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, $δ$13C, $δ$18O, $δ$37Cl, and $δ$34S 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.