Issues of structural control of demantoid mineralization at the Poldnevskoye deposit (Middle Urals)

Abstract

Relevance of the work is due to the need to establish structural control of the mineralization of jewelry demantoid in ultrabasites of the Korkodinsky massif for forecasting, prospecting and exploration of its deposits. Purpose of the work: identification the criteria for structural control at the Poldnevsky demantoid field. Research methodology: analysis of literature data on research topics, studying the geology of the field and the nature of the distribution of mineralization, the conditions of occurrence of mineralized zones, the features of fault tectonics in the quarry of the Poldnevskoye field, the study of thin sections and thin sections by optical methods Results. A geotectonic model for the formation of the Korkodinsky ultrabasite massif represented by a crust-mantle mixture from the depth of 50–60 km is proposed. In the process of rock hoisting, decompression, metamorphism and hydrothermal-metasomatic transformations, decompression fracturing were experienced. The earliest fractures were made by hydrothermal-metasomatic clinopyroxenite, which then also underwent decompression fracturing. Ultrabasic rocks and clinopyroxenites underwent local antigoritization. Later decompression fractures are made by antigorite, clinochrysotile, carbonate, sometimes magnetite and demantoid. All of them were formed under conditions of continuous decrease in allround pressure and synchronous opening of decompression fractures. The largest fractures are oriented subparallel to the wings of the enclosing fault and have a steep fall. The fractures are sinuous and can be arranged en echelon. Mineral-forming fluids were released from the massif as a result of decompression and mixed with meteoric waters penetrating along tensile cracks in the roof of the massif. Late low-temperature serpentinization of the massif is accompanied by the manifestation of “serpentinite tectonics”, which can destroy bunches with demantoid mineralization when superimposition. The demantoid mineralization in weakly serpentinized dunites is preserved from destruction by “serpentinite tectonics”. Conclusion. Since demantoid mineralization is confined to decompression fractures, the possibilities of structural control are very limited. The largest fractures are oriented subconcordantly to the sides of the fault enclosing the massif. This pattern of the distribution of demantoid mineralization in ultrabasites is favorable for the formation of deluvial-alluvial placers, but is unfavorable for development in bedrock depth.