AN EVAPORITIC ORIGIN OF THE PARENT BRINES OF COLOMBIAN EMERALDS - FLUID INCLUSION AND SULFUR ISOTOPE EVIDENCE

The fluids trapped by emerald, dolomite and pyrite in the Colombian em erald deposits consist predominantly of Na-Ca brines with some KCI. Th e similarity of fluid composition in the eastern and western emerald z ones demonstrates the homogeneity of the parent fluids. The Na-Ca-K ch emistry of the brines provides strong evidence for an evaporitic origi n of the parent hydrothermal fluids. Their origin was investigated by a sulphur isotopic study of pyrite that coprecipitated with emerald. T he delta(34)S values of H2S in solution in equilibrium with pyrite fro m six emerald deposits range from 14.8 to 19.4 parts per thousand wher eas sedimentary pyrite from the enclosing black shales yield a delta(3 4)S of -2.4 parts per thousand. The narrow range in delta(34)S(H2S) be tween the different deposits suggests a uniform and probably unique so urce for the sulphide-sulphur. The high delta(34)S(H2S) values suggest the non-participation of magmatic or Early Cretaceous black-shale sul phur sources. Saline diapirs occur in the emeraldiferous areas and the most likely explanation for high delta(34)S involves the reduction of sedimentary marine evaporitic sulphates. Fluid-inclusion and sulphur- isotope data give a typical evaporitic sedimentary signature for Colom bian emerald mineralization. This emerald-deposit type, unique in the world, corresponds to mesothermal deposits (300 degrees C), formed in a sedimentary environment and produced through thermochemical reductio n of sulphate-rich brines to hydrogen sulphide by interaction with org anic-rich strata.