Magnesite microbialites of the Kunduzak occurrence in the weathering crust of the Khalilovsky ultrabasite massif (Southern Urals)

Research subject. Previously unknown microbialitic magnesites of the Kunduzak occurrence of the Khalilovsky ultrabasite massif of the Southern Urals. Aim. On the example of siliceous, phosphate and carbonate biolites, to consider the problem of microbial rock formation and to analyze the role of cyanobacterial biota in this process. Despite successful laboratory experiments on mineralization of cyanobacterial mats, observations in nature show that the productivity of cyanobacteria is not determined by the concentrations of chemical elements in the habitat. Under the same conditions one and the same population of cyanobacteria behaves neutrally or actively accumulates mineral matter. Moreover, the accumulated element is not always dominant in their habitat. Thus, there is every reason to assert that the problem of cyanobacterial rock and ore formation has not been fully solved yet, and additional studies are required for its final closure. Materials and methods. During the geological mapping of the Kunduzak occurrence, samples of magnesites were taken, which were studied by ISP, scanning electron microscopy, as well as by phase, structural and fluorescence X-ray analysis, in addition to studies in transparent sections. The work was carried out in the laboratories of RUDN, IMGRE, PIN RAS, and RGGRU. Results. A rich complex of mineralized bacterial forms, which are a fundamental feature of the structure of magnesites, was revealed. These are fragments of cyanobacterial mats and biofilms, cocciform and bacilliform bacteria, remains of glycocalyx, covers of filamentous microfossils, as well as microbiota of unclear systematic position. The development of biomorphosis in magnesites allows us to draw a parallel with the processes of modern magnesite accumulation associated with the activity of the microbial community of cyanobacterial mats. The latter circumstance becomes an irrefutable argument of biogenic (biolitic) nature of magnesite of the Kunduzak occurrence. Conclusions. In the sediments of the pre-Jurassic weathering crust of ultrabasites, microbialitic magnesites previously undescribed in the literature have been revealed. They belong to the “amorphous” industrial type of ores, and form a bed deposit, which cardinally differs from the neighboring deposits, where magnesite is concentrated in the form of stockwork bodies. The biolitic nature of magnesites is confirmed by a variety of bacterial structures, and an additional feature of these rocks, obviously, are the established anomalies of Dy, Yb and Lu. Microbialitic magnesites were formed in several stages. Their inferred resources are estimated at 130 thousand tons. © 2025 Elsevier B.V., All rights reserved.