Fluid fractionation of tungsten during granite-pegmatite differentiation and the metal source of peribatholitic W quartz veins: Evidence from the Karagwe-Ankole Belt (Rwanda)

被引:113
|
作者
Hulsbosch, Niels [1 ]
Boiron, Marie-Christine [2 ]
Dewaele, Stijn [3 ]
Muchez, Philippe [1 ]
机构
[1] Katholieke Univ Leuven, Geodynam & Geofluids Res Grp, Dept Earth & Environm Sci, Celestijnenlaan 200E, B-3001 Leuven, Belgium
[2] Univ Lorraine, CNRS, CREGU, GeoRessources, Blvd Aiguillettes BP 239, F-54506 Vandoeuvre Les Nancy, France
[3] RMCA, Dept Geol & Mineral, Leuvensesteenweg 13, B-3080 Tervuren, Belgium
关键词
LA-ICP-MS; CENTRAL-AFRICA; TRACE-ELEMENT; PETROLOGIC ASSESSMENT; INTERNAL ZONATION; GATUMBA AREA; ORE-DEPOSITS; CASSITERITE SOLUBILITY; TRIOCTAHEDRAL MICAS; CHEMICAL EVOLUTION;
D O I
10.1016/j.gca.2015.11.020
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
The identification of a magmatic source for granite-associated rare metal (W, Nb, Ta and Sn) mineralisation in metasediment-hosted quartz veins is often obscured by intense fluid-rock interactions which metamorphically overprinted most source signatures in the vein system. In order to address this recurrent metal sourcing problem, we have studied the metasediment-hosted tungsten-bearing quartz veins of the Nyakabingo deposit of the Karagwe-Ankole belt in Central Rwanda. The vein system (992 +/- 2 Ma) is spatiotemporal related to the well-characterised B-rich, F-poor G4 leucogranite-pegmatite suite (986 +/- 10Ma to 975 +/- 8Ma) of the Gatumba-Gitarama area which culminated in Nb-Ta-Sn mineralisation. Muscovite in the Nyakabingo veins is significantly enriched in granitophile elements (Rb, Cs, W and Sn) and show alkali metal signatures equivalent to muscovite of less-differentiated pegmatite zones of the Gatumba-Gitarama area. Pegmatitic muscovite records a decrease in W content with increasing differentiation proxies (Rb and Cs), in contrast to the continuous enrichment of other high field strength elements (Nb and Ta) and Sn. This is an indication of a selective redistribution for W by fluid exsolution and fluid fractionation. Primary fluid inclusions in tourmaline of these less-differentiated pegmatites demonstrate the presence of medium to low saline, H2O-NaCl-KCl-MgCl2-complex salt (e.g. Rb, Cs) fluids which started to exsolve at the G4 granite-pegmatite transition stage. Laser ablation inductively coupled plasma mass-spectrometry shows significant tungsten enrichment in these fluid phases (similar to 5-500 ppm). Fractional crystallisation has been identified previously as the driving mechanism for the transition from G4 granites, less-differentiated biotite, biotite-muscovite towards muscovite pegmatites and eventually columbite-tantalite mineralised pegmatites. The general absence of tungsten mineralisation in this magmatic suite, including the most differentiated columbite-tantalite mineralised pegmatites of the Gatumba-Gitarama area, emphasises the efficiency of fluid saturation to extract crystal-melt incompatible tungsten from the differentiating melt phase. Fluid-melt-crystal partitioning calculations support the concept of a magmatic-hydrothermal fluid source for tungsten and constrain the range of permissible crystal-melt and fluid-melt partition coefficients together with realistic values for water solubility in the parental G4 granitic melt. Consequently, we propose that for highly-differentiated B-rich, F-poor granite systems fluid saturation started prior to or at the granite-pegmatite transition stage resulting in apical to peribatholitic tungsten veins systems that are paragenetically older than the final pegmatite stage. (C) 2015 Elsevier Ltd. All rights reserved.
引用
收藏
页码:299 / 318
页数:20
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