Abstract Hydrothermal fluorite-hematite veins represent a relatively rare vein type, which is sometimes associated with more common, fluorite-barite-quartz‑carbonate veins containing base metal mineralization. The origin and significance of the fluorite-hematite veins, and how they relate to the more common expression of mineralization, is essentially unknown. Here, we present new data illuminating the formation conditions of these “sulfide-barren” fluorite-hematite veins in a mining district known for hydrothermal Pb + Zn + Ag ores for which formation conditions are well understood: the Schwarzwald district in SW Germany. The exemplary Odsbach-Hesselbach vein was chosen to study the fluids involved in fluorite-hematite precipitation through the first, direct investigation of fluid inclusions hosted in hydrothermal hematite using combined infra-red light microscopy, microthermometry and LA-ICPMS. The results indicate that the hematite-precipitating fluids are nearly identical to those observed in the associated fluorite, showing consistent salinities of 24.6–25.0 wt% (NaCl+CaCl2) and homogenization temperatures of 150 to 155 °C. Moreover, the trace element abundances determined by the LA-ICPMS analyses show a composition similar to other inclusions observed in the common mineralization in the mining district, including elevated Pb (>5000 μg/g) and Zn (>2000 μg/g) concentrations at low Cl/Br mass ratios, and sulfur contents mostly below the limits of detection. The results suggest fluid mixing between a continental basement brine and a redbed-derived fluid from the Triassic Buntsandstein. Therefore, the formation of the rare fluorite-hematite veins was probably triggered by a change of the sedimentary fluid endmember representing a shift from the Middle Triassic Muschelkalk fluid, which drove the more common Pb + Zn + Ag mineralization, towards fluids derived from the Buntsandstein. Furthermore, the absence of sulfur is interpreted to have precluded base metal sulfide precipitation, and the formation of hematite points to unusually oxidized conditions and hence towards a lack of a reducing agent, which would also have been required to generate a mineralization of base metal sulfides.

中文翻译：

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陆相基底卤水与红层衍生流体混合形成热液萤石-赤铁矿脉：Schwarzwald 矿区，SW-Germany

摘要 热液萤石-赤铁矿脉是一种相对稀有的脉类型，有时与更常见的含有贱金属矿化的萤石-重晶石-石英-碳酸盐矿脉有关。萤石-赤铁矿脉的起源和意义，以及它们与更常见的矿化表现有何关系，基本上是未知的。在这里，我们提供了新数据，阐明了这些“硫化物贫瘠”萤石 - 赤铁矿脉在以热液 Pb + Zn + Ag 矿石而闻名的矿区的形成条件，该矿区的形成条件非常清楚：德国西南部的 Schwarzwald 区。选择示例性的 Odsbach-Hesselbach 静脉来研究参与萤石-赤铁矿沉淀的流体，使用组合红外光显微镜、显微测温法和 LA-ICPMS 直接研究热液赤铁矿中的流体包裹体。结果表明赤铁矿沉淀流体与在相关萤石中观察到的流体几乎相同，显示一致的盐度为 24.6-25.0 wt% (NaCl+CaCl2)，均质温度为 150 至 155 °C。此外，由 LA-ICPMS 分析确定的微量元素丰度显示出与在矿区常见矿化中观察到的其他包裹体相似的成分，包括升高的 Pb (>5000 μg/g) 和 Zn (>2000 μg/g) 浓度在低 Cl/Br 质量比下，硫含量大多低于检测限。结果表明，大陆基底卤水与来自三叠纪 Buntsandstein 的红层衍生流体之间存在流体混合。因此，罕见的萤石-赤铁矿脉的形成可能是由沉积流体端元的变化引发的，这代表着从驱动更常见的 Pb + Zn + Ag 矿化的中三叠世 Muschelkalk 流体向源自 Buntsandstein 的流体的转变. 此外，没有硫被解释为排除了贱金属硫化物沉淀，赤铁矿的形成表明异常氧化条件，因此缺乏还原剂，这也是产生贱金属硫化物矿化所必需的. 对源自 Buntsandstein 的流体。此外，没有硫被解释为排除了贱金属硫化物沉淀，赤铁矿的形成表明异常氧化条件，因此缺乏还原剂，这也是产生贱金属硫化物矿化所必需的. 对源自 Buntsandstein 的流体。此外，没有硫被解释为排除了贱金属硫化物沉淀，赤铁矿的形成表明异常氧化条件，因此缺乏还原剂，这也是产生贱金属硫化物矿化所必需的.