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

Emerald mineralisation in Colombia is located in two distinct zones along t he borders of the Eastern Cordillera, some 80 km apart. Mineralisation in t he 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 fro m both zones, demonstrates that in each region brines associated with emera ld mineralisation range between two extremes with many samples yielding int ermediate compositions. Fluid 1, found mainly in emerald-hosted fluid inclu sions, is dominated by NaCl with high Cl:Br ratios indicating that the sali nity was derived by dissolution of halite, most probably from the local sal t beds. Fluid 2, found notably in quartz hosted-fluid inclusions, is of sim ilar salinity but contains less Na and significant concentrations of Ca-K-F e-Cl and other cations. It has lower Cl:Br ratios, more comparable with for mation waters, but is inferred to have obtained part of its salinity by hal ite dissolution. Bivariate plots of almost all cations have linear or subli near trends regardless of the mineral hosting the inclusions or the localit y from which the samples originated, demonstrating that mixing of the two s aline fluids has occurred. Because the same two fluids occur in both easter n and western zones, despite the difference in space and time, it is inferr ed that fluid compositions were rock controlled by similar interactions wit h evaporites and black shales in both instances. It is proposed that beryll ium was transported as Be-F complexes in the NaCl-fluid, and was precipitat ed as emerald after mixing with the calcic brine caused precipitation of fl uorite and parisite.