Complex metasomatism of an Archean placer in the Witwatersrand Basin, South Africa; the Ventersdorp contact reef; a hydrothermal aquifer?
Complex metasomatism of an Archean placer in the Witwatersrand Basin, South Africa; the Ventersdorp contact reef; a hydrothermal aquifer?
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南非威特沃特斯兰德盆地太古代砂矿的复杂交代作用；
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
H. Frimmel
中科院分区：
文献类型：
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作者：
V. Gartz;H. Frimmel
Ventersdorp接触礁是南非晚太古代威特沃特斯兰德超群顶部的含金砾岩层位和经济矿体。它与威特沃特斯兰德盆地其他砾岩层（礁）的不同之处在于其上壁面为块状变质玄武岩层序，且沉积后蚀变强烈。对Ventersdorp接触礁的地球化学、岩石学和流体包裹体研究揭示了其多期热液蚀变史。在酸性条件下的区域变质作用之后是沿礁的钾化蚀变。钾蚀作用也延伸到上盘和下盘岩石，随后发生了一个局限于珊瑚礁及其直接接触的绿泥化时期。质量平衡计算表明，后一种蚀变只需要来自外部源的少量流体流入，而钾质蚀变反映了硅质下盘岩石的逐渐脱水和玄武岩上盘的相关水化作用。基于sem -阴极发光成像对石英的显微结构观察，泥绿石化事件归因于2020-Ma Vredefort撞击事件，该事件以微裂缝的形式产生了必要的次生渗透率，特别是在砾岩层内。考虑到Ventersdorp接触礁通常被认为是Witwatersrand盆地蚀变最强烈的礁，本研究建立的相对较小的流体渗透表明，与普遍观点相反，其他含金砾岩层可能经历了更少的外源热液流入。
The Ventersdorp contact reef is an auriferous conglomerate horizon and economic orebody on top of the Late Archean Witwatersrand Supergroup in South Africa. It differs from the other conglomerate beds (reefs) in the Witwatersrand basin in having a massive metabasalt sequence as its hanging wall and by its intense post-depositional alteration. A geochemical, petrologic, and fluid inclusion study of the Ventersdorp contact reef reveals a polyphase hydrothermal alteration history. Regional metamorphism under acidic conditions was followed by potassic alteration along the reef. The potassic alteration, which also extended into the hanging-wall and footwall rocks, was subsequently followed by a chloritization episode that was restricted to the reef and its immediate contacts. Mass-balance calculations suggest that the latter alteration required only minor fluid influx from an external source, whereas the potassic alteration reflects the progressive dehydration of the siliciclastic footwall rocks and associated hydration of the basaltic hanging wall. Based on microtextural observations on quartz using SEM-cathodoluminescence imaging, the chloritization event is ascribed to the 2020-Ma Vredefort impact event that created the necessary secondary permeability in the form of microfractures particularly within the conglomerate bed. Considering that the Ventersdorp contact reef is generally regarded as the most intensely altered reef in the Witwatersrand basin, the relatively minor fluid infiltration established in this study suggests that, contrary to prevailing opinion, the other auriferous conglomerate beds might have experienced even less influx of externally derived hydrothermal fluids.