Features of ore-forming magma and fluid revealed by apatite and zircon geochemistry: A case study from the Huanggang skarn Fe-Sn deposit, NE China
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The large-scale Huanggang Skarn Fe-Sn deposit is located in the southern part of the Great Xing'an Range (SGXR), NE China. The causative granitic rocks in the Huanggang ore district are characterized by consistently high SiO2 contents, low EuN/EuN* (0.02–0.24) and Sr/Y (0.2–1.9). The pre-ore granite porphyry and syn-ore syenogranite have been dated through zircon and monazite U-Pb dating at 144.6 ± 1.4 Ma and 138.6 ± 1.3 to 135.9 ± 0.8 Ma, respectively. The U-Pb dating of garnet from ore-associated skarn reveals that the mineralization was happened at 136.5 ± 1.3 Ma, which is consistent with the emplacement age of the ore-related syenogranite. Zircon geochemistry shows that the pre-ore granite porphyry has relatively low oxygen fugacity (average zircon Ce4+/Ce3+ = 14.74), whereas the syn-ore syenogranite is more oxidized (average zircon Ce4+/Ce3+ = 178.9). We suggest that the elevation of oxygen fugacity was most likely caused by degassing. The textural features indicate that apatite crystals from the pre-ore granite porphyry are magmatic in origin. In contrast, textural and chemical characteristics of apatite grains from syn-ore syenogranite reveal that they have been modified by hydrothermal fluids in different degrees. The apatite in both pre- and syn-ore granites display relatively high F and low Cl contents, and very low SO3 concentrations (i.e., <0.032 wt%), indicate extremely low content of S in the causative magma. Apatite from syn-ore syenogranite display more strong and variable enrichment of LREE relative to HREE compared to those from pre-ore granite porphyry. The extensive present of new REE mineral inclusions (monazite and xenotime) in the altered apatite suggests that the concentrations of Ca and Na must be low in the ore-forming fluid. Apatite from syn-ore syenogranite have higher concentrations of Mo, W, Zn, Sn, and Be than those from pre-ore granite porphyry. Combined with zircon and apatite geochemistry, and whole rock components of the ore-related granites in the Huanggang deposit, we propose that the ore-forming magma has undergone long-term evolution under relatively reduced conditions with extensive fractional crystallization of plagioclase. The reduced and S-poor evolved magma could have promoted the enrichment of Sn and prevented Fe from precipitating as sulfide. This study also reveals that the compositional characteristics of ore-forming fluid, such as the enrichment of ore-forming materials, could be revealed by altered apatite geochemistry.Keywords:
Mineral redox buffer
Ore genesis
Huanren skarn polymetallic deposit,hosted by skarn contact belt between Cambrian limestone and late Yanshanian diorite complex,is located in conjuction of northern part of Liaodong Rift and Taizihe Depression.Typically,skarn minerals are characterized by two main alteration styles: early prograde assemblage and later retrograde assemblage,and there is an obvious change in ore elements from Fe→Cu(Mo) to Zn→Pb upwards.In order to understand physicochemical properties and evolution history of ore-forming fluids,vapor-liquid fluid inclusions from garnet and calcite were studied.The data show that temperatures of homogenization for garnet fluid inclusions range from 376.1 to 450.0 ℃,with an average of 411.6 ℃;whereas those for calcite fluid inclusions range from 122.6 to 170.0 ℃ and 178.3 to 270.2 ℃,with averages of 149.5 ℃ and 204.5 ℃ respectively.Temperatures of final ice melting range between -4.2 and -17.6 ℃.All data we determined are consistent with those of former researcher and are of theoretical and practical significance in comparison with that of other skarn deposits.Based the above,we proposed that ~410 ℃,400~300 ℃,~150 ℃ and ~200 ℃ are main ore-forming temperatures of Fe-Cu(Mo),Cu-Zn and Zn-Pb ore bodies,respectively.Old Pb,S and D-O geochemical data and new REE data indicate that magmatic fluid originates in the upper mantle and carries 2010 年 plentiful ore-forming materials in migration,meteoric water may be involved during later skarn retrograde stage.
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The Huanggangliang deposit in Inner Mongolia is a large-size Fe-Sn polymetallic deposit in the central-southern section of Da Hinggan Ling area, where Jurassic-Cretaceous volcanic-plutonic rocks are widespread. Thus the deposit is regarded as an epigenetic hydrothermal deposit genetically related to the Mesozoic magmatism. Based on a study of geochemical characteristics of this deposit combined with the deposit geology, the authors have reached the following conclusions: (1)the stratabound ore-bearing skarn associated with magnetite ore and micro-disseminated tin is a peculiar example of exhalites;(2)REE geochemical characteristics show that the stratabound skarn in this deposit is different remarkably from the typical magmatic-hydrothermal contact metasomatic skarn but is quite similar to the modern sea floor hydrothermal fluid sedex deposit and associated hydrothermal sedimentary rock, and hence it should genetically belong to the hydrothermal exhalative type;(3)the relationship between carbon and oxygen isotopes in the stratabound ore-bearing skarn is on the whole comparable with that of many sedex-type sulfide ores and associated exhalites,implying a similar genesis for these deposits.
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Abstract On the basis of geological studies of skarn deposits in China and by using thermodynamic models of the solid solution, the coexisting clinopyroxene‐ garnet pair in skarn deposits has been analysed and a coexisting clinopyroxene‐garnet acidometer developed. They can be used to estimate the medium condition under which the skarn was formed. The research on the acidity for the formation of various metallic skarn deposits in China suggests that skarns endowed with dissimilar types of mineralization and occurring in diverse environments differ in the acidity conditions for their formation and in the trend of acidity variation of ore‐bearing fluid. This, coupled with the study of oxygen fugacity of coexisting minerals, makes it possible to establish the oxygen fugacity‐ acidity facies of skarn deposits, which reflect the close genetic relationship between the metallization and the formation of skarn.
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The Baiyinnuo’er zinc-lead deposit (32.74 Mt at 5.44% Zn, 2.02% Pb, and 31.36 g/t Ag), located in the south segment of the Great Xing’an Range, is the largest Zn-Pb deposit in northern China. Skarn and orebodies mainly occur between the different units of the Huanggangliang Formation, or within the contact zone between the intrusions and Permian marble.
Several phases of igneous rocks exposed within the mining areas, and among them the Yanshanian plutonic rocks, which intruded into limestone of the early Permian Huanggangliang Formation, are interpreted to be the source of ore, since their Pb isotope compositions (206Pb/204Pb = 18.25–18.35, 207Pb/204Pb = 15.50–15.56, and 208Pb/204Pb = 38.14–38.32) are highly consistent with the sulfides, including sphalerite, galena, and chalcopyrite (206Pb/204Pb = 18.23–18.37, 207Pb/204Pb = 15.47–15.62, and 208Pb/204Pb = 37.93–38.44). Sulfur isotope values of the sulfides give a narrow δ 34S interval of −6.1 to −4.6‰ (mean = −5.4‰, n = 15), suggesting the ore-forming fluid is of magmatic origin.
Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observation, which are the preore stage (garnet-clinpyroxene-wollastonite-magnetite ± sulfides), the synore stage (sulfides-epidote-quartz-calcite ± garnet), and the postore stage (calcite-chlorite-quartz-fluorite). Several fluid evolution episodes can be inferred from microthermometric results at the Baiyinnuo’er Zn-Pb deposit:
1. Immiscibility: Preore-stage coexistence of halite-bearing brine inclusions (S1-type, ~44 wt % NaCl equiv) and vapor-rich fluid inclusions (V-type) sharing the same homogenization temperatures (~470°C) confirms that fluid unmixing occurred under lithostatic pressures of ~400 bars (~1.5 km), and the brine is considered to account for most prograde skarn minerals (e.g., clinopyroxene).
2. Overpressure trapping: Preore-stage brine inclusions homogenized by halite dissolution (S2-type) postdated the immiscible assemblages. This type of inclusions is characterized by high but variable (minimum) trapping pressures (150–3,000 bars) relative to S1-type inclusions and can be explained as a result of entrapment under overpressuring condition.
3. Boiling: The presence of both vapor and liquid inclusions (i.e., V- and L-type) in the same assemblages within synore-stage quartz, calcite, and sphalerite indicates the occurrence of fluid boiling (~350°C), at hydrostatic pressures of ~150 bars, and depth of ~1.5 km), which resulted in large-scale mineralization in the Baiyinnuo’er Zn-Pb deposit.
4. Mixing with external fluids: Fluid inclusions scattered within postore-stage calcite or secondary trails in synore-stage minerals show low homogenization temperatures (<260°C) and both decreasing (for L-type) and increasing (for CaCl2-bearing inclusions, i.e., Lc-type) trends for salinities as homogenization temperatures decrease, implying addition of both meteoric water (low-temperature, low-salinity) and basinal brines (low-temperature, Ca-rich), respectively.
Systematic fluid inclusion studies also indicate that the mineralization-related fluid is of magmatic origin. Prograde minerals formed during the preore-stage fluid immiscibility while sulfides deposition occurred during the synore-stage fluid boiling. Mixing with external fluids began as the hydrothermal system cooled to <300°C, when the main metal precipitation process had ended.
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Apatite [Ca5(PO4)3(F,Cl,OH)] is present in a wide range of planetary materials, and due to the presence of volatiles within its crystal structure (X-site), many recent studies have attempted to use apatite to constrain the volatile contents of planetary magmas and mantle sources [i.e., 1]. Experimental studies have investigated the apatite-melt partitioning behavior of F, Cl, and OH in basaltic systems [e.g., 2- 3], reporting that apatite-melt partitioning of volatiles is best described as exchange equilibria similar to Fe-Mg partitioning between olivine and silicate melt. However, exchange coefficients may vary as a function of temperature, pressure, melt composition, and/or oxygen fugacity. Furthermore, exchange coefficients may vary in portions of apatite compositional space where F, Cl, and OH do not mix ideally in apatite [3]. In these regions of ternary space, we anticipate that crystal chemistry could influence partitioning behavior. Consequently, we conducted experiments to investigate the effect of apatite crystal chemistry on apatite-melt partitioning of F, Cl, and OH.
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