Late-stage anhydrite-gypsum-siderite-dolomite-calcite assemblages record the transition from a deep to a shallow hydrothermal system in the Schwarzwald mining district, SW Germany
Late-stage anhydrite-gypsum-siderite-dolomite-calcite assemblages record the transition from a deep to a shallow hydrothermal system in the Schwarzwald mining district, SW Germany
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DOI：
10.1016/j.gca.2017.12.002
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发表时间：
2018-02
影响因子：
5
通讯作者：
M. Burisch;B. Walter;A. Gerdes;Maximilian Lanz;G. Markl
中科院分区：
文献类型：
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作者：
M. Burisch;B. Walter;A. Gerdes;Maximilian Lanz;G. Markl
大多数具有经济价值的热液矿脉系统出现在地壳相对较浅的地方，尽管其中许多形成于更深的地方。它们现在的位置是隆起和侵蚀的结果。虽然，它们的形成的许多方面都受到很好的限制，在隆升和侵蚀过程中，这些系统的时间化学演化仍然知之甚少。这些矿脉矿物包括方解石、菱铁矿-铁白云石、菱铁矿-菱镁矿、硬石膏和石膏，形成了德国西南部施瓦兹瓦尔德矿区侏罗纪和第三纪硫化物-萤石-石英-重晶石矿脉中的最后脉石组合。9个地区的样品的矿物结构表明，在这些序列中，矿物沉淀遵循一个循环模式：早期方解石其次是硬石膏或石膏，菱铁矿和/或白云石。对晚期脉体组合的三个连续世代的15个碳酸盐岩样品进行了原位（LA-ICP-MS）U-Pb年龄测定，结果显示其年龄在20 ~ 0.6Ma之间。每个矿物序列形成于一个独特的时期，约2-5 Ma。这些年龄清楚地将这些晚期矿物相与黑森林最年轻的地质幕相关联，黑森林最年轻的地质幕与新生代莱茵河地堑裂谷和基底隆起相关联。根据热力学模型，所观察到的矿物组合的形成需要深源的富Mg-、Fe-和SO 4流体（B），该流体与浅地壳富HCO 3流体（a）进行了间歇性混合。作为流体混合的结果，Mg，Fe和SO 4的浓度暂时增加，并开始形成所观察到的硫酸盐-碳酸盐矿物sequence.This不连续的大规模垂直流体混合可能直接相关的事件与新生代走滑体制的上莱茵河地堑的活动构造。类似地，幕式流体混合是在同一特定脉系统中形成更老的（侏罗纪至早第三纪）Pb-Zn-萤石-石英-重晶石组合的主要关键，尽管涉及不同的流体成分。由于连续的隆起，越来越多的较浅和较冷（150 -70 °C）的流体可能参与这种混合过程。流体成分的相关变化导致脉矿物学从硫化物-石英-萤石-重晶石变为方解石-硬石膏/石膏-菱铁矿-白云石，因为系统被动地上升到更接近地表。
The majority of hydrothermal vein systems of economic interest occur at relatively shallow crustal levels, although many of them formed at significantly greater depths. Their present position is a consequence of uplift and erosion. Although, many aspects of their formation are well constrained, the temporal chemical evolution of such systems during uplift and erosion is still poorly understood. These vein minerals comprise calcite, dolomite-ankerite, siderite-magnesite, anhydrite and gypsum forming the last gangue assemblages in Jurassic and Tertiary sulphide-fluorite-quartz-barite veins of the Schwarzwald mining district, SW Germany. Mineral textures of samples from nine localities reveal that in these sequences, mineral precipitation follows a recurring pattern: early calcite is followed by anhydrite or gypsum, siderite and/or dolomite. This succession may repeat up to three times.In-situ (LA-ICP-MS) U-Pb age dating of 15 carbonates from three subsequent generations of the late-stage vein assemblage yield robust ages between 20 and 0.6 Ma. Each mineral sequence forms in a distinctive period of about 2–5 Ma. These ages clearly relate these late-stage mineral phases to the youngest geological episode of the Schwarzwald, which is associated with the Cenozoic Rhine Graben rifting and basement uplift.Based on thermodynamic modelling, the formation of the observed mineral assemblages required an deeply sourced Mg-, Fe- and SO4-rich fluid (b), which was episodically mixed with a shallow crustal HCO3-rich fluid (a). As a consequence of fluid mixing, concentrations of Mg, Fe and SO4temporarily increased and initiated the formation of the observed sulphate-carbonate mineral sequences.This discontinuous large-scale vertical fluid mixing was presumably directly related to episodes of active tectonics associated with the Cenozoic strike-slip regime of the Upper Rhine Graben. Analogously, episodic fluid mixing is a major key in the formation of older (Jurassic to early Tertiary) Pb-Zn-fluorite-quartz-barite assemblages in the same specific vein systems, albeit involving different fluid compositions.Late-stage hydrothermal (∼20–70 °C) vein assemblages reported in this study record the transition from deep (>2 km) to very shallow (0–1 km) crustal conditions. As a consequence of successive uplift, increasing proportions of shallower and cooler (∼50–70 °C) fluids could take part in such mixing processes. Associated changes in the fluid composition caused the vein mineralogy to change from sulphide-quartz-fluorite-barite to calcite-anhydrite/gypsum-siderite-dolomite, as the system passively ascended closer to the surface.