The “Exceptionally Fresh” Udachnaya-East Kimberlite: Evidence for Brine and Evaporite Contamination

Abstract

The composition of the serpentine-free Udachnaya-East kimberlite containing alkali carbonate, gypsum, halite, and other Na-, Cl-, and S-rich minerals has been the basis for a model of alkali-rich primary kimberlite melt. The interpretation of these minerals as mantle-derived, however, contradicts geology and hydrogeology of the Yakutian kimberlite province, as well as petrographic, geochemical, and isotopic evidence. The Udachnaya-East pipe is similar to many other southern Yakutian kimberlites, which emplace through 2-km-thick evaporite-bearing terrigenous carbonate sediments saturated with brines. A secondary origin of Na-, Cl-, and S-rich minerals in the southern Yakutian kimberlites is supported by (1) a regional correlation between the geology and hydrogeology of the local country rocks and the kimberlite mineralogy, in particular the difference between southern and northern Yakutian kimberlites; (2) a restriction of halite or gypsum mineralization to certain depth horizons where pipes intersect country rock strata with similar mineralogy; (3) the localization of the highest abundances of Na–Cl–S-bearing minerals at a depth interval that correlates across three magmatic phases of kimberlites and coincides with the roof of the aquifer carrying Na brines; (4) the presence of evaporite xenoliths and veins of halite, gypsum, and carbonate cutting through kimberlite and xenoliths; (5) crystallization of halite and alkali carbonate after serpentine and other groundmass minerals as evidenced by the rock textures; (6) geochemical evidence for crustal contamination, including high bulk CO2 and CaO content, the absence of correlation between bulk Na2O and any geochemical parameters, as well as initial Sr ratios, δ13C, δ18O, δ37Cl, and δ34S intermediate between crustal and mantle values. We propose that the Udachnaya-East kimberlites acquired high Na, S, and Cl contents by interaction with buried Cambrian Na–Ca–Cl brines or assimilating evaporite xenoliths from the 500-m-thick Chukuck suite formed in the Daldyn-Markha carbonate bank.