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首页> 外文期刊>Economic geology and the bulletin of the Society of Economic Geologists >Structural Controls on Orogenic Au Mineralization During Transpression: Lupa Goldfield, Southwestern Tanzania
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Structural Controls on Orogenic Au Mineralization During Transpression: Lupa Goldfield, Southwestern Tanzania

机译：压变过程中造山型金矿化的结构控制：坦桑尼亚西南部的卢帕金矿

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摘要

Au mineralization in the western Lupa goldfield, southwestern Tanzania was associated with transpression and reverse sinistral slip along a network of steeply S dipping shear zones with non-Andersonian geometries. Slip was accommodated by: (1) frictional failure and sliding during emplacement of quartz ± Au-bearing veins; and (2) crystal plasticity and fluid-assisted diffusive mass transfer. The Kenge mineral system is situated along a NW-SE-trending shear zone and is characterized by ≤ 10-m thick, Au-bearing fault-fill veins hosted by well-developed phyllosilicate-rich mylonites. The broadly contemporaneous Porcupine mineral system is situated along an ENE-WSW- to E-W-trending shear zone, which is characterized by narrow, discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith. Au mineralization at Porcupine occurs within steeply dipping fault-fill and subhorizontal extension/oblique-extension veins. Three-dimensional frictional reactivation theory provides a self-consistent explanation for the different vein styles at Kenge and Porcupine and extends the classic fault valve model to the general case of oblique slip along multiple, arbitrarily oriented shear zones. Analysis of the differential stress required for frictional reactivation suggests the following: (1) the Kenge shear zone was intrinsically weaker than the Porcupine shear zone, consistent with the lack of well-developed mylonites at Porcupine; and (2) frictional reactivation of the Kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures, whereas frictional reactivation of the Porcupine shear zone occurred under near-lithostatic fluid pressures. We hypothesize that near-lithostatic pore fluid pressures relieved effective normal stresses at grain-grain contacts, helping to preserve intragranular and fracture porosity at the Porcupine orebody. As such, these pore spaces may be important mierostructural sites for Au mineralization. Low effective normal stresses can also explain the poorly developed phyllosilicate-rich mylonites and limited degree of shear zone weakening at Porcupine.
机译：坦桑尼亚西南部卢帕金矿区西部的金矿化与沿非S型陡峭S剪切带网络形成的反压和逆向左旋滑移有关。滑移是由以下因素引起的：（1）在放置石英±含金静脉的过程中发生摩擦破坏和滑动; （2）晶体可塑性和流体辅助扩散传质。 Kenge矿物系统位于NW-SE趋势剪切带上，其特征是由发育良好的富含页硅酸盐的my石构成的≤10 m厚，含金断层脉。广泛同时期的豪猪矿物质系统沿ENE-WSW-E-W趋势剪切带分布，其特征是硅质化和非叶状花岗岩样原岩中狭窄的，不连续的淀粉样剪切带。豪猪的金矿化发生在陡倾的断层充填和水平下延伸/斜延伸脉内。三维摩擦复活理论为肯格（Kenge）和豪猪（Porcupine）的不同静脉样式提供了自洽的解释，并将经典的故障阀模型扩展到了沿多个任意定向的剪切带的倾斜滑动的一般情况。对摩擦再活化所需的差应力的分析表明：（1）肯格剪切带本质上比豪猪剪切带弱，这与豪猪缺乏发达的黑lon石相符; （2）肯格剪切带的摩擦活化是在超静水压力下但在准静态的孔隙流体压力下发生的，而豪猪剪切带的摩擦活化是在近静态的流体压力下发生的。我们假设接近恒压的孔隙流体压力减轻了谷物与谷物接触时的有效法向应力，从而有助于保持豪猪矿体的粒内孔隙和裂缝孔隙。因此，这些孔隙空间可能是金矿化的重要微观结构部位。有效法向应力低也可以解释富含豪硅的层状硅酸盐发育不佳和豪猪的剪切带减弱程度有限。

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