Jinggui Sun

The reduction of oxidized felsic magmas at upper-crustal levels plays an important role in the formation of some porphyry Cu deposits with reduced ore assemblages dominated by pyrrhotite and chalcopyrite. However, the timing of this reduction process relative to volatile exsolution, which is critical to understanding the genesis of this variety of porphyry system, is unclear. The Ermi porphyry Cu system in Northeast China preserves robust petrological evidence that the onset of pyrrhotite and chalcopyrite precipitation was triggered by the reduction of the early oxidized magmatic-hydrothermal system through reduced mafic melt replenishment in the upper crust. This mafic melt is preserved as mafic microgranular enclaves (MMEs) that host fine-grained hornblende, which crystallized at oxygen fugacity (fO2) of NNO −0.78 ± 0.22 (where NNO is the nickel−nickel oxide buffer). The fO2 values estimated from hornblende crystals in the quartz diorite and granodiorite hosts to MMEs show that this replenishment event significantly decreased the fO2 of the magmatic system from NNO +1.0 to NNO <0. Biotite crystals from these intrusive rocks and MMEs also yielded magmatic fO2 values of NNO <0. Based on the hornblende thermobarometer and hygrometer, the parental magma to the Ermi Cu deposit mainly evolved at pressures <3 kbar and contained 4.38−5.94 wt% water. In addition, the water-saturated and oxidized Ermi magma experienced fluid exsolution during differentiation, as evidenced by the lower Cl contents (0.94−0.03 wt%) of apatite. The transition from sulfate-dominated to sulfide-dominated (S2−) fluids occurred when the system was recharged by the reduced mafic melt. We attribute the pyrrhotite-dominated ore assemblage in many porphyry Cu deposits to a decrease in the S6+/S2− ratio of the magmatic-hydrothermal systems, which resulted from the addition of reduced materials (e.g., magmas and organic carbon-rich sedimentary rocks) to the oxidized magmatic systems in the shallow crust. This reduction process could also have led some of the Cu to be lost from the magmatic-hydrothermal systems by early sulfide segregation prior to fluid ascent and ore deposition.

The Jiapigou area is a world‐famous gold mining district. Recent studies have revealed that large‐scale mineralization occurred between 180 and 172 Ma, which is temporally associated with the Wudaoliuhe intrusive rocks. The Wudaoliuhe intrusive rocks are mainly divided into two stages, including Early Jurassic monzogranite (stage I) and small amounts of Middle Jurassic diorite, granodiorite and monzogranite (stage II), which are present as stocks in the stage I monzogranite and Archean supracrustal rocks. Zircon U–Pb ages suggest that the magma emplacement of the stage I monzogranite occurred at 186.1 ± 1.2 Ma, whereas stage II diorite, granodiorite and monzogranite were emplaced at 176.3 ± 2.4 Ma, 175.0 ± 1.4 Ma and 174.4 ± 1.1 Ma, respectively. The geochemical characteristics reveal that stage I and monzogranite and stage II granodiorite and monzogranite have high SiO ₂ and Al ₂ O ₃ contents, low MgO, and CaO contents, A/CNK ratios (1.01–1.10), are enriched in LILEs and LREEs, depleted in HFSEs and HREEs and exhibit moderate negative Eu anomalies. The stage II diorite has high SiO ₂ , Al ₂ O ₃ , CaO and MgO contents, A/CNK ratios (0.93–0.96), is enriched in LILEs, lacks HFSEs and has significant Eu anomalies. The εHf(t) values of the stage I monzogranite are −5.5 to −11.3. The εHf(t) values of stage II diorite, granodiorite and monzogranite are −5.7 to −4.7, −6.9 to −9.7, and −7.7 to −10.4, respectively. These geochronology and geochemistry characteristics indicate that the deep geological process in the Early Jurassic was large‐scale partial melting of the crust. With the evolution of the magma chamber and the addition of mantle‐derived materials, some residual magma transformed into intermediate–basic magma. The magma intruded upwards and formed the stage II diorite, granodiorite and monzogranite. The large‐scale mineralization in the Jiapigou mining district (JMD) coincided with an extensional‐transition process triggered by the subduction of the palaeo‐Pacific Ocean. The metallogenic process coincided with partial melting of the continental lower crust, and mantle‐derived magma participated in the late stage of magma chamber evolution. These findings highlight that crustal partial melting and mantle‐derived material input were critical factors controlling the large‐scale Au mineralization in the JMD.

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.