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首页> 外文期刊>Journal of Volcanology and Geothermal Research >The geology and emplacement of the volcaniclastic infill at the Voorspoed Group Ⅱ kimberlite (orangeite) pipe, Kroonstad Cluster, South Africa
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The geology and emplacement of the volcaniclastic infill at the Voorspoed Group Ⅱ kimberlite (orangeite) pipe, Kroonstad Cluster, South Africa

机译：南非克伦斯塔德群的VoorspoedⅡ类金伯利岩（橙色）管火山碎屑岩的地质特征和位置

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摘要

The Voorspoed Group II kimberlite pipe is atypical in terms of South African kimberlite pipes. Located in the central region of the Kaapvaal Craton (South Africa), the Voorspoed pipe is one of six pipes comprising the Kroonstad Kimberlite Cluster. Reconstruction of the palaeo-stratigraphy at the time of emplacement, of the Kroonstad kimberlites, indicates that significant post-emplacement erosion has occurred and the pipes are currently exposed at ～1600 m depth from the original land surface. The volcaniclastic kimberlite (VK) infilling the pipe is distinctly different from typical tuffisitic kimberlite infilling other South African type kimberlite pipes. Three textural volcaniclastic kimberlite varieties are observed: massive volcaniclastic kimberlite (MVK), bedded volcaniclastic kimberlite (BVK) and fine-grained volcaniclastic kimberlite (fgVK) layers. The BVK and fgVK are volumetrically insignificant and the bulk of the pipe is infilled with MVK. The MVK can be further sub-divided into seven varieties. The two dominant varieties are described in detail, which include: main MVK (mMVK) and the juvenile-rich MVK (jrMVK). The MVKs are typically massive, clastic, poorly sorted and supported by an interclast material consisting of juvenile and xenocrystic crystals in a mud-rich matrix. Magmaclasts within the MVKs are spherical, crystalline with a typical coherent hypabyssal texture and differ-ent mineralogical varieties are typically juxtaposed. BVK units are composed of several normally graded beds of volcaniclastic material. fgVK layers are well sorted relative to the MVKs and all constituents are typically <2 mm in size. The BVK and fgVK can be deposited only within an open vent in order to account for the sorting of the components. A distinct basalt and sandstone rich unit (bsBreccia), which is essentially devoid of kimberlite material, is also observed. Three volcaniclastic zones are identified based on consistent vertical variation in the internal stratigraphy of the infill: outer east/west zone with a sequence bsBreccia-BVK-mMVK/softMVK; a cen-tral zone with a sequence mMVK-BVK-jrMVK-mMVK and an outer north zone with a uniform mMVK sequence. The evidence suggests that the volcaniclastic infill has formed through complex processes involving re-working of extra-crater tephra back into the vent and coeval pyroclastic deposition. It is also likely that there is signifi-cant mixing of the resedimented and pyroclastic deposits. Distinction between deposits formed through re-working of material through repeated explosive eruptions within the vent and that formed through varying de-grees of re-working extra-crater tephra from the surface (with possible coeval pyroclastic eruption) is probably impossible. For this reason we interpret the mMVK as being deposited by dominantly resedimentation of tephra whereas the jrMVK has been deposited dominantly by pyroclastic processes. The presence of BVK and fgVK de-posited deep within the pipe through slumping of the crater margins at the surface suggests that a crater ～2000 m in depth was periodically open. The identification of the complex, layered, variable internal structure of the volcaniclastic infill at the Voorspoed has significant implications for diamond ore evaluation in South Africa. Typical South Africa kimberlite pipes are infilled with tuffisitic kimberlite, which are characterised by their homogenous nature over large vertical inter-vals (> 1000 m). This is not the case at the Voorspoed pipe where MVK layers are variable in thickness (<10m up to 230 m).
机译：就南非金伯利岩烟斗而言，Voorspoed Group II金伯利岩烟斗是非典型的。 Voorspoed管道位于南非Kaapvaal Craton的中部地区，是构成Kroonstad Kimberlite集群的六支管道之一。安置时的古地层，克朗斯塔德金伯利岩的重建表明，发生了严重的安置后侵蚀，并且管道目前暴露于距原始陆地表面约1600 m的深度。填充管道的火山碎屑金伯利岩（VK）与填充其他南非型金伯利岩管道的典型凝灰岩金伯利岩明显不同。观察到三种质地的火山碎屑金伯利岩变种：块状火山碎屑金伯利岩（MVK），层状火山碎屑金伯利岩（BVK）和细粒火山碎屑金伯利岩（fgVK）层。 BVK和fgVK在体积上微不足道，并且在管道的大部分中填充了MVK。 MVK可以进一步细分为七个品种。详细介绍了两个主要变体，包括：主要MVK（mMVK）和富含青少年的MVK（jrMVK）。 MVK通常是块状，碎屑状，分类不善，并被富含泥质基质中的由幼年和异种晶体组成的间质物质支撑。 MVKs中的乳突质是球形的，具有典型的连贯性下丘肌纹理的晶体，并且通常将不同的矿物学种类并列。 BVK单元由几个通常分级的火山弹性材料床组成。 fgVK层相对于MVK进行了很好的分类，所有成分的大小通常小于2 mm。 BVK和fgVK只能放置在一个开放的通风孔中，以便考虑组分的分类。还观察到一个独特的富含玄武岩和砂岩的单元（bsBreccia），基本上不含金伯利岩质。根据填充物内部地层的一致垂直变化确定了三个火山碎屑带：东，西外层带，序列为bsBreccia-BVK-mMVK / softMVK；一个具有mMVK-BVK-jrMVK-mMVK序列的中央区域和一个具有统一mMVK序列的北北部区域。有证据表明，火山碎屑充填物是通过复杂的过程形成的，其中包括将火山口特非拉火山重新加工回火山口，以及同时期的火山碎屑沉积。再沉积和火山碎屑沉积物也可能有明显的混合。通过在排气孔内反复爆炸而对材料进行再加工而形成的沉积物与通过对地表火山口特非拉表面进行不同程度的重新沉积而形成的沉积物（可能伴有时代的火山碎屑喷发）可能是不可能的。由于这个原因，我们将mMVK解释为主要由麻风再沉积而沉积，而jrMVK则主要由热碎屑作用沉积。 BVK和fgVK的存在是由于表面的火山口边缘的塌陷而沉积在管道深处，这表明深度为〜2000 m的火山口是周期性开放的。 Voorspoed火山碎屑填充物复杂，分层，可变的内部结构的识别对南非的钻石矿石评估具有重要意义。南非典型的金伯利岩管道充满凝灰岩金伯利岩，其特点是在较大的垂直区间（> 1000 m）内具有均质性。 Voorspoed管道的情况并非如此，MVK层的厚度是可变的（<10m到230 m）。

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