Genesis of the Zhijiadi Ag-Pb-Zn Deposit, Central North China Craton: Constraints from Fluid Inclusions and Stable Isotope Data
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Abstract:
The Zhijiadi Ag-Pb-Zn deposit is located in the central North China Craton. Fluid inclusions (FIs) studies indicate three types of FIs, including aqueous, aqueous-carbonic, and daughter mineral-bearing multiphase inclusions. The daughter minerals in FIs are mainly composed of marcasite, chalcopyrite, calcite, and dolomite. Microthermometric data show that the homogenization temperature and salinity of FIs decrease gradually from early to late stages. Homogenization temperatures from early to main to late stages span from 244 to 334°C, from 164 to 298°C, and from 111 to 174°C, respectively, while their salinities are 4.0–9.9 wt.% NaCl equiv., 0.5–12.7 wt.% NaCl equiv., and 0.2–8.8 wt.% NaCl equiv., respectively. Trapping pressures drop from 203–299 MPa (the early stage) to 32–158 MPa (the main stage). The dropping of pressure and temperature and mixing and/or dilution of ore-forming fluids result in the formation of ore deposit. Combined with C-O-S-Pb isotopic compositions, the initial ore-forming fluids and materials were likely derived from a magmatic system. As a whole, we proposed that this deposit belongs to medium-low temperature hydrothermal deposit related to volcanic and subvolcanic magmatism strictly controlled by the fault zones.Keywords:
Ore genesis
Magmatic water
The Talate ore field is located in the Abagong polymetallic metallogenic belt of the Altay Orogen, NW China. Lenticular ore bodies occur in the Kangbutiebao Formation, a package of intermediate‐felsic marine volcanic rocks and terrigenous clastic sedimentary‐carbonate rocks. Skarn alteration (mainly garnet) is present in both ore and wall rocks, especially the carbonate rocks. The mineral assemblages and cross‐cutting relationships of veins allow the alteration and mineralization process to be divided into four stages. From early to late, these are the early skarn (E‐skarn), the late skarn with quartz–magnetite veins (QM), the quartz–sulphide (QS) and the quartz–carbonate (QC) assemblages. Quartz crystals are important gangue minerals in the latter three stages, in which four distinct compositions of fluid inclusions are identified based on petrography, microthermometry and laser Raman microspectroscopy, namely aqueous inclusions (W‐type), pure CO 2 inclusions (PC‐type), CO 2 ‐rich inclusions (C‐type) and daughter mineral‐bearing inclusions (S‐type). Microthermometric data and laser Raman analyses show that the quartz crystals from the QM stage contain all four inclusion types, with the W‐type being predominant. Homogenization temperatures range between 271 and 426 °C. The salinities of the W‐ and C‐type fluid inclusions range from 0.5 to 22.4 wt.% NaCl eqv., whereas the S‐type fluid inclusions in the QM stage range from 31 to 41 wt.% NaCl eqv. Daughter minerals in the fluid inclusions include halite, sylvite, pyrite and calcite. Quartz from the QS stage (main mineralization stage) contains the W‐, C‐ and PC‐type inclusions, which are homogenized at temperatures of 204–269 °C, with salinities of 0.2–15.6 wt.% NaCl eqv. Only W‐type fluid inclusions have been identified in the QC stage. These yielded homogenizing temperatures of 175–211 °C and salinities of 1.1–9.9 wt.% NaCl eqv. The C‐type fluid inclusions of the main (QS) mineralization stage yield trapping pressures of 107–171 MPa, corresponding to a depth of 4–6 km. The sulphur isotope values (−1.7‰ and −6.6‰) imply that the QS stage may not be directly associated with the early skarn (−7.4‰) and quartz–magnetite stages (−4.8‰ and −5.0‰), though the QS stage is probably dominated by magmatic‐hydrothermal fluids. 40 Ar/ 39 Ar isotope plateau ages of 227.6 and 214.1 Ma for biotite separated from the QM and QS stages are significantly younger than the host Kangbutiebao Formation ( ca . 410 Ma). The Talate Pb–Zn(–Fe) deposit is interpreted to be an unusual skarn‐type system formed in a continental collision orogeny. Copyright © 2014 John Wiley & Sons, Ltd.
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Carbon-rich or carbonic fluids are identified in some orogenic-type gold deposits worldwide. Their nature, origin, evolution, and relations with gold mineralization are still poorly understood. The Tokuzbay large-size orogenic-type gold deposit in Chinese Altai is characterized by enormous CO2-rich fluid inclusions (FIs), providing an ideal case to investigate the genetic relationship between carbon-rich hydrothermal systems and orogenic-type gold mineralization. The mineralization process in the Tokuzbay includes four stages: (1) stage I quartz-magnetite veins; (2) stage II quartz-pyrite veins; (3) stage III quartz-polymetallic sulfide veins; and (4) stage IV quartz-carbonate veins. Based on the microscopic investigation, cathodoluminescence (CL) imaging, Raman spectrum analysis, and mircothermometric measurements, three types of FIs are identified in the veins, including the N2-bearing carbonic or CO2-rich inclusions (PC-type), the CO2-H2O inclusions (C-type), and the H2O-dominated or aqueous inclusions (W-type). The FIs in the stage I minerals are dominated by PC- and C-types, with minor amount of W-type, and occur as primary and pseudosecondary within quartz grains. These inclusions show a salinity of 5.0—9.5 wt% NaCl eqv. and homogenize to liquid phase during 272℃ to 399℃, indicating their entrapment from an initially homogeneous carbon-rich CO2-H2O-NaCl metamorphic fluid system. From early to late stages, the PC- and C-type FIs are getting infertile, while the W-type FIs become more abundant, indicating the escapes of CO2 and compositional transition from CO2-rich to H2O-rich system. Moreover, the decreasing in temperature (272–399 ℃, 238—326 ℃, 173—267 ℃ and 135—182 ℃ from stages I to IV, respectively) and salinity (5.0—9.5, 2.6—9.2, 2.3—7.8 and 1.4—4.3 wt% NaCl eqv. from stages I to IV, respectively), together with the decrease of the δ18O values, suggests an input of meteoric water during mineralization. In the main mineralization stages (II and III), some FIs show divergent homogenization direction (liquid vs vapor) at similar homogeneous temperatures, yielding quite different salinities. For example, the coexisting W-type and C-type FIs yield similar homogenization temperatures, and the liquid-rich and vapor-rich FIs divergently homogenize to liquid and vapor at similar temperatures with different salinities. This indicates that the fluid effervescence drove phase separation and caused the CO2 escape from the fluid system which promoted the gold precipitation at the ductile-to-brittle transition zone that revealed by the depth (8—11 km) calculated by the C-type FIs of the immiscible inclusion assemblages. The δ13C values of the CO2 extracted from fluid inclusions range from –7.6 to –22.8‰, suggesting that the CO2 might be derived from the hydrolysis of organic matter in the source sediments during metamorphism, rather than simply sourced from magma or the mantle. This understanding is supported by the widespread observation of the low δ13C values from most of orogenic-type gold deposits hosted in sedimentary rocks worldwide.
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