Spinel-group minerals in peridotites of the guli and boruryakh intrusions (Meimecha-Kotuy Province, Northern Siberia)

The Guli and Bor-Uryakh massifs, a part of the Siberian Large igneous province (LIP) are mafic-ultramafic intrusive complexes, with strong alkaline affinity. They contain deposits of apatite and are also known to be source rocks of Os-Ir-Ru placers. These massifs are of great interest for petrologists worldwide, as they are composed of an unusual variety of rocks (dunites/olivinites, shonkinites, melilitites, alkali syenites and carbonatites) and being coeval with Siberian trap volcanic rocks, including diamondiferous kimberlites. Since mineralogical approaches based on spinel-group minerals have been proved to be efficient in constraining origin of the ultramafics, we present the first descriptive study of chromite and magnetite mineralization, observed in olivine-dominated rocks of the Guli and Bor-Uryakh intrusions. In dunites of Guli massif spinel-group minerals are dominated by Mg-poor chromite (FeMg)Cr2O4 and Cr-Ti-rich magnetite FeFe2O4, while in Bor-Uryakh massif spinel-group minerals are predominantly magnetite with only minor Mg-poor chromite. These minerals form either small euhedral inclusions in olivine or large subhedral to anhedral grains in serpentinized fractures and interstitial space. The latter type of grains can have intricated irregular shape and contain inclusions. We also observed abundant Cr-magnetite lamellae in olivine and chromite/magnetite micro-grains within olivine-hosted multiphase inclusions. Spinel (MgAl2O4) is occasionally found in intergrowths with chromite and magnetite. The obtained data show that spinel-group minerals in the massifs do not correspond to primary-magmatic varieties and suggest extensive alteration during post-magmatic processes. Textural and chemical evidence of substantial modification of initially-cumulative lithologies of Guli and Bor-Uryakh massifs favors meta-magmatic origin for these massifs.