Exotic rock block from the Koshava gypsum mine, Northwest Bulgaria: Petrography, geochemistry, mineralogy and melting phenomena

Abstract

The described exotic rock block (60×80×13–15 cm) was found at 290 m depth in a lower–middle Badenian gypsum layer in the Koshava mine, NW Bulgaria, near the Danube River. It is greyish-black, granular, with layered structure and layers composed of α-quartz rosettes covered with organic matter (kerogen-like type with high contents of Ge, Mo and B), wood relicts with chalcedony replacement, and porous lenses with compact accumulation of organic matter. The block is coated with quartz crust, up to 2 cm thick, with regmaglypt-like forms, also replaced by quartz. Aside from the surface, melting phenomena were also observed inside the quartz rosettes and especially in the wood relicts and porous lenses. The melted drops are actually crystallized chalcedony. The organic matter accumulations contain Si-organic zoned micrometre-sized spherules. Fe silicides were found in the organic matter of all parts of the block, in which hapkeite was determined by X-ray analysis. Other detected minerals include graphite, cristobalite, coesite, skeletal and framboidal pyrite, moassanite, magnetite, suessite, sphalerite and minerals formed in the gypsum lagoon (gypsum, celestine, barite, calcite, halite and clays). The geological position of the block in the gypsum without any other sediments, the extensive melting phenomena with melted spherules, crushed quartz, its enrichment in 18O isotope and the presence of coesite suggest that it is shock ejecta, in certain aspects resembling the large Muong Nong-type tektites, but its characteristics could be the basis for distinguishing it as a new tektite type. The fact that it was found in a gypsum layer of early–middle Badenian age points to its probable association with the Ries-Steinheim impact event, despite the long distance between them (~1100 km).