Native Metals in Impact Melt Rocks of the Boltysh Impact Structure

UDC 549.3 : 523.681.8

https://doi.org/10.15407/mineraljournal.37.04.058

E.P. Gurov, V.V.Permyakov

Institute of Geological Sciences of the NAS of Ukraine

55- b, О. Honchar Str., Kyiv, Ukraine, 01601

E-mail: yevgeniy.gurov@gmail.com

Native Metals in Impact Melt Rocks of the Boltysh Impact Structure

Language: Russian

Mineralogical journal 2015, 37 (4): 58-67

Abstract: The complex of accessory native metals and alloys was determined by the electron microprobe analyses of impact melt rocks of the Boltysh structure in the Ukrainian Shield. Our study established a variety of minerals represented by native metals, alloys, oxides, sulfides, phosphates, and silicates, formed during several stages of cooling and solidification of the thick impact melt sheet. Two main units of impact melt rocks can be distinguished in the impact melt sheet of the Boltysh structure. Hialine impact melt rocks with a glassy matrix and microliths of feldspars and orthopyroxene compose the lower unit of the melt sheet at the intervals 792—657 m in borehole No 11475 and 736—653 m in borehole No 50. Microcrystalline impact melt rocks with microliths of feldspars and completely chloritized pyroxene form the upper unit of the impact melt sheet at intervals from 657 m to its surface at 573 m in borehole No 11475 and from 653 to 594 m in borehole No 50. The accessory mineralization of zinc-copper alloy (natural brass) with zinc content 35.8—37.5 wt. %, copper-silver alloy with copper content 26.8 wt. %, and native gold occurs in impact melt rocks of the lower unit of the sheet that do not contain any traces of secondary alteration process. Isometric, oval, or drop-like shapes of these minerals and occurrence in glassy matrix of impact melt rocks without any traces of alteration evidence for their crystallization in a residual melt before its solidification. Formation of brass with zinc contents of about 35—40 wt. % takes place at temperatures of about 900 oC, and crystallization of native gold occurs at temperatures of about 1032 oC, while tempe ratures of solidification of glassy matrix is estimated of about 780 oC. Native metals: copper, platinum, cuproplatinum, silver and iron were discovered in hydrothermally altered impact melt rocks of the upper unit of the sheet that was undergo to chloritization. The shapes of native metals in form of microveins and close association with chlorite evidence for their crystallization after the complete solidification of impact melt by the hydrothermal alteration. Native copper, platinum, cuproplatinum, silver, and iron occur within the upper unit of the hydrothermally altered impact melt rocks of the sheet in the form of veins and vein-like aggregates in the matrix and at the margins of chlorite grains. Such shapes of these native metals evidence for their origin after the complete solidification of impact melt during hydrothermal alteration. Temperatures of this process are estimated in the range of 300—350 oC. Mineralization of native platinum, cuproplatinum, copper, silver and iron in impact melt rocks of siliceous composition with SiO2 content of 68 wt. % seems to be uncommon and is probably caused by admixture of a material of the asteroid that formed the Boltysh impact structure. Geochemical investigations of impact melt rocks of the Boltysh structure showed their low enrichment in some platinum group elements, nickel, chromium and some others.

Keywords: impact structure, impact melt rock, native copper, native platinum, cuprous silver, chloritization.

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