Chemical composition of garnet from the Xintianling skarn W deposit in southern Hunan and its geological significance

Abstract

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.