Owing to the superimposition of water-rock interaction and external fluids, magmatic source signatures of ore-forming fluids for vein-type tin deposits are commonly overprinted. Hence, there is uncertainty regarding the involvement of magmatic fluids in mineralization processes within these deposits. Tourmaline is a common gangue mineral in Sn deposits and can crystallize from both the magmas and the hydrothermal fluids. We have therefore undertaken an in situ major, trace element, and B isotope study of tourmaline from the Yidong Sn deposit in South China to study the transition from late magmatic to hydrothermal mineralization. Six tourmaline types were identified: (1) early tourmaline (Tur-OE) and (2) late tourmaline (Tur-OL) in tourmaline-quartz orbicules from the Pingying granite, (3) early tourmaline (Tur-DE) and (4) late tourmaline (Tur-DL) in tourmaline-quartz dikelets in the granite, and (5 and 6) core (Tur-OC) and rim (Tur-OR), respectively of hydrothermal tourmaline from the Sn ores. Most of the tourmaline types belong to the alkali group and the schorl-dravite solid-solution series, but the different generations of magmatic and hydrothermal tourmaline are geochemically distinct. Key differences include the hundredfold enrichment of Sn in hydrothermal tourmaline compared to magmatic tourmaline, which indicates that hydrothermal fluids exsolving from the magma were highly enriched in Sn. Tourmaline from the Sn ores is enriched in Fe³⁺ compared to the hydrothermal tourmaline from the granite and displays trends of decreasing Al and increasing Fe content from core to rim, relating to the exchange vector Fe³⁺Al_–1. This reflects oxidation of fluids during the interaction between hydrothermal fluids and the mafic-ultramafic wall rocks, which led to precipitation of cassiterite. The hydrothermal tourmaline has slightly higher δ¹¹B values than the magmatic tourmaline (which reflects the metasedimentary source for the granite), but overall, the tourmaline from the ores has δ¹¹B values similar to those from the granite, implying a magmatic origin for the ore-forming fluids. We identify five stages in the magmatic-hydrothermal evolution of the system that led to formation of the Sn ores in the Yidong deposit based on chemical and boron isotope changes of tourmaline: (1) emplacement of a B-rich, S-type granitic magma, (2) separation of an immiscible B-rich melt, (3) exsolution of an Sn-rich, reduced hydrothermal fluid, (4) migration of fluid into the country rocks, and (5) acid-consuming reactions with the surrounding mafic-ultramafic rocks and oxidation of the fluid, leading to cassiterite precipitation.

中文翻译：

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华南伊东锡矿床岩浆-热液成矿过程：电气石原位化学和硼同位素变化的洞察

由于水-岩相互作用和外部流体的叠加，脉状锡矿床成矿流体的岩浆源特征通常被套印。因此，关于岩浆流体在这些矿床内的成矿过程中的参与存在不确定性。电气石是锡矿床中常见的脉石矿物，可以从岩浆和热液中结晶。因此，我们对华南伊东锡矿床中的碧玺进行了原位主量元素、微量元素和 B 同位素研究，以研究晚期岩浆成矿向热液成矿的转变。鉴定了六种电气石类型：（1）早期电气石（Tur-OE）和（2）来自平营花岗岩的电气石-石英球体中的晚期电气石（Tur-OL），(3) 早期电气石 (Tur-DE) 和 (4) 晚期电气石 (Tur-DL) 在花岗岩中的电气石-石英脉管中，以及 (5 和 6) 核心 (Tur-OC) 和边缘 (Tur-OR)，分别来自 Sn 矿石的热液碧玺。大部分电气石类型属于碱类和硫镁石固溶体系列，但不同世代的岩浆和热液碧玺在地球化学上是不同的。主要区别包括与岩浆碧玺相比，热液碧玺中 Sn 的富集是 100 倍，这表明从岩浆中溶出的热液流体中 Sn 含量很高。来自锡矿石的电气石富含铁 大部分电气石类型属于碱类和硫镁石固溶体系列，但不同世代的岩浆和热液碧玺在地球化学上是不同的。主要区别包括与岩浆碧玺相比，热液碧玺中 Sn 的富集是 100 倍，这表明从岩浆中溶出的热液流体中 Sn 含量很高。来自锡矿石的电气石富含铁 大部分电气石类型属于碱类和硫镁石固溶体系列，但不同世代的岩浆和热液碧玺在地球化学上是不同的。主要区别包括与岩浆碧玺相比，热液碧玺中 Sn 的富集是 100 倍，这表明从岩浆中溶出的热液流体中 Sn 含量很高。来自锡矿石的电气石富含铁³⁺与花岗岩中的热液碧玺相比，显示出从核心到边缘减少 Al 和增加 Fe 含量的趋势，这与交换矢量 Fe ³⁺ Al _{–1 相关}。这反映了在热液流体与基性-超基性围岩相互作用期间流体的氧化，导致锡石沉淀。热液碧玺的 δ ¹¹ B 值略高于岩浆碧玺（这反映了花岗岩的变沉积源），但总体而言，矿石中的碧玺具有 δ ¹¹B 值与花岗岩相似，表明成矿流体为岩浆成因。我们根据电气石的化学和硼同位素变化确定了导致伊东矿床形成锡矿的系统岩浆-热液演化的五个阶段：(1) 富 B、S 型花岗岩岩浆的侵位, (2) 不混溶的富 B 熔体的分离，(3) 富锡的还原热液出溶，(4) 流体迁移到围岩中，以及 (5) 与周围基性岩发生消耗酸的反应-超镁铁质岩石和流体氧化，导致锡石沉淀。