The physical and chemical evolution of fluids in rare-element granitic pegmatites associated with the Lacorne pluton, Québec, Canada

Abstract

Co-magmatic rare-element (Be, Li, Nb, Ta) granitic pegmatites associated with the Lacorne monzogranite pluton of the Preissac-Lacorne batholith (Québec) range from a least evolved beryl-type in the pluton through a transitional beryl + spodumene-type to a most evolved spodumene-type in mafic country rocks. They possess internal lithologic fabrics and primary fluid inclusions that provide evidence for saturation of the magma with aqueous fluids immediately after the formation of aplite in the beryl pegmatite, and pene-contemporaneously with formation of the beryl + spodumene and spodumene pegmatites. The early fluids are represented by aqueous liquid-vapor (Type 1a) and aqueous liquid-vapor-solid (Type 2) inclusions in beryl from beryl and beryl + spodumene pegmatites and are estimated to have been trapped between 500 and 550 °C and at 3.5 Kb. On the basis of microthermometric measurements, analyses of the compositions of the precipitates from decrepitated inclusions and the nature of the solids (trapped), these fluids are interpreted to have been NaCl-dominated (≤16 wt% NaCl eq.) and to have contained appreciable Fe and dissolved CO2. In spodumene pegmatite, the equivalent fluid, which was trapped at 450 to 500 °C, contained significant Mn, Li and Cs in place of Fe, consistent with corresponding trends in the chemical evolution of the host rock.Volatile-rich, mainly secondary, aqueous fluid inclusions (Type 1b) trapped in beryl are interpreted to reflect temporary sharp drops in pressure later in the crystallization history of the beryl-bearing pegmatites due to local fluid overpressures that led to brecciation and consequent pressure release. These inclusions have high salinity, are CaCl2-rich and are considered to represent magmatic hydrothermal fluids that acquired their calcium as a result of their interaction with the host Lacorne pluton. Incursion of CaCl2-rich fluids into the spodumene pegmatites is interpreted to have occurred earlier in their crystallization history, i.e., upon emplacement of the pegmatites, as shown by the fact that a high proportion of primary inclusions in spodumene in these pegmatites contains considerable calcium, and calcium contents are generally higher in inclusions in spodumene than in beryl. The inclusions in the spodumene pegmatites are thought to have acquired their calcium through interaction of the fluids with the amphibolites (metabasalts) that host these intrusions.The crystallization of the pegmatites terminated with the entrapment of low-salinity aqueous‑carbonic fluids (Type 3 inclusions) in quartz (6–10 wt% NaCl eq.), which most likely formed as a result of the unmixing of the magmatic fluid into separate aqueous and carbonic fluids, probably at temperatures between 250 and 350 °C.The results of this study show that aqueous fluid saturation can take place at an early stage of pegmatite emplacement, and that magma evolution is matched by changes in the solute chemistry of the exsolving aqueous fluid. They also show that evolution to a more volatile-rich composition (H2O + F + Li) may have contributed to the increased mobility of later magma batches, thereby helping to explain the regional zonation of the pegmatites, from beryl-type within the pluton to spodumene-type in the adjacent amphibolite.