional Paleo-Pacific slab roll-back. This study highlights the reliability and viability of garnet U-Pb dating for constraining the timing of skarn formation, and utilizing its age with textural and trace element concentrations could well reconstruct the ore-forming process of skarn deposits.

Introduction

Skarn type deposits widely occur throughout the world and have contributed to the world’s supply of base and precious metals including Fe, W, Au, Cu, Zn, Mo, Sn, Au, and Ag (e.g., Meinert et al., 2005). Skarns are usually dominated by calc-silicate minerals (such as garnet and pyroxene) and commonly occur at the contact between igneous intrusions and carbonate-rich rocks (Meinert et al., 2005, Gevedon et al., 2018). In previous studies, skarn formation ages were commonly constrained by zircon U-Pb ages of nearby or possible ore-causative igneous rocks (Meinert, 1993, Xie et al., 2011), the U-Pb ages of accessory minerals, such as monazite (Schaltegger et al., 2005), titanite (Li et al., 2010) and allanite (Deng et al., 2014), or the Re-Os ages of molybdenite (Stein et al., 2001). However, these methods date the timing of magmatism or late-stage metasomatism rather than the onset of hydrothermal activity (Meinert et al., 2005). Therefore, it is essential to obtain direct geochronology data of skarns for establishing robust genetic links between igneous bodies and economically important ores, and further refining ore deposit models (Deng et al., 2017, Seman et al., 2017, Gevedon et al., 2018, Li et al., 2021).

Garnet is commonly the earliest characteristic paragenetic mineral crystallized during skarn formation and is often voluminous in skarns, making garnet crystallization an accurate capture of the onset of the hydrothermal event (Gevedon et al., 2018). Typically, the temperature of skarn formation (350–650 °C, Bowman, 1998) is far below the U-Pb closure temperature of garnet (0.5 cm diameter garnet grain > 850 °C, Mezger et al., 1989), which suggests that garnet U-Pb age will not be affected by most hydrothermal, metamorphic, and metasomatic processes. Therefore, garnet is considered to be an ideal and trustworthy geochronometer (Li et al., 2021). However, variable and low U concentrations (typically < 1 ppm), high common-Pb concentrations, coupled with the presence of U-rich mineral inclusions (such as crystalline uranium, zircon, monazite, and allanite) in garnet would limit the wide application of in situ garnet U-Pb dating (Dewolf et al., 1996, Meinert et al., 2001). Recently, andradite-rich skarn garnet has been reported to contain relatively high U concentrations but negligible common-Pb compared with other garnets, and in situ techniques such as LA-ICP-MS can target inclusion-free areas to mitigate the effect of mineral inclusions and acquire high-quality data (Deng et al., 2017, Gevedon et al., 2018, Li et al., 2019, Zhang et al., 2019, Zhang et al., 2019b). Collectively, LA-ICP-MS U-Pb dates of andradite-rich garnets can provide reliable and robust geochronological data for direct constraints on the time and history of magmatism and hydrothermal processes (Deng et al., 2017, Seman et al., 2017).

The Haobugao skarn Zn-Pb deposit is located in the southern Great Xing’an Range (SGXR), NE China, and contains reserves of 0.29 million metric tonnes (Mt) Zn at an average grade of 4.24% and 0.15 Mt Pb at an average grade of 2.25%, with subordinate Fe, Cu and Ag, and minor Mo and Sn (Sun et al., 2018, Shu et al., 2021). Previous studies determined the molybdenite Re-Os ages from 144 ± 4 to 137 ± 2 Ma (Wan et al., 2014, Liu et al., 2017, Wang et al., 2018a), which were within the uncertainty of zircon U-Pb ages from 145 ± 1 to 139 ± 2 Ma for the ore-related granite (Li, 2015, Li et al., 2016, Liu et al., 2017, Liu et al., 2018, Wang et al., 2019, Wang et al., 2018a). The temporal and spatial relationships suggest a genetic association between Zn-Pb mineralization and the emplacement of granitoids in the Haobugao deposit. However, there still lack for a direct constraint on the timing of the skarn event as well as its geochronological relationship with both granitic magmatism and base metal mineralization in this district. In this contribution, we applied LA-ICP-MS U-Pb dates of andradite to firstly identify the direct timing of the Haobugao skarns. Combined with the zircon LA-ICP-MS U-Pb ages of granitoids and ID-N-TIMS Re-Os ages of molybdenite from this deposit, we could establish a temporal sequence from magmatism, skarn to ore mineralization. The trace elements and REE concentrations of garnet detected by LA-ICP-MS also provide further insight into the physicochemical conditions of hydrothermal fluids.