Spherules with “titanium” shells in debris of carbonate composition among metagravelites of the black shale sequence of the Neoproterozoic Akshiryak Ridge (Middle Tien Shan)

Research subject. Various carbonate spherical formations having shells of different compositions and forming clusters in the composition of limestone fragments in metagravelites of the flyschoid black shale section of the Vendian hanging wall of the ore-bearing structure of the Kumtor deposit. Aim. To assess the entire diversity of the population of carbonate spherules with similar shells, from the point of view of establishing possible variants of the nature of their formation and the material composition of the spherule shells. Materials and methods. A series of sections of the Vendian flyschoid black shale sequence of the hanging wall of the ore-bearing structure of the Kumtor deposit with horizons of specific metagravelites containing carbonate fragments with clusters of spherules was studied. Samples of metagravelites from borehole cores were studied by optical methods, with diagnostics of mineral formations in thin sections, polished sections and polished sections. A scanning electron microscope (SEM) was used to work with one of the samples. Individual areas of the polished thin section were analyzed to obtain energy-dispersive X-ray spectra (EDS) and multi-level elemental EDS maps. Results. Metagravelites are observed as a marker horizon in the composition of the terrigenous rhythmite sequence – flyschoid interbedding of carbonaceous phyllites, psammitic phyllites, and polymictic sandstones with carbonate cement in the section of rocks of the hanging wall of the ore-bearing structure of the Kumtor gold deposit. In the composition of most carbonate fragments of metagravelites, various spherical formations with individual sizes, external shells and, often, internal structure were found. They form clusters of different density, the distribution of which does not go beyond the carbonate fragments in metagravelites. In the composition of the studied external shells of spherules, the presence of titanium in the mineral form of rutile has been established to date. Conclusions. The observed spherules belong to syn-, diagenetic formations of pelitomorphic carbonate sediments, destroyed and redeposited later as gravel-sized limestone fragments in terrigenous sediments of a later age, which were subsequently metamorphosed. Based on their appearance and size, such spherule clusters were initially classified as peloids in carbonate rocks. However, the presence of specific external shells in all spherules, the diversity of their shapes and sizes, and, in some cases, their internal structure suggest the presence of objects of biological nature among them. The formation of mineral “titanium” shells around spherules within carbonate fragments occurred either during regional metamorphism or is associated with diagenesis of carbonate sediments containing spherules of biogenic origin with elevated titanium contents. These mineral shells contributed to the preservation of spherule shapes during subsequent transformations of terrigenous rocks under greenschist metamorphism.