The nature of the Witwatersrand hinterland; conjectures on the source area problem
The nature of the Witwatersrand hinterland; conjectures on the source area problem
复制标题
威特沃特斯兰德腹地的自然风光；
DOI：
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发表时间：
1990
期刊：
影响因子：
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通讯作者：
F. Meyer
中科院分区：
文献类型：
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作者：
L. Robb;F. Meyer
虽然威特沃特斯兰德盆地的Au和U矿床通常被认为是改性砂矿，但源区的性质和Au和U的最终来源仍然是一个主要的谜。在威特沃特斯兰德腹地，太古代基底的一些穹状露头出露，揭示了各种花岗岩类和少量（<20%）绿岩带成分的存在。此外，在盆地附近钻探的130多个钻孔的样品揭示了早元古代沉积和火山盖层之下的地下太古代基底的特征。花岗岩地下基底通常以普遍的热液蚀变为特征，明显表现为绿柱石化、绢云母化、碳酸盐化和云英岩化，以及各种与脉有关的蚀变，包括石英+或-钾长石+或-钾长石+或- CO 3 +或-硫化物+或-萤石共生体。这些热液蚀变花岗岩似乎已被保存在早元古代盖层之下，但很大程度上已从地表露头被侵蚀。热液蚀变花岗岩，据信是<或= 3,050马，在威特沃特斯兰德腹地的地表花岗岩类和巴伯顿山地的整个花岗岩范围内，Au和U都显著富集。威特沃特斯兰德腹地富集的热液蚀变花岗岩的存在支持了盆地成矿作用在第一阶段占主导地位的观点。例如砂矿过程。Au和U的显着浓度是唯一的中央兰德集团的威特沃特斯兰德盆地表明，在序列的上部的沉积物挖掘了肥沃的来源不存在于内陆地区在早期的沉积循环。该源区的侵蚀残留物以热液蚀变花岗岩为代表，这些花岗岩被认为是一套高级花岗岩类，其上部和顶部区域矿化有Au和U，易于侵蚀。这些花岗岩的侵位以及伴随的高水平热液蚀变的广泛区域的发展与威特沃特斯兰德沉积作用是同步的。热液蚀变花岗岩不仅贡献了富Au和U碎屑在威特沃特斯兰德，但他们的侵位可能已被链接因果关系的沉积序列的上部的沉积。
Although the Au and U deposits of the Witwatersrand basin are generally regarded as modified placer in origin, the nature of the source area and the ultimate origin of Au and U still remain a major enigma. A number of domal outcrops of the Archean basement are exposed in the Witwatersrand hinterland, revealing the presence of a diverse suite of granitoids and a minor (<20%) greenstone belt component. In addition, samples from over 130 boreholes drilled adjacent to the basin have revealed the character of subsurface Archean basement beneath early Proterozoic sedimentary and volcanic cover. The granitic subsurface basement is commonly characterized by pervasive hydrothermal alteration evident as propylitization, sericitization, carbonation, and greisenization, as well as a variety of vein-related alterations comprising quartz + or - K-feldspar + or - chlorite + or - CO 3 + or - sulfide + or - fluorite parageneses. These hydrothermally altered granites appear to have been preserved beneath early Proterozoic cover but have largely been eroded from surface outcrops. The hydrothermally altered granites, which are believed to be < or = 3,050 Ma, are significantly enriched in Au and U with respect to both surface granitoids from the Witwatersrand hinterland and the entire range of granites from the Barberton Mountain Land.The presence of enriched hydrothermally altered granites in the Witwatersrand hinterland lends support to the view that mineralization processes in the basin were dominated in the first instance by placer processes. The remarkable concentrations of Au and U that are unique to the Central Rand Group of the Witwatersrand basin indicate that the sediments in the upper portions of the sequence tapped a fertile source not present in the hinterland during earlier cycles of sedimentation. The erosional remnants of this source area are represented by the hydrothermally altered granites which are considered to have been a suite of high-level granitoids, the upper portions and roof zones of which were mineralized in Au and U and easily accessible to erosion. The emplacement of these granites, and the development of extensive zones of attendant, high-level, hydrothermal alteration, was synchronous with Witwatersrand sedimentation. The hydrothermally altered granites not only contributed to the Au- and U-rich detritus in the Witwatersrand, but their emplacement may have been linked causally to the deposition of the upper part of the sedimentary sequence.