Ngubevu金矿和贱金属矿(南非Tugela Terrane Natal冲断层前缘)的微观断裂:它们与宏观尺度的变形一致吗?它们告诉我们什么?

IF 1.2 4区 地球科学 Q2 GEOLOGY
N. S. Busakwe, E. Carranza, O. Chagi, L. Hoyer
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引用次数: 0

摘要

在Tugela Terrane的Natal冲断层前缘(NTF),缺乏对剪切带中贵金属和贱金属矿化的结构控制的多尺度特征。本文通过对已知剪切带及其周围的微裂缝的分析,有助于当前对NTF中剪切带主导的贵金属和贱金属矿化的结构控制的理解,该剪切带主导了NTF Ngubevu地区的矿脉金和贱金属成矿。基于控制地质结构的应力在一系列地理尺度上的空间分布是分形的(即具有尺度不变性或自相似性)的概念,本研究(a)确定从Ngubevu金矿的微裂缝推断出的应力是否与NTF中与区域到局部地质结构相关的应力一致,以及(b)解释这些推断和已知应力所暗示的矿化控制。根据显示Ngubevu定向矿脉材料样品中微观裂缝趋势的玫瑰图,推断出了基于最大水平应力(SHmax)和最小水平应力的每个样品的相对微观应力场。然后将其与NTF现有研究得出的局部到区域尺度的应力场方向进行比较。Ngubevu样品中微观断裂SHmax的北东北-西南偏南方向与Tugela Terrane NTF中区域-区域规模逆冲断层SHmax的西北偏北-东南偏南方向一致。同样,Ngubevu东部样品中微裂缝的西-西北-东-东南方向与局部规模S1叶理的一般东-西方向一致。这些方位与D1逆冲事件有关,该事件是由北向北东北方向的区域逆冲构造引起的。从区域-区域尺度逆冲断层SHmax的西北偏北-东南偏南方向到矿脉物质中微断裂SHmax的东北偏北-西南偏南方向的顺时针变化证明了早期提出的区域-区域规模逆冲会聚顺时针旋转的概念。这表明Ngubevu的V1静脉可能在D1事件后期形成。样品中的微裂缝方向(主要来自Ngubevu西部)描绘了走滑状态,这与D2左旋挤压一致,D2左旋挤压影响了早期形成的东西走向结构。这表明,Ngubevu的一些V1矿脉,特别是其西部的V1矿脉,受到D2左旋挤压的影响(例如,通过一些V1矿脉中的金的再活化)。这项研究中的新发现对于地质测绘、矿化控制的识别以及勘探不足地区的矿产勘探至关重要,这些地区的地质条件允许(即代表有利于形成某些类型矿化的过程)。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Micro-fractures in the Ngubevu Gold and Base Metal Mines (Natal Thrust Front, Tugela Terrane, South Africa): Are they consistent with macro-scale deformation, and what do they tell us?
In the Natal Thrust Front (NTF) of the Tugela Terrane, there is a lack of multi-scale characterisation of structural controls on shear-zone hosted precious and base metal mineralisation. This paper contributes to the current understanding of the structural controls on shear-zone-hosted precious and base metal mineralisation in the NTF with insight derived from the analyses of micro-fractures at and around the known shear-zone-hosted lode gold and base metal mineralisation in the Ngubevu area of the NTF. Based on the notion that the spatial distribution of stresses that control geological structures is fractal (i.e., possessing scale-invariance or self-similarity properties) across a range of geographic scales, this study (a) determines whether the stresses inferred from micro-fractures in the gold mines in Ngubevu are consistent with the stresses associated with regional- to local-scale geological structures in the NTF, and (b) interprets the mineralisation controls implied by these inferred and known stresses. From rose diagrams showing trends of micro-fractures in oriented vein material samples from Ngubevu, the relative micro-scale stress field per sample based on the maximum horizontal stress (SHmax) and the minimum horizontal stress (SHmin) were inferred. These were then compared to the local- to regional-scale stress field orientations derived from existing studies in the NTF. The north-northeast–south-southwest orientations of SHmax of micro-fractures in samples from Ngubevu are consistent with the north-northwest–south-southeast orientations of SHmax of regional- to district-scale thrust faults in the NTF of the Tugela Terrane. Likewise, the west-northwest–east-southeast orientations of micro-fractures in samples from the eastern part of Ngubevu are consistent with the general east–west orientations of local-scale S1 foliation. These orientations are associated with a D1 thrusting event, which was due to north to north-northeast-directed regional thrust tectonics. The clockwise change from the north-northwest–south-southeast orientations of SHmax of regional- to district-scale thrust faults to the north-northeast–south-southwest orientations of SHmax of micro-fractures in vein material attests to an earlier posited notion of clockwise rotation of regional- to district-scale thrust vergence. These suggest that V1 veins in Ngubevu were likely formed late in the D1 event. The orientations of micro-fractures in samples, mostly from the western part of Ngubevu, depict a strike-slip regime, which is consistent with D2 sinistral transpression that affected the earlier-formed east–west-trending structures. This suggests that some V1 veins in Ngubevu, particularly those in its western part, were affected by the D2 sinistral transpression (e.g., by remobilisation of gold in some of the V1 veins). The novel findings in this study are crucial to geological mapping, recognition of mineralisation controls and mineral prospecting in poorly-explored regions where the geology is permissive (i.e., represents processes that are favourable) for the formation of certain type(s) of mineralisation.
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来源期刊
CiteScore
3.60
自引率
5.60%
发文量
18
审稿时长
>12 weeks
期刊介绍: The South African Journal of Geology publishes scientific papers, notes, stratigraphic descriptions and discussions in the broadly defined fields of geoscience that are related directly or indirectly to the geology of Africa. Contributions relevant to former supercontinental entities such as Gondwana and Rodinia are also welcome as are topical studies on any geoscience-related discipline. Review papers are welcome as long as they represent original, new syntheses. Special issues are also encouraged but terms for these must be negotiated with the Editors.
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