Geology, Geochemistry, and Fluid Inclusions of the Gejiu Tin-Polymetallic Field, People's Republic of China
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The Gejiu tin field in southern China consists of six major deposits and many minor ones containing more than 120 million tons (pre-mining resource) at 1% Sn and significant amounts of Cu, Ag, Zn, and Pb. It is one of the largest tin fields of primary deposits in the world. Mineralization is the result of the intrusion of granitic plutons into Permian and Triassic sedimentary rocks, which are dominantly limestone, dolomitic limestone, and dolomite. Five (mostly peraluminous) granitic intrusives (64-115 Ma) are present in the area. The largest orebodies are spatially and temporally related to the Laoka (principally), Beipaotai, and Marsong granites. Tin mineralization is mainly within greisens developed at the outermost zone of a skarn zonal sequence and are mineralogically dominated by fluorite, quartz, and micas. The deposits are the result of volatile-rich ore solutions that evolved late in the plutonic crystallization history. The solutions produced metamorphic skarns as well as ore skarns, both of which later became "greisenized" skarns. Gejiu is the largest example of what has been, up to now, a style of mineralization reported only in minor amounts.Keywords:
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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 Handan-Xingtai district, situated in the central part of the North China craton, is one of the most important concentrations of Fe skarns in China. Baijian is the largest Fe skarn deposit in this district with significant Fe reserves being newly identified. This deposit is spatially related with a monzodiorite stock intruding the Middle Ordovician evaporate-bearing marine carbonates, with Fe mineralization occurring in the contact zone or within carbonate wall rocks. This paper conducts a comprehensive investigation encompassing geological, mineralogical, geochronological, and stable isotope analyses of the Baijian deposit. The goal is to provide insights into its formation and mineralization processes and offer a broader understanding of regional Fe metallogeny. The skarn mineralogy in the Baijian deposit is predominantly characterized by Mg-rich minerals such as diopside, tremolite, serpentine, and phlogopite. Magnetite is the dominant metallic mineral, featuring low Ti contents (<0.11 wt%) and high Fe concentrations (>66.59 wt%), indicative of a hydrothermal origin. The majority of the magnetite trace element data are plotted in the skarn field on the Al + Mn versus Ti + V diagram. Pyrite, a notable component in ores, exhibits considerable variations in Co and Ni concentrations, with Co/Ni ratio generally higher than unity. Phlogopite 40Ar–39Ar dating constrains the formation of the Baijian Fe skarn deposit at ca. 128 Ma, aligning with zircon U-Pb ages (128.8 ± 0.9 Ma) of the associated monzodiorite. This temporal congruence suggests a genetic relationship between the magmatism and skarn mineralization. Combined with previous published geochronological data, this study identifies an increasing trend in Fe mineralization intensity within the Handan-Xingtai district, spanning from ca. 137 to 128 Ma. Geological and oxygen isotopic evidence advocates for a magmatic origin of the ore-forming fluids at the Baijian deposit. The δ18O values of these fluids experience elevation through interaction with carbonate wall rocks. The pronouncedly high δ34S values of pyrite (>16.1 ‰) in the Baijian magnetite ores underscore a substantial sulfur contribution from sulfate in evaporate beds. Drawing on geological, mineralogical, and isotopic evidence, the study suggests that the interaction between magmatic fluids and evaporate-bearing carbonate rocks plays an important role in magnetite precipitation at the Baijian deposit. This interaction serves to reduce fluid acidity and facilitate the oxidation of ferrous iron (Fe2+). The Fe skarn deposits in Handan-Xingtai district are mostly hosted in middle Ordovician evaporite-bearing carbonate strata with ore-related sulfides exhibiting strong 34S enrichment (δ34S > 10 ‰). The interaction of magmatic fluids with evaporate-bearing carbonates is likely a common process responsible for magnetite deposition in the Fe skarn deposits.
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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 The Baba Ali skarn deposit, situated 39 km to the northwest of Hamadan (Iran), is the result of a syenitic pluton that intruded and metamorphosed the diorite host rock. Rare earth element (REE) values in the quartz syenite and diorite range between 35.4 and 560 ppm. Although the distribution pattern of REEs is more and less flat and smooth, light REEs (LREEs) in general show higher concentrations than heavy REEs (HREEs) in different lithounits. The skarn zone reveals the highest REE-enriched pattern, while the ore zone shows the maximum depletion pattern. A comparison of the concentration variations of LREEs (La–Nd), middle REEs (MREEs; Sm–Ho) and HREEs (Er–Lu) of the ore zone samples to the other zones elucidates two important points for the distribution of REEs: 1) the distribution patterns of LREEs and MREEs show a distinct depletion in the ore zone while representing a great enrichment in the skarn facies neighbouring the ore body border and decreasing towards the altered diorite host rock; 2) HREEs show the same pattern, but in the exoskarn do not reveal any distinct increase as observed for LREEs and MREEs. The ratio of La/Y in the Baba Ali skarn ranges from 0.37 to 2.89. The ore zone has the highest La/Y ratio. In this regard the skarn zones exhibit two distinctive portions: 1) one that has La/Y >1 beingadjacent to the ore body and; 2) another one with La/Y < 1 neighbouring altered diorite. Accordingly, the Baba Ali profile, from the quartz syenite to the middle part of the exoskarn, demonstrates chiefly alkaline conditions of formation, with a gradual change to acidic towards the altered diorite host rocks. Utilising three parameters, Ce/Ce*, Eu/Eu* and (Pr/Yb) n , in different minerals implies that the hydrothermal fluids responsible for epidote and garnet were mostly of magmatic origin and for magnetite, actinolite and phlogopite these were of magmatic origin with low REE concentration or meteoric water involved.
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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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Indium (In), one of the strategic critical metal elements, is preferred to be enriched in the skarn system, but the enrichment regularity in terms of temporal and spatial distribution in a specific skarn Pb-Zn deposit remains unclear. To address the problem, we conducted a systematic geological, mineralogical, and trace element geochemical investigation on the Baoshan In-rich Pb-Zn polymetallic deposit, Hunan Province. This deposit is situated in the central part of the famous Nanling W-Sn-Pb-Zn polymetallic metallogeny belt, South China. Three ore districts in the Baoshan deposit with distinct economic element associations are divided, including the Central District (dominated by Cu-Mo), West District (dominated by Pb-Zn), and North District (dominated by Pb-Zn). They comprise a typical skarn system, including the outcropped granodiorite porphyry, typical skarn mineral assemblages, and Pb-Zn-hosting Carboniferous limestone. We systematically sampled the representative Pb-Zn ores in two ore districts (West and North) and three underground levels (-270 m, −230 m, and −190 m). Four types of sphalerites with different mineral associations and mineralogical characteristics were identified, that is, Sp1a and Sp1b (formed in the early sulfide stage), as well as Sp2a and Sp2b (formed in the late sulfide stage). Our EPMA (Electron Probe Micro-Analyzer) and LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometer) analyses reveal the four types of sphalerite have distinct chemical compositions. Such as, the mean value of indium is 3524 ppm for Sp1a, 142 ppm for Sp1b, 48.03 ppm for Sp2a, and 87.98 ppm for Sp2b in a decreasing order, and a similar trend of calculated temperatures ranging from 336 to 135 °C using GGIMFis thermometers can be obtained. Further LA-ICP-MS trace elements mapping show that the core of Sp1a is relatively enriched in more In, Cu, Sn, and Ag contents. Spatially, the sphalerites in the deep contain a higher indium content (mean = 1581 ppm) than those in the shallow (107.20 ppm) in the vertical profile. In the planar, indium is more enriched in the West District (mean = 1090 ppm) than in the North District (60.92 ppm). The indium distribution regularity reflects that the metals-carrying magmatic hydrothermal fluids flow from Central District, through the West District, to the North District. Collectively, we conclude that indium prefers to be enriched in the earlier stages, higher temperature, and deeper space during the sphalerite crystallization in the Baoshan skarn system, and therefore highlight the deep space of Baoshan West District is a promising target for indium exploration. This new finding maybe shed light on the scientific understanding on indium enrichment and associated exploration strategies in the similar skarn Pb-Zn metallogenic systems.
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