The Poldnevskoye deposit of demantoid (Middle Urals): Geology and mineralogy

Abstract

Research subject. The results of studying the geology and mineralogy of the Poldnevskoye deposit of demantoid are presented. Material and methods. The factual material was collected during feld research. Demantoid samples were pro vided by mine owners. Rock and vein minerals were studied in hand specimens, polished specimens and thin sections; the chemical composition was determined by X-ray fluorescence spectroscopy, scanning electron microscopy, electron probe X-ray spectral microanalysis, inductively coupled plasma mass spectrometry with laser sampling (LA-ICP-MS).Results. The Korkodinsky ultrabasic massif containing the deposit experienced strong decompression and syndecompression mineral formation during the ascent. At an early stage, veins of clinopyroxenite formed in dunite, which also experienced decompression cracking. Then the rocks underwent antigoritization and the appearance of vein antigorite, which was replaced by veins of clinochrysotile (+ magnetite ± carbonate ± demantoid). This was followed by the formation of lizardite (+ magnetite ± carbonate ± demantoid), which also developed along the earlier vein serpentines. Demantoid is represented by rounded grains and rounded grain aggregates with the signs of growth under the conditions of allround extension. The predominant color is brownish-green, yellow-green, rarely green and dark green. Green and dark green demantoids contain Cr2O3 0.52–2.3 wt %. In the central part of some grains and demantoid aggregates, a brown color is observed, which binds to TiO2, the content of which reaches 1 wt %. The demantoids of the Poldnevskoye deposit bear typomorphic features of the Ural-type demantoids (inclusions of the “horse’s tail” type). The distribution of demantoid is nested. The length of the veins with demantoid is frst meters. Their pillar-like shape is assumed. Conclusion.Demantoids from nests in serpentinized massive or brecciform dunites are more intact; demantoid nests in strongly serpentinized rocks were tectonic crushed and contain little gemstone.