Emerald mineralisation in Colombia: fluid chemistry and the role of brine mixing

Abstract

Emerald mineralisation in Colombia is located in two distinct zones along the borders of the Eastern Cordillera, some 80 km apart. Mineralisation in the western zone has been dated at ca. 35 Ma whereas in the eastern zone it is 30 Ma older. Crush leach analysis of the electrolyte chemistry of fluid inclusions contained in emerald, quartz, calcite, dolomite and fluorite from both zones, demonstrates that in each region brines associated with emerald mineralisation range between two extremes with many samples yielding intermediate compositions. Fluid 1, found mainly in emerald-hosted fluid inclusions, is dominated by NaCl with high Cl:Br ratios indicating that the salinity was derived by dissolution of halite, most probably from the local salt beds. Fluid 2, found notably in quartz hosted-fluid inclusions, is of similar salinity but contains less Na and significant concentrations of Ca–K–Fe–Cl and other cations. It has lower Cl:Br ratios, more comparable with formation waters, but is inferred to have obtained part of its salinity by halite dissolution. Bivariate plots of almost all cations have linear or sublinear trends regardless of the mineral hosting the inclusions or the locality from which the samples originated, demonstrating that mixing of the two saline fluids has occurred. Because the same two fluids occur in both eastern and western zones, despite the difference in space and time, it is inferred that fluid compositions were rock controlled by similar interactions with evaporites and black shales in both instances. It is proposed that beryllium was transported as Be–F complexes in the NaCl-fluid, and was precipitated as emerald after mixing with the calcic brine caused precipitation of fluorite and parisite.