A fluid inclusion study of the Little Nahanni LCT-Type Pegmatite Group, NWT Canada: implications for the nature and origin of fluids in LCT-Type Pegmatites and Pegmatite evolution

Abstract

The Cretaceous (82 Ma) Little Nahanni Pegmatite Group , Canada, is a 15 km long swarm of LCT-type pegmatite dikes that intrude Precambrian to Lower Cambrian siliciclastic metapelites and carbonate rocks. The dykes are dominated by a primary assemblage of quartz, spodumene, K-feldspar ± muscovite lacking both graphic textures and core zones, but characterized by combtextured spodumene and K-feldspar intergrowths. Varying degrees of albitization and lesser phyllic alteration accompany ore-grade oxide enrichment (Ta-Nb and Sn, respectively).A comprehensive fluid inclusion study using a wide variety of methods (petrography , CL, microthermometry, SEM/EDS evaporate mound analysis, LA ICP-MS, laser Raman spectroscopy, δ13C) on minerals (quartz, K-feldspar, albite, spodumene, muscovite, garnet, and tourmaline) from barren and mineralized samples indicate the fluid inclusions are secondary and appear to post-date pegmatite crystallization. The presence of secondary fluid inclusions may instead represent entrapment during the late-stage (sub-solidus to post crystallization) metasomatic evolution stage of the pegmatites. Three fluid types occur, and listed in paragenetic order are: (A) an aqueous-carbonic fluid (≤1 wt. % eq. NaCl) with varying XCO2 (0.1 to 0.78), purity (TmCO2= -57o to -61oC), and density; (B) a low-salinity (≤2 wt. % eq. NaCl) aqueous fluid; and (C) an aqueous-methane-nitrogen fluid. SEM-EDS analysis indicates that the fluids are Na (-K) - dominant, but with significant S and enrichment in transition and chalcophile elements. Fluid inclusion extracts have δ13C values (-9.7 to -27.7‰; n=7) which indicates exchange of the fluids with the immediate, carbon-bearing wall rocks occurred. Collectively the data indicate: (1) fluids were trapped during the later, metasomatic stage of pegmatite evolution; (2) fluids were trapped at <2.5 kbars and <350°C with pressure fluctuating between lithostatic and hydrostatic; (3) variable degrees of fluid:rock interaction occurred with the exocontact zone, resulting in varying XCO2and CO2:CH4 values; and (4) metasomatism and rare metal mineralization occurred in the presence of meteoric water. These findings indicate that the LNPG pegmatites are not a closed system, some of the carbonic component of H2O-CO2 fluids is exotic, and that low-salinity fluids of meteoric origin infiltrated during the subsolidus stage of pegmatite evolution.