Jilong Han

The North China Craton (NCC), one of the oldest cratons worldwide, may provide information on the evolution and geodynamic processes of the early Earth, especially during the pre-Mesoarchean period. Many ancient zircons have been discovered in the Jiapigou terrane of the northeastern NCC on the basis of our recent studies, providing an excellent opportunity to trace the early crustal evolution trend of the NCC. Here, we present a detailed study of the petrography, mineralogy, zircon U–Pb dating and Lu–Hf isotopes of supracrustal rocks (biotite schist) obtained from the Jiapigou terrane. Geochronology combined with the internal structures and Th/U ratios of the zircons reveal that the zircons acquired from the supracrustal rock can be divided into the following two types: magmatic zircons and metamorphic zircons. Among the magmatic zircons, the youngest zircon age (2.49 Ga) is considered to represent the time at which the protolith of the supracrustal rock (i.e., Neoarchean) crystallized, whereas the others were likely captured or inherited from their magma sources. The zircon Hf isotopes reveal that unexposed Hadean–Paleoarchean crust (4.18–3.57 Ga) is present beneath the Jiapigou terrane, and its growth history can be traced back to the Hadean period. Moreover, the evidence derived from this and previous studies indicates that the Jiapigou terrane underwent two crustal recycling events (3.37–3.20 Ga and ~2.96 Ga) during the Paleoarchean, two crustal reworking episodes (2.53 Ga and 2.49 Ga) during the Neoarchean, and later metamorphism at 2.41 Ga. Thus, the Jiapigou terrane has undoubtedly recorded multiple episodes of early crustal growth and/or reworking that are similar to, but not limited to, those of the northern and southern margins of the NCC.

The Jiapigou mining district, a world-famous gold-producing district with a capacity that greatly exceeds 180 t Au, has a mining history longer than 200 years. The large amount of Jurassic Au mineralization in this district significantly differs from that in other districts of the North China Craton (130–115 Ma). However, the deep-seated dynamic processes and mechanisms that triggered the unique Jurassic mineralization in the Jiapigou district are poorly understood. Here, we present new data on the geology, petrography, and zircon U–Pb and Lu–Hf isotopes of the typical Huangnihe pluton in the Jiapigou district to address the above issues. The results revealed the following: (1) The Huangnihe pluton comprises mainly fine-grained granite and porphyritic granite, which were emplaced at 187 ± 2 Ma (n = 13) and 166 ± 2 Ma (n = 15), respectively. (2) The Hf isotope data indicate that the two episodes of granites exhibit distinct origins: the former (εHf(t) = −1.4 to +5.3; T_DM2 = 1784–1181 Ma) originated from juvenile lower crust, whereas the latter (εHf(t) = −14.9 to −9.7; T_DM2 = 2987–2518 Ma) was derived from Archean crust. (3) On the basis of published geochemical data, the estimated crustal thicknesses of the Jiapigou district ca. 187 Ma, ca. 175 Ma, and ca. 166 Ma ranged from 45 to 52 km, 43 km, and 58 to 63 km, respectively. Combined with regional observations, the results of this study further reveal the following: (1) The Jurassic magmatism in the Jiapigou district can be subdivided into three episodes: 187–186 Ma, ca. 175 Ma, and 166–165 Ma. (2) The crust in the Jiapigou district gradually thickened during the Jurassic and underwent partial melting during multiple episodes of Paleo-Pacific Plate subduction, thereby generating arc-like calc-alkaline (ca. 187 Ma), adakite-like (ca. 175 Ma), and adakite magmas (ca. 166 Ma) that were emplaced to form corresponding granitoids. Moreover, syn-ore magma mixing between the ca. 175 Ma adakite-like felsic magma and mantle-derived mafic magmas was considered a crucial process in magma evolution. This process in turn promoted the enrichment of ore-forming elements within the magma system, which significantly contributed to the formation of the large Au mineralization in the Jiapigou district.

The Jiapigou mining district (>180 t Au) is an important gold district in China. For a long time, the ore genesis of the gold deposits in the Jiapigou district has been a subject of controversy and differing opinions, which has severely hindered metallogenic theories and mineral exploration. Here we present a comprehensive investigation including geology, fluid inclusions (FIs), and H–O–S isotopic data for the Naizhigou deposit in the Jiapigou district to elucidate the sources of orefluids and metals, as well as the metallogenic mechanism. The results show the following: (1) The Naizhigou deposit is characterized by quartz vein-type ores and is hosted in the Middle Jurassic granitic pluton. Native gold and sulfides were mainly deposited in the second stage (quartz–polymetallic sulfides) compared with the first (quartz–pyrite–molybdenite) and third (quartz–calcite) stages. (2) The FI studies indicated that the orefluids evolved from the early–main-stage CO₂–H₂O–NaCl system to the late-stage H₂O–NaCl system and have homogenization temperatures of 289–363, 210–282, and 124–276 °C and salinities of 4.1–20.9, 5.8–16.4, and 6.1–12.7 wt% NaCl equivalent, respectively. Fluid boiling and fluid mixing collectively controlled the precipitation of gold and ore-forming elements. (3) The δD values of the FIs hosted in quartz from the three stags range from −81 to −75 ‰, from −99 to −86 ‰, and from −110 to −101 ‰, while δ¹⁸O_water values of these FIs range from 5.3 to 5.9 ‰, from 1.1 to 5.2 ‰, and from −2.1 to −0.7 ‰, respectively. Pyrite samples from the three stages in the Naizhigou deposit have δ³⁴S values of 2.1 to 2.5 ‰, 3.1 to 4.3 ‰, and 3.8 to 3.9 ‰, respectively. The stable isotopes indicate that the orefluids and metals mainly originated from magma. A comparative study of regional observations reveals that the Naizhigou deposit is a magmatic-related mesothermal gold deposit, rather than a metamorphism-related orogenic gold deposit. The estimated ore-forming depths are 4.0–20.7 km, with exhumation depths of 4.1–5.5 km, which indicated that the deposit has been well preserved. Regionally, the new exploration strategies should place greater emphasis on work concerning ore-related plutons, ore-controlling faults, and hydrothermal alteration.

Skarn Au deposits exist in the circum-pacific metallogenic belt. Interestingly, the Da’anhe Au deposit is the only independent skarn gold deposit in the Lesser Xing’an Range. To determine the metallogenic mechanism and prospecting potential of the Da’anhe deposit, we performed skarn mineralogy, fluid inclusion (FI) and H-O isotope analyses. The results show the following: (1) The Da’anhe deposit is a calcareous reduced skarn Au deposit that formed between an Early Jurassic gabbroic diorite and the Permian Tumenling Formation marble. Its metallogenic process includes five stages: the early skarn stage (Stage I₁), late skarn stage (Stage I₂), early quartz-sulfide stage (Stage II₁), late quartz-sulfide stage (Stage II₂) and quartz-carbonate stage (Stage II₃). Gold precipitated in Stage II₁ and Stage II₂. (2) The initial ore-forming fluid was derived from magmatic water and featured a high temperature and intermediate to high salinity. After boiling and mixing, the fluid eventually changed to a low-temperature and low-salinity reducing fluid dominated by meteoric water. (3) The formation depth of the Au orebodies was 2.27–3.11 km, and the orebodies were later lifted to the surface (<500 m). The potential for finding skarn Au deposits in the study area is limited. (4) The distinctive nature of the ore-related magma (i.e., source, reducing conditions and high water content) was key to the formation of the Da’anhe skarn gold deposit.

Ocean-continent subduction drives sequential geologic events, including upwelling of mantle magmas and delamination of thickened lithosphere, which lead to crustal thickness variations and metallic enrichment or depletion at crustal roots. This paper estimates the crustal thickness in eastern Heilongjiang and Jilin Provinces based on La/Yb ratios of crust-derived I-type granites formed by Mesozoic Palaeo-Pacific slab westward subduction beneath Eurasia and assesses the related hydrothermal Cu mineralization. During the flat subduction phase (210–160 Ma), the continental crust dramatically thickened from 32 ± 6 km to 58 ± 6 km, indicating that much mafic magma accumulated under the ancient crust; notably, 210–177 Ma granitoids were generated in the thinner crust because of their normal arc natures, and subsequent remelting of mafic cumulates in the thicker crust formed adakitic magmas (~177–165 Ma) with low MgO (0.03–1.38 wt.%). From 140 to 95 Ma, the steep subduction of the dense slab drove regional decompression and consequent delamination and thinning of the lithosphere from ~58 km to 23 km, inducing explosive volcanic activity along with limited hydrothermal Cu deposition. Most Cu mineralization was associated with normal arc magmas; therefore, we speculate that two key factors were responsible for the small volume of Cu deposition: (1) thinner crust under an extensional setting failed to suppress voluminous pyroclastic eruptions, causing the dissipation of volatiles and preventing a large magma reservoir with long-lasting shallow differentiation; (2) root removal of thickened crust led large volumes of sulphide cumulates, if present, to founder into the mantle rather than remain in the crust, which greatly depleted Cu in the ascending melts.

The ~100,000 km² granitoid belt in the Lesser Xing’an-Zhangguangcai Ranges (LXZR) is an important component of the Phanerozoic granitic provinces (known as the ‘granitoid ocean’) in NE China. To reveal the mechanisms of Mesozoic magmatism and crustal growth in the LXZR, we present systematic geologic data, lithofacies information, zircon U-Pb ages, Hf isotopic, and element geochemical data for the Dong’an granites in the northern LXZR. The results show the following: (1) the Dong’an granites can be subdivided into three lithofacies, namely, medium- to coarse-grained alkali-feldspar granite, fine-grained alkali-feldspar granite, and granitic aplite, which were emplaced during the Early Jurassic (187–178 Ma); (2) the Dong’an granites are characterized by high SiO₂ and total alkali contents and low Fe₂O₃^t contents and Mg# values, and they are enriched in LILEs (e.g., Rb and K) and depleted in HFSEs (e.g., Nb and Ta); and (3) the zircons from these granites yield ε_Hf(t) values of −2.4 to +5.7 and two-stage model ages of 772–1542 Ma. These results indicate that the Dong’an granites were emplaced during the Early Jurassic, are highly fractionated I-type (HFI) granites and were likely derived from partial melting of the thickened lower crust. Regionally, the Dong’an granites and the Early-Middle Jurassic intrusive rocks (201–163 Ma; 90,000 km²) in the LXZR show a trend of comagmatic evolution controlled by fractional crystallization, and they were successively emplaced during magma evolution. The HFI granites that are geochemically similar to the Dong’an granites formed in the later stage of magma evolution. Combined with regional geological observations, we conclude that the granitic rocks in the LXZR formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. This process may record an important episode of crustal accretion in the LXZR.