Geochemical characteristics of rare earth elements and their implications for the Huachanggou gold deposit in Shaanxi Province, China
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Rare-earth element
Ore genesis
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Gold deposits in the Taihang Mountains, northern China, mainly consist of quartz sulfide veins in granitoid plutons. This paper describes the geological setting of the gold deposits, and presents the results of microthermometric, Fourier transform infrared spectra, and stable isotope analyses of ore—forming fluids for the purpose of examining the characteristics of these fluids. The ore—forming fluid was of high temperature (up to 380°C) and high salinity (33–41 wt% NaCl equiv.), represented by type I inclusions (with daughter minerals). This fluid evolved to low salinity at low temperatures recorded in type II (liquid-rich) and III inclusions (vapor—rich). Primary type II inclusions coexist with type III inclusions in quartz. Type III inclusions have almost the same homogenization temperatures as type II inclusions. This probably reflects boiling. The secondary fluid inclusions homogenized at lower temperatures, and have lower salinities than primary inclusions. Based on microthermometric data, we propose that the high—temperature fluid that separated from residual magma corresponded to the ore—forming fluid represented by type I inclusions. This fluid mixed with meteoric water in the upper part of the granitic pluton and was diluted. The diluted fluid boiled, probably due to abrupt pressure decrease, and formed liquid—rich type II inclusions and vapor—rich type III inclusions. The deposition of sulfide minerals and gold probably occurred during boiling.
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Introduction: The Zhangjiapingzi gold deposit, located at the western margin of the Yangtze Craton and controlled by the Jinhe-Chenghai deep fault, is a newly discovered super-large gold deposit in the Danba-Mianning metallogenic belt. The gold ore bodies are hosted in the Middle Triassic (T 2 ) altered dolomite (Dm), and have two types of mineralization: altered rock type and quartz vein type. Previous studies on this deposit are rare, especially on the ore-fluid characteristics, which limit the understanding on the ore genesis. Methods: This study focused on fluid inclusions in quartz from altered rocks, and used microthermometry and laser Raman spectroscopy to investigate the properties and sources of ore-forming fluids, and to determine the ore genetic type. Results and discussion: The results show that the fluid inclusions are mainly CO 2 -H 2 O-NaCl inclusions, with medium temperature (220–300°C), low salinity (<10%), medium-low density (0.79–1.01 g/cm 3 ) and high contents of CO 2 and CH 4 , resembling typical orogenic gold ore fluids. We suggest that the Zhangjiapingzi is best classified as orogenic type, and our findings provide new insights into the fluid origin and metallogenesis of orogenic gold deposits in the Danba-Mianning metallogenic belt.
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The Dongfengbeishan molybdenum deposit is located in northeastern Tianbaoshan ore concentration area in Yanbian Prefecture, Northeast China. Almost all discovered orebodies are controlled by faults and fractures in biotite quartz diorite, and characterized by veins and veinlets. The deposit experienced three metallogenic stages, namely quartz-molybdenite stage, quartz-polymetallic sulfide stage and quartz-calcite stage. Fluid inclusions in ore-bearing quartz veins can be classified into liquid-rich phase, gas-rich phase, daughter mineral-bearing three-phase and minor CO2-bearing three-phase occasional pure CO2 inclusions. Fluid boiling occurred during ore-forming evolution and might have played a very important role in metal elements precipitation. Besides H2 O vapor, the gas phase in different types of inclusions, analyzed by Laser Raman Spectrum, also contains small amounts of CO2 and CH4,which indicates that the ore-forming fluid belongs to the H2O-Na Cl±CO2±CH4magmatic fluid system with medium-high temperature and medium-high salinity. The discovered vein molybdenum orebodies in the Dongfengbeishan molybdenum deposit resulted from the porphyry molybdenum mineralization, which implies that the metallogenic porphyry intrusion should have occurred under the biotite quartz diorite. The veinlet disseminated molybdenum mineralization has important guiding implications for further theoretical research and geological prospecting.
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The Jidong area is a vital gold province in North China, including plenty of metamorphic rock-hosted gold deposits. Whether ore-forming fluids are magmatic or metamorphic remains controversial. The genesis for gold mineralization in this gold province is ambiguous. The Fenghuangshan gold deposit lies on the eastern fragment of the Jidong gold province. In this paper, we present a detailed study on ore geology, fluid inclusions, and stable isotopes for the deposit to illustrate the characteristics of ore-forming fluids and ore genesis. The deposit occurs as gold-bearing quartz vein and disseminated- and veinlet-type mineralization. The main orebody named 7# is hosted in migmatitic granite, controlled by NE trending secondary fracture system. Four hydrothermal mineralization stages were identified, including: (Ⅰ) white quartz, (II) pyrite-quartz, (III) quartz-polymetallic sulfide, and (IV) calcite-chlorite-quartz. The Stage II and III belong to main gold mineralization stage. Fluid inclusions from different stages show that a lowering trend of fluid inclusion homogenization temperature was observed. They ranged from 222 °C to 407 °C (early-stage), 144 °C to 375 °C (main-stage), and 86 °C to 174 °C (late-stage), respectively. The salinities of the ore-forming fluids were typically low, ranging from 0.83 to 15.47 wt% NaCl equivalent. On the basis of Laser Raman analysis and fluid inclusion microthermometry, the ore-forming fluids belonged to a low- to moderate-temperature, low-salinity H2O-NaCl-CO2 system. Fluid immiscibility gave rise to rapid gold deposition. The early-stage δD and δ18OH2O values from quartzs ranged from –96.4‰ to –92.5‰ and from 1.74‰ to 2.44‰, whereas main-stage fluids had δ18OH2O values of –4.10‰ to –3.50‰ and δD values of –91.5‰ to –89.5‰. The early- and main-stage sulfide δ34S values varied from –2.0‰ to + 2.2‰, with average values of + 0.4‰. H-O-S-isotope data indicated that the primary ore-forming fluids were magmatic water mixing meteoric water during mineralization, and ore-forming materials were derived from a homogeneous magmatic source. Ore geology, fluid inclusions, and stable isotopes illustrate that the Fenghuangshan gold deposit is a low- to moderate-temperature magmatic-hydrothermal gold deposit. Future deep-seated and surrounding gold exploration should center on this area.
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