Potassium sulfides in kimberlite-hosted chloride-"nyerereite" and chloride clasts of Udachnaya-East Pipe, Yakutia, Russia

Abstract

Chloride-bearing clasts from the Udachnaya-East pipe kimberlites, Yakutia, Russia, contain abundant to accessory rasvumite, KFe2S3, and djerfisherite, K6Na0-1(Fe,Ni,Cu)(24)S26Cl, which have been previously described in the groundmass of the host kimberlite. Rasvumite occurs in chloride-"nyerereite" and carbonate clasts, where it forms prismatic crystals (up to 5 mm) or is associated with djerfisherite in a rim around pyrrhotite. Djerfisherite is the omnipresent sulfide in all types of chloride-containing clasts, in which it rims pyrrhotite and forms individual xenomorphic to subhedral grains and, more rarely, octahedral crystals at the contact with host kimberlite. In chloride clasts and the host kimberlites, the following sequence of crystallization was found: pyrrhotite - rasvumite - djerfisherite, reflecting an increase in the activity of Cl in the evolved melt. The composition of rasvumite from the Udachnaya-East pipe is close to ideal KFe2S3, with low abundances of Rb, Cs and Na (< 0.9, < 0.2 and <0.15 wt.%, respectively). Analyses of the rasvumite by the LA-ICP-MS technique indicates the presence of T1 (95-480), Ba (110-215), Pb (25-190), Te (up to 60), Se (20-135) and Co (25-100 ppm). These data also indicate compositional differences between unaltered rasvumite and its oxidized rim; the levels of Na, Mg, V, Mn, Ni, As, Sr, Sb and W increase by an order of magnitude. Unlike djerfisherite in the kimberlite groundmass, that in the chloride clasts is generally characterized by higher amounts of Fe and lower amounts of Cu, with low contents of Rb (< 2000 ppm), Cs (< 100 ppm), and Tl (< 330 ppm). Sulfides of potassium in chloride-bearing clasts are considered to be primary phases crystallized from an evolved kimberlitic melt.