Igneous geology and the evolution of hydrothermal systems in some sub-volcanic tin deposits of Bolivia
Igneous geology and the evolution of hydrothermal systems in some sub-volcanic tin deposits of Bolivia
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DOI：
10.1144/gsl.sp.1977.007.01.14
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发表时间：
1977
期刊：
影响因子：
--
通讯作者：
J. Grant;C. Halls;W. Avila;G. Ávila
中科院分区：
文献类型：
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作者：
J. Grant;C. Halls;W. Avila;G. Ávila
概要 玻利维亚东科迪勒拉山脉南部的锡矿床由复杂的矿脉系统和普遍的浸染状矿化组成，这些矿化位于第三纪晚期喷发中心或其边缘的强烈热液蚀变区内。在一些中心，侵蚀完全去除了火山上部结构，仅保留了侵入岩（Llallagua、San Pablo de Morococala）。在其他情况下，侵入岩和同时代的火山岩都被保存下来（乔罗尔克、奥鲁罗、波托西），而在某些情况下，只有部分被侵蚀的火山上部结构暴露出来，并且没有明确的侵入岩已知（乔卡亚、塔塔西）。总的来说，所有这些矿床的地质关系表明，矿化是陆地成层火山内部较深处区域产生的热液系统的产物。与矿化关系最密切的火成岩是强烈蚀变的石英斑岩。它们的原始成分可能是流纹英安石。在保存的种群中，它们是复杂的物体，垂直截面通常是圆锥形的，随着深度的增加而变窄。它们显示了侵入、火成角砾岩和爆炸性热液角砾岩等几个阶段的证据，这些阶段早于主要矿脉系统的形成。硅酸盐蚀变组合包括石英-电气石、电气石-绢云母和绢云母-粘土，它们通常从中心向外依次呈现粗略的同心分带。有时，在火山上部结构残余物处会出现青盘蚀变的最外层光环。改变是普遍存在的，并且不受主要静脉的控制。低品位锡石矿化分布在整个内部区域。 Chorolque 的流体包裹体研究表明，热液系统是由复杂化学成分的高盐水或熔体的分离引发的，这些盐水或熔体在 500°C 以上的温度下形成。当温度降至 450°C 以下并发生间歇性沸腾时，普遍的石英-电气石蚀变、喷口火成岩的初始破裂以及邻近沉积岩的角岩化就完成了。在此阶段沉积了广泛浸染的锡石。主要石英锡石矿脉的最早生长阶段发生在 400°C 左右的温度下，流体的成分与最早的流体相似，但已被大幅稀释（约 40 wt% 氯化钠当量）。当温度降至 200°C 以下且盐度下降时，矿脉继续生长。大多数锡石沉积似乎发生在 300–250°C 的温度范围内，并且可能伴随着流体盐度的大幅下降。流体包裹体研究获得的数据，结合矿化体的总体几何形状，表明火山中心的热液过程最初是由围岩静压力和流纹英安石岩浆分异部分含水流体残余物压力之间的平衡破坏所控制的，导致普遍的水力压裂、角砾化和蚀变，其中早期生成的锡石矿化是一个组成部分。在次火山库顶部热液耗尽后，矿化火山结构的上层似乎已经稳定下来。然后，热液活动的焦点必须转移到更深的地方，从那里，火山基础设施和周围基底的构造应力的相互作用以及平行脉断层系统中热液流体压力的积累控制了矿化。与热液焦点后退相关的双峰型矿化概念可以使人们更清楚地了解次火山系统中的异热或“望远镜”类型的矿物和温度分区。然而，从流体包裹体研究中获得的温度数据表明，矿物共生并不是温度分区的明确标准，并且在组合合成变得有意义之前必须独立考虑两者。
Synopsis The tin deposits of the southern part of the Cordillera Oriental of Bolivia consist of complex vein systems and pervasive disseminated mineralization within zones of strong hydrothermal alteration in, or at the margins of, eruptive centres of late Tertiary age. At some centres erosion has removed the volcanic superstructure completely and only intrusive rocks are preserved (Llallagua, San Pablo de Morococala). At others, both the intrusives and coeval volcanics are preserved (Chorolque, Oruro, Potosi), whereas in some cases only the partially eroded volcanic superstructure is exposed and no unequivocally intrusive rocks are known (Chocaya, Tatasi). In general, the geological relationships at all these deposits suggest that the mineralization is the product of hydrothermal systems generated in the inner, deeper regions of terrestrial strato-volcanoes. The igneous rocks most closely associated with mineralization are strongly altered quartz porphyries. Their original composition was probably rhyodacite. Where stocks are preserved, they are complex bodies, often conical in vertical section, narrowing with depth. They show evidence of several phases of intrusion, igneous brecciation and explosive hydrothermal brecciation, which pre-date the formation of the major vein systems. Silicate alteration assemblages include quartz-tourmaline, tourmaline-sericite and sericite-clay, which often show rough concentric zonation in that order outward from the centre. An outermost aureole of propylitic alteration is sometimes present where there are remnants of the volcanic superstructure. Alteration is pervasive and not controlled by the major veins. Low-grade cassiterite mineralization is dispersed throughout the inner zones. Fluid inclusion studies at Chorolque show that the hydrothermal system was initiated by the separation of a highly saline brine, or melt, of complex chemistry, which formed at temperatures above 500°C. The pervasive quartz-tourmaline alteration, initial fracturing of the igneous rocks of the vent and the hornfelsing of the adjacent sedimentary rocks were accomplished while temperatures fell to below 450°C and intermittent boiling took place. Widespread disseminated cassiterite was deposited during this phase. The earliest stages of growth of the major quartz-cassiterite veins took place at temperatures of around 400°C, from a fluid the composition of which was similar to that of the earliest fluid, though substantially diluted (about 40 wt % NaCl equivalent) Vein growth continued while temperatures fell to below 200°C and salinity decreased. Most cassiterite deposition seems to have taken place in the temperature range 300–250°C and may have been accompanied by a major decrease in the salinity of the fluids. The data obtained from the fluid inclusion studies, taken in conjunction with the gross geometry of the mineralization, suggest that hydrothermal processes at the volcanic centres were initially controlled by failure of the balance between confining lithostatic pressure and the pressure of the hydrous fluid residuum in the differentiated parts of the rhyodacite magma, leading to pervasive hydraulic fracturing, brecciation and alteration, of which the early generation of cassiterite mineralization was an integral part. The upper levels of the mineralized volcanic structures appear to have stabilized following the hydrothermal exhaustion of the apical portions of the sub-volcanic stocks. The focus of hydrothermal activity must then have been relocated at greater depths from which mineralization was controlled by the interplay of tectonic stresses in the volcanic infrastructure and surrounding basement and the build-up of hydrothermal fluid pressure in parallel vein-fault systems. This concept of a bimodal style of mineralization related to a retreating hydrothermal focus can lead to a clearer understanding of the xenothermal or ‘telescoped’ type of mineral and temperature zonation in sub-volcanic systems. Temperature data obtained from fluid inclusion studies show, however, that mineral paragenesis is not an unambiguous criterion of temperature zonation and that the two have to be considered independently before a combined synthesis becomes meaningful.