REE contents, REE minerals and Sm/Nd isotopes of granite- and unconformity-related fluorite mineralization at the western edge of the Bohemian Massif: With special reference to the Nabburg-Wölsendorf District, SE Germany
REE contents, REE minerals and Sm/Nd isotopes of granite- and unconformity-related fluorite mineralization at the western edge of the Bohemian Massif: With special reference to the Nabburg-Wölsendorf District, SE Germany
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DOI：
10.1016/j.oregeorev.2011.06.003
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发表时间：
2011-09
影响因子：
3.3
通讯作者：
H. Dill;B. T. Hansen;B. Weber
中科院分区：
文献类型：
--
作者：
H. Dill;B. T. Hansen;B. Weber
萤石脉型矿床产于沿着德国东南部波希米亚地块的西缘，从Bayerischer Wald到Frankenwald。它们侵位在各种寄主岩石岩性中，与华力西晚期花岗岩和晚古生代低级变质沉积岩一样古老。位于德国巴伐利亚州东北部的Nabburg-Wölsendorf矿区曾经是德国最大的矿区，也是世界上最重要的矿区之一。少量萤石也集中在Pingarten附近基底岩石上不整合的二叠纪-中生代硅质沉积物中。本文研究了萤石的稀土元素变化、伴生稀土矿物及其Sm/Nd同位素比值。原生蓝黑色萤石早期从热液中沉淀出来，与围岩没有明显的相互作用，且贫稀土元素和钇。次生绿色、白色和黄色萤石是较老萤石再活化作用的产物，是较粘稠流体与围岩作用的产物，富含REE + Y。独居石-（Ce）是稀土元素再沉积成萤石的主要来源。在一个双交会图，在这项研究中，设计和使用的Ce-，Eu异常和偏度（分馏）的稀土元素模式，地质和水文演化的萤石脉成矿有关的花岗岩和不整合面，这是水力参考面的萤石成矿。首先，蓝色萤石形成于不整合面下方相当深的地方。含铀矿物丰富的深部脉状矿化中，只有恶臭萤石才有。这些铀矿物对蓝色萤石的晶格造成了一定的辐射损伤，变成了黑色。根据萤石中稀土元素的变化规律和Sm/Nd年龄（萤石年龄：约1000 ~ 10000 μ m），研究了萤石中稀土元素的变化规律。270 Ma），可以重建二叠纪古地理。有人认为，在纳堡-沃尔森多夫区，一个西北-东南方向的涌浪从一个广阔的平原中脱颖而出。其次，沿着波西米亚地块西缘由绿色、白色和黄色萤石组成的萤石脉矿化是由于与相对于蓝色和黑色萤石的REE和Y增加以及LREE与HREE的强烈分馏相关的再活化造成的。尽管成矿流体来源于流体，但后者的稀土元素特征越来越“不整合”（Eh>0）。稀土元素配分模式、Sr-和Sm/Nd同位素表明，成矿流体主要来源于花岗质源岩，受变质沉积岩的影响较小。铈矿-（Ce）形成于萤石成矿的初始阶段，蓝萤石的晶格中容纳了少量稀土元素。在绿色、白色和黄色萤石的沉淀过程中，由于稀土元素在萤石的结构中的容纳量不断增加，使萤石的颜色由蓝色经绿色变为白色，稀土矿物不能继续自行发育。在萤石矿物组合的表生蚀变过程中再次形成稀土矿物，终止了整个F-REE循环。菱铁矿是表生成因，指示中性至弱碱性的大气环境。随后，florencite演变下的化学风化的酸性制度。Sm-，Nd-和Sr同位素的交叉图中的不同数据阵列，同样，表明这种细分成萤石起源于流体与花岗岩和片麻状岩石在深度和那些正在形成的近端和远端的不整合。
Fluorite vein-type deposits occur along the western edge of the Bohemian Massif in SE Germany, from Bayerischer Wald to Frankenwald. They are emplaced in various host rock lithologies, as old as Late Variscan granites and low-grade metasediments of late Paleozoic age. The Nabburg-Wölsendorf mining district in NE Bavaria, Germany, used to be the largest in Germany and one of the most important in the world. A minor proportion of fluorite is also concentrated in the Permo-Mesozoic siliciclastic sediments resting unconformably upon the basement rocks near Pingarten. Fluorite is investigated in the present study for its REE variation, the REE minerals associated with it and its Sm/Nd isotope ratios. The primary blue-black fluorites precipitated from aqueous hydrothermal solutions during early stages without noticeable interaction with wall rocks and were poor in REE and Y. The secondary green, white and yellow fluorites formed from remobilization of older fluorite and through the interaction of more viscous fluids with the wall rocks and they are abundant in REE plus Y. Monazite–(Ce) is the major source for the REE re-deposited into fluorite. In a double cross plot, designed during this study and using the Ce-, Eu anomalies and the skewness (fractionation) of the REE pattern, the geological and hydrological evolution of fluorite vein mineralizations are related to the granite and the unconformity, which is the hydraulic reference plane for the fluorite mineralization. First, blue fluorite formed at considerable depth below the unconformity. Fetid fluorite is exclusive to the deep-seated vein mineralization with abundant uranium minerals. These uranium minerals caused some radiation damage to the lattice of the blue fluorite and turned into black. Based upon the variation of REE in fluorite and the Sm/Nd dating (fluorite age: ca. 270Ma) the Permian paleogeography can be reconstructed. It is suggested that in the Nabburg-Wölsendorf District a NW–SE oriented swell stood out from a vast peneplain. Second, fluorite vein mineralization composed of green, white and yellow fluorite along the western edge of the Bohemian Massif resulted from remobilization associated with an increase in REE and Y relative to the blue and black fluorites and a strong fractionation of LREE vs. HREE. The REE signature of the latter fluorites got increasingly more “unconformity-related” (Eh>0), notwithstanding the fluid source which the mineralizing fluids originated from. The REE patterns, Sr- and Sm/Nd isotopes suggest that mineralizing fluids mainly derived from a granitic source rock with little impact from metasedimentary rocks. Cerite–(Ce) formed during the initial stages of fluorite mineralization when little REE were accommodated in the lattice of blue fluorite. During precipitation of green, white and yellow fluorite, REE minerals failed to continue developing on their own, because REE were increasingly accommodated in the structure of fluorite changing its color from blue via green into white. REE minerals were again formed during supergene alteration of fluorite mineral assemblages, terminating the entire F-REE cycle. Rhabdophane is of supergene origin and indicative of meteoric conditions that are neutral to slightly alkaline. Subsequently, florencite evolved under a more acidic regime of chemical weathering. The distinct data arrays in the cross plots of the Sm-, Nd- and Sr isotopes, likewise, indicate this subdivision into fluorite originating from fluids that interacted with granitic and gneissic rocks at depth and those being formed proximal and distal to the unconformity.