The nature of orthomagmatic hydrothermal fluids in the Oka carbonatite, Quebec, Canada: Evidence from fluid inclusions

Abstract

Fluid inclusions in apatite, monticellite, and calcite from the Oka carbonatite complex generally comprise either aqueous liquid + vapour or aqueous liquid + vapour + halite. Some inclusions also contain a variety of trapped solids, including calcite, strontianite, nahcolite, and mirabilite. Microthermometry and Raman spectroscopy indicate that CO 2 is not an important constituent of these fluids. Microthermometric data also indicate that the hydrothermal fluids liberated from the carbonatitic magma had a wide range of salinities (4.2 to 49 eq. wt% NaCl) and densities (0.77 to 1.26 g/cm 3). Most of the fluids had relatively low concentrations of SO 4 2− and CO 3 2− (<6 wt%); however, the nature of the trapped minerals and the phase behaviour of some inclusions indicate that some fluids may have had higher concentrations of SO 4 2−, CO 3 2− and/or HCO 3 − Textural relationships indicate that the crystallization sequence for the host minerals was monticellite → apatite → calcite, which, when combined with fluid inclusion data from the three minerals, indicates that fluid salinity decreased and fluid density increased during progressive crystallization of the magma. The density increase is thought to have been mainly caused by decreasing temperature. Isochoric projections indicate that the fluids were exsolved at minimum pressures of 4 to 8 kb. These data provide a basis with which to assess the involvement of orthomagmatic aqueous fluids in the formation of hydrothermal rare element mineralization associated with carbonatites.