An evaporitic origin of the parent brines of Colombian emeralds: fluid inclusion and sulphur isotope evidence

Abstract: The fluids trapped by emerald, dolomite and pyrite in the Colombian emerald deposits consist predominantly of Na-Ca brines with some KC1. The similarity of fluid composition in the eastern and western emerald zones demonstrates the homogeneity of the parent fluids. The Na-Ca-K chemistry of the brines provides strong evidence for an evaporitic origin of the parent hydrothermal fluids. Their origin was investigated by a sulphur isotopic study of pyrite that coprecipitated with emerald. The 634S values of H2S in solution in equilibrium with pyrite from six emerald deposits range from 14.8 to 19.4 %O whereas sedimentary pyrite from the enclosing black shales yield a 634S of -2.4 %O. The narrow range in 634S~2s between the different deposits suggests a uniform and probably unique source for the sulphide-sulphur. The high 634S~2s values suggest the non-participation of magmatic or Early Cretaceous black-shale sulphur sources. Saline diapirs occur in the emeraldiferous areas and the Fluid-inclusion and sulphur-isotope data give a typical evaporitic sedimentary signature for Colombian emerald mineralization. This emerald-deposit type, unique in the world, corresponds to mesothennal deposits (300'C), most likely explanation for. high 634S involves the reduction of sedimentary marine evaporitic sulphates. fomicd in a sedimentaiy environment and produced through thermochemical reduction of sulphate-rich brines to hydrogen sulphide by interaction with organic-rich strata