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首页> 外文期刊>South African Journal of Geology: Being the Transaction of the Geological Society of South African: Syndie die Verhandelinge van die Geologiese Vereniging van Suid-Afrika >Early Permian diamond-bearing proximal eskers in the Lichtenburg/Ventersdorp area of the North West Province, South Africa
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Early Permian diamond-bearing proximal eskers in the Lichtenburg/Ventersdorp area of the North West Province, South Africa

机译：早期二叠纪钻石近端ESKES在南非西北省的Lichtenburg / Ventersdorp地区

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摘要

Diamond-bearing gravels of the Lichtenburg-Ventersdorp area of the North West Province are associated with north-south orientated sinuous 'runs' that occur almost entirely on a flat erosional surface of the Malmani dolomites (Transvaal Supergroup) at some 1,500 m elevation. East to west, this dolomite plain measures 150 km, and north south it is on average 40 km wide. This unconformity, which first developed before the Pretoria Group sedimentation over a period of at least 80 Myr, is marked by siliceous breccias (palaeo-karst infill) and conglomerates (reworked breccias). It was exhumed in pre-Karoo and post-Gondwana times. Glacial pavements and remnants of thin Lower Karoo sediments are also found on this polyphase surface. The gravels that make up these 'runs' and sinkholes directly or indirectly linked to these runs, are coarse-grained, very poorly-sorted, and are best described as diamictites. The 'runs' are narrow, elongated, generally positive ridges that meander across the dolomite surface and are up to 30 km long and between 80 to 300 m wide. They have always been regarded as post Cretaceous drainage features linked to southward-flowing river systems. Diamonds were discovered in these 'runs' and they have produced some 12 million carats. However, no Cainozoic fossils or artefacts have ever been found in almost 90 years of mining. From new field evidence, geomorphological studies, age dating from inclusions in diamond and zircon and clay analyses, it is proposed that these coarse-grained runs represent proximal palaeoeskers of the last deglaciation of the Dwyka continental ice sheet, that are preserved on this ancient 'palimpsest' surface. The age of the deposit is constrained by two populations of agate within the diamictites that are linked to two separate volcanic units of the Pretoria Group. In addition, the youngest crustal zircon ages from the gravels are 1 Ba, but mantle zircons from Lichtenburg suggest that these have been derived from Cambrian age kimberlites. Analysis of inclusions in diamond support a Neoproterozoic to Cambrian source for the diamonds, so the absence of diamonds from Mesozoic kimberlites and Cainozoic fossils within the gravels support the conclusion that the runs are of Karoo age.
机译：西北省南部Lichthburg-Ventersdorp面积的钻石砾石与北方导向的蜿蜒的“奔跑”有关，几乎完全在Malmani Dolomites（Transvaal Supergroup）的平坦侵蚀表面，在约1,500米的海拔。东向西，这款白云石平原占地面积150公里，北南部平均宽40公里。这种无关，首次在至少80 myR的时间段内进行比较群体沉积之前开发的，由硅质Breccias（Palaeo-karst infill）和集团（重新加工的Breccias）标记。它被挖掘在卡罗前和底坑湾时间。在该多相表面上也发现了冰川路面和薄的karoo沉积物的残余物。弥补这些“跑步”和下沉到这些运行的砾石的砾石是粗糙的，非常差的，并且最好被描述为Difictites。 “跑步”是狭窄的，细长的，一般正脊，蜿蜒穿过白云石表面，长达30公里，宽度为80至300米。他们一直被视为与南风河流系统相关的白垩纪排水特征。钻石在这些“奔跑”中被发现，他们已经生产了1200万克拉。然而，在近90年的采矿中没有发现CainoicoOce化石或人工制品。从新的现场证据，地貌研究，钻石和锆石和粘土分析中的夹杂物的年龄，提议这些粗糙的持续时间代表Dwyka Continental Iceplope的最后一个古老古玩，在这座古代保留了近年的古玩。 Palimpsest'表面。矿床的年龄受到与比较罗碧组织两种单独的火山单位相关的玛瑙血液的约束。此外，来自砾石的最年轻的地壳锆石是1个BA，但来自Lichtenburg的地幔锆石表明这些是来自寒武纪时期金伯利特的。钻石中夹杂物的分析支持NeoProterozoice对钻石的寒武纪来源，因此砾石内的中生代金伯利岩和Cainoicoice化石的钻石的缺失支持跑步是karoo年龄的结论。

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