We present a petrographic and geochemical study of tourmaline from the Triassic Chacaltaya Sn-polymetallic district in the Cordillera Real of Bolivia. Tourmaline is associated with greisens, breccias, and veins, which occur around the Triassic Chacaltaya peraluminous granitic stock hosted by Silurian metasedimentary rocks. Three main petrographic types of hydrothermal tourmaline have been identified: pre-ore greisen-related (Tur-1), syn-ore breccia-related (Tur-2), and syn-ore vein-related (Tur-3). The three types of tourmaline belong to the alkali group and have Fe-rich compositions mostly close to the schorl end member. Overlapping Fe/(Fe + Mg) ratios suggest broadly similar compositions of the hydrothermal fluids during the deposition of tourmaline. The most notable differences in minor and trace element contents include relative enrichment in Zn and Li in Tur-1 and relative enrichment in Ca, Sc, V, Cr, Sr, Sn, Y, Cs, Be, and Zr in Tur-3, with Tur-2 showing intermediate compositions between those of Tur-1 and Tur-3. The progressive enrichment in Sn from Tur-1 (avg = 14 ppm) through Tur-2 (avg = 311 ppm) and Tur-3 (avg = 476 ppm) indicates an increase of Sn concentrations in the hydrothermal system coinciding with cassiterite deposition in breccias and veins. The transition from high Li and Zn contents in Tur-1 to elevated Ca, Sr, V, and Cr contents in Tur-3 is interpreted as reflecting interaction between a hydrothermal fluid of magmatic origin and the metasedimentary country rocks. Strong and relatively steady positive Eu anomalies in all tourmaline types suggest dominantly reduced hydrothermal conditions. In situ δ¹⁸O and δ¹¹B analyses of greisen-related Tur-1 reveal crystallization in isotopic equilibrium with magmatic water derived from a peraluminous S-type granite. In contrast, higher δ¹⁸O values of breccia-related Tur-2 and vein-related Tur-3 indicate crystallization in isotopic equilibrium with a fluid of metamorphic origin or a magmatic fluid that variably interacted with the metasedimentary host rocks. Geochemical modeling reproduces interactions between a fluid of magmatic origin and the host metasedimentary rocks at moderate water/rock ratios between 0.1 and 0.5. We conclude that cassiterite mineralization in the Chacaltaya district was formed primarily through interaction between B-Sn–rich magmatic fluids and the metasedimentary country rocks.

中文翻译：

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通过原位化学和电气石 δ 18 O-δ 11 B 成分追踪三叠纪 Chacaltaya 地区（玻利维亚雷亚尔山脉）的锡矿化

我们对玻利维亚雷亚尔山脉三叠纪查卡尔塔亚锡多金属区的电气石进行了岩相学和地球化学研究。电气石与云英岩、角砾岩和矿脉有关，这些矿物出现在志留纪变质沉积岩所含的三叠纪 Chacaltaya 过铝质花岗岩储层周围。热液电气石的三种主要岩相类型已被确定：前矿云英岩相关 (Tur-1)、同矿角砾岩相关 (Tur-2) 和同矿脉相关 (Tur-3)。这三种电气石均属于碱性电气石，富铁成分大多接近 schorl 端元。重叠的 Fe/(Fe + Mg) 比率表明电气石沉积过程中热液的成分大致相似。微量元素和微量元素含量最显着的差异包括 Tur-1 中 Zn 和 Li 的相对富集以及 Tur-3 中 Ca、Sc、V、Cr、Sr、Sn、Y、Cs、Be 和 Zr 的相对富集，其中Tur-2显示出介于Tur-1和Tur-3之间的中间组成。从 Tur-1（平均值 = 14 ppm）到 Tur-2（平均值 = 311 ppm）和 Tur-3（平均值 = 476 ppm），Sn 逐渐富集，表明热液系统中 Sn 浓度的增加与锡石沉积相一致。角砾岩和矿脉。从 Tur-1 中高 Li 和 Zn 含量到 Tur-3 中 Ca、Sr、V 和 Cr 含量升高的转变被解释为反映了岩浆起源的热液与变质沉积围岩之间的相互作用。所有碧玺类型中强烈且相对稳定的正铕异常表明热液条件显着减少。与灰岩相关的 Tur-1 的原位 δ ¹⁸ O 和 δ ¹¹ B 分析揭示了与来自过铝质 S 型花岗岩的岩浆水的同位素平衡结晶。相比之下，角砾岩相关的 Tur-2 和脉相关的 Tur-3 较高的 δ ¹⁸ O 值表明与变质成因的流体或与变沉积岩主岩发生不同相互作用的岩浆流体处于同位素平衡状态下的结晶。地球化学模型以 0.1 至 0.5 之间的适度水/岩比再现了岩浆起源流体与宿主变沉积岩之间的相互作用。我们得出的结论是，查卡尔塔亚地区的锡石矿化主要是通过富含 B-Sn 的岩浆流体与变沉积围岩之间的相互作用形成的。