Hu Fangfang

Fluid Inclusion and Carbon-Oxygen Isotope Studies of the Hujiayu Cu Deposit, Zhongtiao Mountains, China: Implications for Syn-metamorphic Copper Remobilization

The Hujiayu Cu deposit, representative of the “HuBi-type” Cu deposits in the Zhongtiao Mountains district in the southern edge of the North China Craton, is primarily hosted in graphite-bearing schists and carbonate rocks. The ore minerals comprise mainly chalcopyrite, with minor sphalerite, siegenite [(Co, Ni)3S4], and clausthalite [Pb(S,Se)]. The gangue minerals are mainly quartz and dolomite, with minor albite. Four fluid inclusion types were recognized in the chalcopyrite-pyrite-dolomite-quartz veins, including CO2-rich inclusions (type I), low-salinity, liquid-dominated, biphase aqueous inclusions (type II), solid-bearing aqueous inclusions (type III), and solid-bearing aqueous-carbonic inclusions (type IV). Type I inclusion can be further divided into two sub-types, i.e., monophase CO2 inclusions (type Ia) and biphase CO2-rich inclusions (with a visible aqueous phase), and type III inclusion is divided into a subtype with a halite daughter mineral (type IIIa) and a subtype with multiple solids (type IIIb). Various fluid inclusion assemblages (FIAs) were identified through petrographic observations, and were classified into four groups. The group-1 FIA, consisting of monophase CO2 inclusions (type Ia), homogenized into the liquid phase in a large range of temperatures from −1 to 28°C, suggesting post-entrapment modification. The group-2 FIA consists of type Ib, IIIb and IV inclusions, and is interpreted to reflect fluid immiscibility. The group-3 FIA comprises type II and IIIa inclusions, and the group-4 FIA consists of type II inclusions with consistent phase ratios. The group-1 and group-2 FIAs are interpreted to be entrapped during mineralization, whereas group-3 and group-4 FIAs probably represent the post-mineralization fluids. The solid CO2 melting temperatures range from −60.6 to 56.6° C and from −66.0 to −63.4° C for type Ia and type IV inclusions, respectively. The homogenization temperatures for type II inclusions range from 132 to 170°C for group-3 FIAs and 115 to 219°C for group-4 FIAs. The halite melting temperatures range from 530 to 562°C for type IIIb and IV inclusions, whereas those for type IIIa inclusions range from 198 to 398°C. Laser Raman and SEM-EDS results show that the gas species in fluid inclusions are mainly CO2 with minor CH4, and the solids are dominated by calcite and halite. The calcite in the hosting marble and dolomite in the hydrothermal veins have δ13CV-PDB values of −1.2 to 1.2‰ and −1.2 to −6.3‰, and δ18OV-SMOW values of 14.0 to 20.8 ‰ and 13.2 to 14.3 ‰, respectively. The fluid inclusion and carbon-oxygen isotope data suggest that the ore-forming fluids were probably derived from metamorphic fluids, which had reacted with organic matter in sedimentary rocks or graphite and undergone phase separation at 1.4–1.8 kbar and 230–240°C, after peak metamorphism. It is proposed that the Hujiayu Cu deposit consists of two mineralization stages. The early stage mineralization, characterized by disseminated and veinlet copper sulfides, probably took place in an environment similar to sediment-hosted stratiform copper mineralization. Ore minerals formed in this precursor mineralization stage were remobilized and enriched in the late metamorphic hydrothermal stage, leading to the formation of thick quartz–dolomite–sulfides veins.

Fluid Evolution in the Sanshandao Gold deposit, Jiaodong Peninsula, China

The Jiaodong gold province located in the Jiaodong Peninsula of eastern China is the most important goldproducing district and is the host for several world-class gold deposits (>100 t gold) in the country (Zhou and Lü, 2000; Qiu et al., 2002; Fan et al., 2003; Hu et al., 2013). Gold deposits here were divided into two types according to ore occurrence, referred to as “Linglong-type” and “Jiaojia-type” (Qiu et al., 1988; Goldfarb and Santosh, 2014). The Linglong-type lode gold mineralization is characterized by massive auriferous quartz veins with narrow alteration halos and usually occurs in subsidiary secondor third-order faults. The Jiaojia-type disseminated and stockwork gold mineralization is usually surrounded by broad alteration zones and generally develops along major first-order regional faults. The Jiaodong gold province hosts dozens of gold deposits. Although most of them have been extensively described, evolution of the ore-forming fluids within the scope of 4000 meters deep from the surface has not been investigated. This study attempts to evaluate the nature and evolution of the ore-forming fluid in the Sanshandao gold deposit from fluid inclusion and stable isotope analysis.

Ore-forming Fluids and Ore Genesis in the World-class Mesozoic Gold Province, Jiaodong Peninsula, Eastern China

The Jiaodong Peninsula is currently the most important gold province in China, with a total gold ore reserve of >1300 tons (Li et al. 2007). Seven world-class gold deposits (> 100 t gold), eight large gold deposits (20 to 100 t gold) and more than one hundred middle to small gold deposits (< 20 t gold) have been discovered in the peninsula (Fig. 1) during the past three decades, accounting for about 25% of China’s gold reserves (Fan et al. 2003). It is located along the southeastern margin of the North China Craton, which is the largest and oldest (3.8– 2.5 Ga) craton in China. Mesozoic granitoids, occupying >50 percent of the northwestern part of the Jiaodong Peninsula, intrude Precambrian basement rocks that have undergone amphibolite to granulite facies metamorphism. The majority of gold resources (>95%) are hosted by these granitoids, making the Jiaodong gold province one of the largest granitoid-hosted gold provinces recognized in the world. The Jiaodong Peninsula occupies the easternmost edge of the Eastern Block of the North China Craton (Zhai and Santosh) and is geologically divided into the southeastern Ludong terrane and the northwestern Jiaobei terrane by the Mishan fault. The Ludong terrane petrotectonically belongs to the northern margin of the South China Block, which records a subduction history associated with a Triassic collisional event. The Jiaobei terrane petrotectonically belongs to the North China Craton. Almost all gold deposits are hosted in the Jiaobei terrane, and more than 80% of the gold reserves are concentrated in the Zhaoyuan-Laizhou gold belt (Fig. 1). The Precambrian basement in the Jiaobei terrane is principally defined by the Archean Jiaodong Group and the Paleoproterozoic Fenzishan and Jingshan Groups. Mesozoic magmatic rocks are widely exposed in the Jiaobei terrane and two main periods of magmatism are recognized, Jurassic and Early Cretaceous. The Jurassic magmatic activity is represented by the crustallyderived Linglong, Luanjiahe, and Kunyushan granitoids, emplaced at 160–150 Ma (Yang et al. 2012). In the Early Cretaceous, extensive magmatism took place through strong crust-mantle interaction, including formation of widespread granitoids (130–126Ma, and 113–110 Ma), mafic to felsic volcanic rocks in the Jiaolai Basin (130– 110 Ma), and numerous mafic dikes (124–122 Ma) with less commonly at 110 to 102 Ma (Yang et al. 2012; Cai et al., 2013). The Mesozoic granitoids are hosts for most gold deposits. Gold deposits in the peninsula can be divided into three mineralized belts from west to east, which include Zhaoyuan-Laizhou, Penglai-Qixia, and Muping-Rushan (Fig. 1). Each belt is separated by Jurassic to Cretaceous volcanic-sedimentary basin. Gold deposits have been classified as the Linglong-type and the Jiaojia-type, both of which are essentially fault controlled. The Linglongtype mineralization is characterized by massive auriferous quartz veins hosted in subsidiary secondor third-order faults cutting Mesozoic granitoids, whereas the Jiaojiatype mineralization consists of disseminatedand stockwork-style ores located in regional faults, which are enveloped by broad alteration halos.