Trace-element composition of pyrite and its implications for hydrothermal process within the Mesoproterozoic metasedimentary sequences of the São Francisco Craton, northeastern Brazil

M. Melo, Éder Carlos Moreira, F. Simplicio, G. Queiroga, Letícia Garcia D’Agostim, M. P. Castro
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Abstract

The distribution of trace elements in pyrite has been documented for the first time in quartz veins hosted in the Mesoproterozoic metasedimentary sequence of the Tombador Formation, São Francisco Craton, northeast Brazil. In this study, Electron Microprobe Analyses (EPMA) were used to determine the trace-element compositions of pyrite in these hydrothermal quartz veins. Three pyrite types have been distinguished and interpreted from petrographical relationships and trace-element patterns. Pre-existing pyrite (Py1), derived from the host-rock quartzite, is Ni-poor with concentrations varying from 600 – 6,100 ppm. Elongated syn-tectonic pyrite (Py2) has similar trace-element composition to the Py1, with Ni amounts ranging between 830 and 7,870 ppm. In contrast, possibly post-tectonic, euhedral to subhedral hydrothermal pyrite (Py3), contains higher contents of Ni (7,970 – 26,120 ppm). Mafic and/or metasedimentary rocks from the Espinhaço Supergroup were probably the source of Ni for this fluid-flow event. Fluid generation is related to  the devolatilization of the base of the thickened crust, with migration of fluids by preexisting structures. Several shear zones and large-scale NNW-trending folds were developed during the inversion of the Espinhaço basin, as a result of the ca. 0.6 Ga Brasiliano orogenic event. Regional fluid movement through the crust at this time is supported by several mineralized veins and hydrothermal deposits in the São Francisco Craton and adjacent Neoproterozoic belts.
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巴西东北部 o Francisco克拉通中元古代变质沉积层序中黄铁矿微量元素组成及其热液作用意义
首次记录了巴西东北部 o Francisco克拉通中元古界Tombador组变质沉积层序石英脉中黄铁矿微量元素的分布。本文采用电子探针分析(EPMA)方法测定了这些热液石英脉中黄铁矿的微量元素组成。从岩相关系和微量元素模式上区分并解释了三种黄铁矿类型。原生黄铁矿(Py1)来源于寄主岩石英岩,其镍含量在600 - 6100 ppm之间。细长型同构造黄铁矿(Py2)的微量元素组成与Py1相似,镍含量在830 ~ 7870 ppm之间。相比之下,可能是构造后,自面状到亚面状热液黄铁矿(Py3)含有较高的Ni含量(7,970 ~ 26,120 ppm)。espinhaarso超群的基性和/或变质沉积岩可能是本次流体流动事件的镍源。流体的产生与增厚地壳基底的脱挥发有关,流体通过先前存在的构造运移。在大约0.6 Ga的Brasiliano造山事件的影响下,espinhao盆地在反转过程中发育了多个剪切带和大规模的nnw向褶皱。在弗朗西斯科克拉通和邻近的新元古代带中,若干矿化脉体和热液矿床支持了这一时期通过地壳的区域流体运动。
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