An evaporitic origin of the parent brines of Colombian emeralds: fluid inclusion and sulphur isotope evidence
Giuliani, Gaston; Cheilletz, Alain; Arboleda, Carlos; Carrillo, Victor; Rueda, Felix; Baker, James H.
European Journal of Mineralogy Volume 7 Number 1 (1995), p. 151 - 166
published: Feb 8, 1995
manuscript accepted: Sep 14, 1994
manuscript received: Jan 17, 1994
ArtNo. ESP147050701012, Price: 29.00 €
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 δ34S values of H2S in solution in equilibrium with pyrite from six emerald deposits range from 14.8 to 19.4 ‰ whereas sedimentary pyrite from the enclosing black shales yield a δ34S of -2.4 %o. The narrow range in δ34SH2S between the different deposits suggests a uniform and probably unique source for the sulphide-sulphur. The high δ34SH2S values suggest the non-participation of magmatic or Early Cretaceous black-shale sulphur sources. Saline diapirs occur in the emeraldiferous areas and the most likely explanation for high δ34S involves the reduction of sedimentary marine evaporitic sulphates. 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 mesothermal deposits (300°C), formed in a sedimentary environment and produced through thermochemical reduction of sulphate-rich brines to hydrogen sulphide by interaction with organic-rich strata.