Genesis of the vein-type tungsten mineralization at Nyakabingo (Rwanda) in the Karagwe-Ankole belt, Central Africa
Genesis of the vein-type tungsten mineralization at Nyakabingo (Rwanda) in the Karagwe-Ankole belt, Central Africa
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DOI：
10.1007/s00126-015-0608-x
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发表时间：
2016-02-01
影响因子：
4.8
通讯作者：
Muchez, Ph.
中科院分区：
文献类型：
--
作者：
Dewaele, S.;De Clercq, F.;Muchez, Ph.
卢旺达中部钨矿带Nyakabingo的脉型钨矿存款位于复杂的Bumbogo背斜构造的东侧。赋矿岩石由砂岩、石英岩和黑色含黄铁矿的变质岩交替组成。钨矿化石英脉有两种类型：平行层理石英脉和与层理成高角度石英脉，称之为横切脉。这两种矿脉类型被解释为形成于挤压变形事件的晚期。这两种矿脉类型都与小的蚀变带有关，包括硅化、电气石化和白云母化。在脉边缘的白云母晶体测年，最大年龄为992.4 +/- 1.5 Ma。该年龄在误差范围内，与专门的G4花岗岩（即，986 +/- 10 Ma）。含钨矿物形成于两个不同的阶段。第一相为白钨矿和块状黑钨矿，第二相为白钨矿之后的铁橄榄石假像。这些矿物出现在块状石英脉演化的后期，有时甚至出现在横切脉的裂缝中。矿石矿物从具有低至中等盐度（0.6-13.8当量）的H2O-CO2-CH 4-N-2-NaCl-（KCl）流体中沉淀。wt% NaCl），以及247至344摄氏度之间的最低捕集温度。石英脉被富硫化物脉横切。基于相似的地质背景、矿物学特征、稳定同位素和流体成分，认为两种类型的钨矿化石英脉形成于同一成矿事件。鉴于G4花岗岩和矿化石英脉之间的年龄重叠，以及卢旺达W矿床与花岗岩侵入体的典型关联，W起源于地球化学专业化的G4花岗岩。强烈的水-岩相互作用和变质流体的混合在很大程度上覆盖了原始的岩浆-热液特征。
The vein-type tungsten deposit at Nyakabingo in the central Tungsten belt of Rwanda is located in the eastern flank of the complex Bumbogo anticlinal structure. The host rock is composed of alternating sequences of sandstones, quartzites, and black pyritiferous metapelitic rocks. Two types of W-mineralized quartz veins have been observed: bedding-parallel and quartz veins that are at high angle to the bedding, which are termed crosscutting veins. Both vein types have been interpreted to have been formed in a late stage of a compressional deformation event. Both vein types are associated with small alteration zones, comprising silicification, tourmalinization, and muscovitization. Dating of muscovite crystals at the border of the veins resulted in a maximum age of 992.4 +/- 1.5 Ma. This age is within error similar to the ages obtained for the specialized G4 granites (i.e., 986 +/- 10 Ma). The W-bearing minerals formed during two different phases. The first phase is characterized by scheelite and massive wolframite, while the second phase is formed by ferberite pseudomorphs after scheelite. These minerals occur late in the evolution of the massive quartz veins, sometimes even in fractures that crosscut the veins. The ore minerals precipitated from a H2O-CO2-CH4-N-2-NaCl-(KCl) fluid with low to moderate salinity (0.6-13.8 eq. wt% NaCl), and minimal trapping temperatures between 247 and 344 A degrees C. The quartz veins have been crosscut by sulfide-rich veins. Based on the similar setting, mineralogy, stable isotope, and fluid composition, it is considered that both types of W-mineralized quartz veins formed during the same mineralizing event. Given the overlap in age between the G4 granites and the mineralized quartz veins, and the typical association of the W deposits in Rwanda, but also worldwide, with granite intrusions, W originated from the geochemically specialized G4 granites. Intense water-rock interaction and mixing with metamorphic fluids largely overprinted the original magmatic-hydrothermal signature.