Diamondiferous kimberlites from recently explored Upper Muna Field (Siberian Craton): petrology, mineralogy and geochemistry insights

Abstract

Abstract Petrographic, geochemical and mineralogical characteristics of diamond deposits from the Upper Muna field have been investigated. Geochemically, diamondiferous kimberlites from Upper Muna belong to the most widespread Fe–Mg-rich rocks in the Yakutian kimberlite province (average FeO total = 8.4 wt%, MgO = 32.36 wt%, TiO 2 = 1.6 wt%). Striking mineralogical features of Upper Muna kimberlites are: (1) abundance of monticellite and perovskite in the groundmass; (2) rare occurrence of Mg-ilmenite; (3) abundance of phlogopite megacrysts (up to 8 cm across); and (4) coexistence of low-Cr (0.1–4 wt% Cr 2 O 3 , with 0.8–1.2 wt% TiO 2 ) and high-Cr (3–8 wt% Cr 2 O 3 , with 0.1–0.6 wt% TiO 2 ) garnet megacrysts with contrasting rare earth element patterns. The compositional features of groundmass minerals, the relatively low CaO and CO 2 contents in kimberlites and few deuteric alteration in Upper Muna kimberlites suggest high-temperature melt crystallization during pipe emplacement. Based on the compositional data of garnet and Cr-diopside from megacrysts and peridotites, we suggest a poor Cr dunite–harzburgitic and lherzolitic mantle source beneath the Upper Muna field where Cr-diopside crystallized within a wide pressure and temperature range (40–65 kbar and 900–1350°С). The mineral geochemistry, trace element distribution and Sr–Nd isotope variations of Upper Muna kimberlites are typical for group I kimberlites and reflect a deep-seated asthenospheric (convective mantle) source for the kimberlites.