Features of composition in spinel subgroup minerals from picrites of the Anabar-Ujinsky interfluve

There are numerous diamond placers on the Eastern Anabar region, but primary sources have not been identified yet. However, non-diamondiferous kimberlite and similar alkaline-ultramafic rocks of picrite-alneite composition, with diamond indicator minerals are widespread in the region. The paper presents a comprehensive study of spinel subgroup minerals, which are typical minerals of small bodies of alkaline-ultramafic rocks of the Tomtor field. The main goal of the work was to determine the typochemical features of spinel subgroup minerals from alkaline-ultramafic bodies of the region and to evaluate their potential role in the formation of mineral halos associated with alluvial diamond. It was found that chemical composition of spinel subgroup minerals are characterized by a wide range of Cr₂O₃ (23.6–57.4 wt. %), Al₂O₃ (1.5–33.5 wt. %) and MgO (8–18.2 wt. %) contents. A group of grains with abnormally high TiO₂ contents (3–7.6 wt. %) was identified. In the classification diagrams, the compositions form a trend from chromite to pleonast, typical for rocks crystallized in shallower level than kimberlites. The presence of zoned grains, in which Cr₂O₃ content decreases from the center to the edge, is associated with saturation of the melt with Al 2 O 3 and crystallization conditions changing. The similarity of the composition of studied high-TiO 2 Cr-spinels with those from the diamond-bearing Carnian tuffites of the Bulkur anticline was established. The results indicate that picrite bodies can be considered as one of the sources of Cr-spinel in placers and intermediate reservoirs of the region.