Tracing fluid saturation during pegmatite differentiation by studying the fluid inclusion evolution and multiphase cassiterite mineralisation of the Gatumba pegmatite dyke system (NW Rwanda)

Abstract

Aqueous fluid saturation during the internal evolution of phosphorus- and boron-rich pegmatites in NW Rwanda has been investigated using petrography, fluid inclusion microthermometry, and microanalysis of minerals and fluid inclusions (Raman microspectroscopy and LA-ICP-MS). The Gatumba dyke system (GDS) in Rwanda has been selected for this study because it hosts a multiphase cassiterite mineralisation, in wall, core and internal replacement zones, suggesting an advancing influence of magmatic-hydrothermal conditions during dyke solidification. As such, multiphase cassiterite precipitation is applied as a tracer for magmatic to hydrothermal crystallisation processes in granitic pegmatites. The GDS consists of a group of six major LCT (lithium-caesium-tantalum) family dykes, which all belong to the most differentiated rare-element pegmatites in the Gatumba-Gitarama field. Petrography identified a well-developed internal anatomy consisting of a border, wall, intermediate and a quartz core zone. In addition, replacement zones developed as cleavelandite after perthitic microcline units in the intermediate zone and as muscovite-quartz pockets in the intermediate and wall zones. The latter replacement zone display muscovitisation reactions with the formation of greisens. Texturally, the greisen pockets replace the secondary cleavelandite units. During internal differentiation, multiphase cassiterite mineralisation formed in 1) large microcline crystals of the wall zone (Cst1), 2) assemblage with quartz, F-poor montebrasite, and carbonate- and boron-enriched Mn-fluorapatite in the core zone (Cst2), and 3) the greisen replacement units in the wall and intermediate zones (Cst3). Fluid inclusion micro-analyses demonstrate that a saline aqueous H2O-NaCl-KCl-(CO2,N2) L1-type fluid (∼20 wt.% NaCl and ∼3 wt.% KCl) was saturated during onset of crystallisation of the wall and intermediate zones and precipitation of the disseminated, magmatic-hydrothermal Cst1 mineralisation in and around large perthitic K-feldspars. Isochore reconstructions of this L1 fluid indicate crystallisation conditions of 535–560 °C and 5.1–5.6 kbar for the GDS. Extensive microcline fractionation and subsequent replacement reactions of cleavelandite after microcline in the intermediate zone were induced by a Li-enriched and more Na-depleted H2O-NaCl-LiCl-(CO2,N2) L2-type fluid, which was present during solidification of the core zone and present during precipitation of Cst2 phase mineralisation (6–12 wt.% NaCl, 1–10 wt.% LiCl). Late-stage, hydrothermal Cst3 mineralisation in the greisen pockets precipitated from a Cs-enriched H2O-NaCl-(KCl, CO2,N2) L3-type fluid (∼15 wt.% NaCl) distinctly after the formation of the cleavelandite replacement units. Subsolidus Cst3 precipitation is dominantly driven by metasomatic, hydrolytic fluid-rock reactions. Combining textural, microthermometric and fluid compositional data with reported Sn melt-fluid partitioning and solubility data demonstrate that primary cassiterite (i.e. Cst1 and Cst2) in the wall and core zone of the GDS precipitated from the exsolved saline aqueous L1-and L2-fluid present at the interface of the highly-fractionated melt phase. Fast disequilibrium growth of (near)-anhydrous mineral assemblages likely caused the formation of flux-rich and, at least locally, water-saturated melt compositions at the crystallisation front of the large crystals. This study emphasises the importance of local magmatic-hydrothermal conditions and the presence of an immiscible aqueous fluid phase during the internal crystallisation and cassiterite mineralisation of rare-element pegmatites.