The formation and trace elements of garnet in the skarn zone from the Xinqiao Cu-S-Fe-Au deposit, Tongling ore district, Anhui Province, Eastern China
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Andradite
Grossular
Metasomatism
Bornite
Trace element
Ore genesis
Compared to calcareous skarn Au-Cu deposits, magnesian skarn (Mg-skarn) Au-Cu deposits are scarce; hence the early evolution of the ore-fluids to late mineralization for the kinds of deposits has been inadequately investigated. Located at the southern margin of the Bangong-Nujiang metallogenic belt, the Galale Au-Cu deposit contains 32.59 tonnes of gold (average grade 2.04 g/t) and 105,600 tonnes of copper (average grade 0.66 %). The orebodies are hosted in an Mg-skarn with associated minerals, including olivine, pyroxene, garnet, and serpentine, which developed in the contact zone between the granodiorite and dolomite of the Jiega Formation. However, the genetic relationship between the Mg-skarn, granodioritic magma, and Au-Cu mineralization remains unclear. The contribution of early stage ore-forming fluids to late Au-Cu mineralization is still vague. In this study, we present the major and trace element compositions, and U–Pb isotope data for garnets obtained from the Galale deposit. We used the data to define the timing, nature, and evolution of early ore-forming fluid and described the genesis of the deposit. The color, at macro- and micro-scales, and chemical composition of the garnets can be used to divide them into two categories, namely early Al-rich Grt-I type garnets, and late stage Fe-rich Grt-II type garnets. In practice, Grt-I (And29.5∼56.1Gro41.3∼68.0Pra1.6∼2.8) and Grt-II (And96.7∼99.9Gro0∼2.7Pra0.1∼0.7) are grossular-andradite solid solution system garnets. Based on the chondrite-normalized REE patterns, Grt-I can be further divided into two sub-classes: Grt-I-1 domain in the core, enriched in HREEs and U, and Grt-I-2 material in the rim, relatively depleted in HREEs. Grt-II is enriched in LREEs and depleted in HREEs. Eu anomalies are slightly negative for the Grt-I-1 domain, weakly positive for the Grt-I-2 domain, and strongly negative for the Grt-II type garnets. The variations in texture and composition in garnets indicated that the formation of Grt-I garnets occurred during diffusive metasomatism in a relatively closed system, in a moderately oxidized ore-forming fluid setting. The formation of Grt-II occurred in a strongly oxidized setting during advective metasomatism in an open system. The enrichment of REE and U in Grt-I garnets was mainly controlled by a "menzerite" type substitution mechanism, partially explaining the strong positive correlation between Mg and REE concentrations. In the Grt-II type garnets, enrichment may also have been co-controlled by coupling substitution and adsorption. In situ U–Pb dating of garnet shows that Grt-I-1 and Grt-I-2 material has isotopic ages of 92 ± 2 Ma (2σ, MSWD = 0.85) and 91 ± 2 Ma (2σ, MSWD = 0.96), respectively. The results were consistent with the previous ages obtained from the granodiorite and Molybdenite using Re–Os isotopic dating within error. This indicates that magma, skarn alteration, and mineralization were spatially and temporally related. In addition, it indicates that the Galale deposit is a typical Mg-skarn Au-Cu deposit. In the earliest stages of deposit development, ore-forming fluids with a high oxygen fugacity (fO2) have inherited their characteristics from the magma. The crystallization of many magnetites during the oxide stage has decreased oxygen fugacity in the metallogenetic system. This has resulted in a relatively low fO2 fluid for Au-Cu precipitation. The study demonstrates that the ore-fluid fO2 plays a vital role in metal migration, hence Au-Cu mineralization in Mg-skarn deposits.
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新田岭矿床是南岭钨锡成矿带中的一个大型矽卡岩型钨矿床，产于骑田岭岩体东北部与石炭系碳酸盐地层的接触带位置。本文运用LA-ICP-MS技术对该矿床矽卡岩中的石榴子石进行了系统的成分分析，获得了其主量、微量和稀土元素含量。结果显示，新田岭矿床中的石榴子石属于钙铁榴石-钙铝榴石固溶体系列（And24Gro66-And71Gro22），石榴子石的端元成分在富钙铝榴石和富钙铁榴石之间变化。稀土元素的配分模式也同时出现了左倾、Eu负异常和右倾、Eu正异常两种类型，暗示新田岭矿床石榴子石结晶过程中热液流体存在不同的氧化还原环境和水/岩比条件，这也与其晶体中是否出现振荡环带相对应。将不同矽卡岩型矿床中石榴子石的W、Sn含量进行对比显示，含W矿化的矽卡岩型矿床中石榴子石的W、Sn含量整体上显著高于不含W矿化的矿床，指示石榴子石中的W、Sn含量在一定程度上具有预测矽卡岩型矿床成W矿潜力的作用。此外，石榴子石中Fe、Eu、U等元素的含量还可以进一步区分矽卡岩W矿床中的伴生金属元素类型（包括W-Mo、W-Sn、W-Cu-Fe和W-Mo-Cu-Fe等）。本文研究表明，石榴子石的成分特征不仅可以指示矽卡岩的成矿环境，还可用于评估矽卡岩中金属（尤其是W）的成矿潜力，具有一定的理论意义和应用价值。;Xintianling is a large skarn W deposit in the Nanling W-Sn metallogenic belt, and it is located in the contact zone between the northeastern Qitianling pluton and the Carboniferous carbonate rocks. In this study, the typical skarn mineral, garnet, has been analyzed by LA-ICP-MS to quantify the concentrations of major elements, trace elements, as well as rare earth elements (REEs). The results reveal that the garnet from the Xintianling deposit mainly belongs to the andradite-grossular solid solution (And24Gro66 to And71Gro22), which shows a transitional trend dominated by either andradite or grossular. The REEs of the garnet show two distinct patterns, one is characterized by HREE enrichment and LREE depletion with negative Eu anomaly, and the other is featured with LREE enrichment and HREE depletion with positive Eu anomaly. These differences suggest that the garnet crystals grow under variable redox conditions and water/rock ratios in the hydrothermal system, which has also been reflected in the oscillatory zones. A compilation of available data from other skarn deposits combined with the results from this study indicates that W and Sn concentrations of garnet from skarn deposits with economic W mineralization are significantly higher than those without W mineralization, implying that the W and Sn concentrations could probably predict the potential economic metals (especially W) in a skarn system. Furthermore, the results also indicate that the Fe, Eu and U concentrations of garnet can provide useful information for distinguishing the associated metal(s) of a W skarn system, including W-Mo, W-Sn, W-Cu-(Fe) and W-Mo-Cu-(Fe). In conclusion, the current study shows that the compositional characteristics of garnet have certain guiding significance in understanding the ore-formation conditions and evaluating mineralization potentials of metals in a skarn system, especially for these W skarns like Xintianling.
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A newly discovered tungsten ore district containing more than 300,000 tons of WO3 in southern Anhui Province has attracted great attention. The Zhuxiling W (Mo) deposit in the district is dominated by skarn tungsten mineralization. This paper conducted in suit EPMA and LA-ICPMS spot and mapping analysis of the skarn mineral garnet to reveal the evolution of fluids, metasomatic dynamics, and formation conditions of skarn. Two generations of garnet have been identified for Zhuxiling W (Mo) skarn: 1) Gt-I generation garnet is isotropic, Al-rich grossular without zoning. As a further subdivision, Gt-IB garnet (Adr19-46Grs49-77 (Sps+Pyr+Alm)4-5) contains significantly high content of Ti and Mn compared with Gt-IA garnet (Adr3-42Grs53-96 (Sps+Pyr+Alm)1-5). 2) Gt-II generation garnet is anisotropic, Fe-rich andradite with oscillatory zoning. Gt-II garnet displays compositional changes with a decrease of Fe and an increase of Mn from proximal skarn (Gt-IIA) to distal skarn (Gt-IIB) with the presence of subcalcic garnet for Gt-IIB type (Sps+Pyr+Alm = 56–68). The presence of pyrrhotite associated with subcalcic garnet indicates a relatively reduced skarn system. Gt-I grossular is overall enriched in Cr, Zr, Y, Nb, and Ta compared with the Gt-II andradite, and both W and Sn strongly favor Fe-rich garnet compared with Al-rich garnet. Gt-IA grossular garnet presents a REE trend with an upward-facing parabola peaking at Pr and Nd in contrast to low and flat HREE, and Gt-IB grossular garnet has a distinct REE pattern with enriched HREE. Gt-IIA andradite garnet displays a right-dipping REE pattern (enriched LREE and depleted HREE) with a prominent positive Eu anomaly (Eu/Eu* = 3.6–15.3). In contrast, Gt-IIB andradite garnet shows depleted LREE and enriched HREE with a weak positive Eu anomaly (Eu/Eu* = 0–6.0). The incorporation and fractionation of REE in garnet are collectively controlled by crystal chemistry and extrinsic factors, such as P–T–X conditions of fluids, fluid/rock ratios, and mineral growth kinetics. Major and trace elements of two generations of garnet combined with optical and textural characteristics suggest that Gt-I Al-rich grossular garnets grow slowly through diffusive metasomatism under a closed system, whereas Gt-II Fe-rich andradite represent rapid growth garnet formed by the infiltration metasomatism of magmatic fluids in an open system. The Mn-rich garnet implies active fluid–rock interaction with Mn-rich dolomitic limestone of the Lantian Group in the district.
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This paper presents in situ microanalysis data of garnets form the Jinshandian iron skarn deposit, Hubei Province. Studies show that garnets from the Jinshandian iron skarn deposit can be divided into two stages, i.e., early Al-rich garnets mainly with grossular and grossular-andradite series, and late Fe-rich garnets dominated by andradite, with the variation implying the increase of the fluid oxidation. Compared with the early garnets, the late stage are rich in large ion lithophile elements and high field strength elements as well as REE. Early grossular shows typical HREE-enrichment and LREE-depletion features, while smaller fractionation between HREE and LREE characterizes grossular-andradite series. Total REE content, δEu and HREE/LREE fractionation degrees of Fe-rich garnet samples vary from sample to sample, and are even different in different parts of a single garnet grain, suggesting that the process of its formation was not stable, and fluid properties changed greatly, possibly due to the addition of evaporate minerals from the wall rocks. Garnet in situ microanalysis research also suggests that the evaporate minerals added into the Jinshandian skarn system had features of heterogeneity and periodicity.
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