Composition, structure and formation conditions of the Yarlykap complex of jasper of the West Magnitogorsk paleovolcanic belt (Southern Urals)

Introduction. This study examines the structural position and genesis of the Middle-Devonian Yarlykap jasper complex and associated manganese mineralization (Southern Urals).

Materials and methods. The studied sites are Gubaidullino and Mamilya potential manganese ore occurrences. They are located in the West Magnitogorsk paleovolcanic belt and are confined to the Middle Devonian sealing wax-red and grayish-yellowish jaspers and tuff sandstones of the Yarlykap formation. The Yarlykap formation is distributed as narrow extended bands and outliers stretching along the Irendyk mountain ridge, Southern Urals. The age of the Yarlykap formation is defined as the Eiffelian, which is proved by conodont finds.

Our complex study includes geochemical, geophysical (magnetic and electrical exploration) and structural (measurements of mesostructure elements).

Results. It was shown that the rock association at both sites of the Yarlykap formation underwent a single stage of deformation, while the jaspers experienced dislocations similar in type and intensity.

 Structurally, the group of Mamiliya ore occurrences is generally confined to a monocline complicated by folded-thrust mesostructures of the north-northeast strike and western vergence. It is assumed that the Yarlykap formation is limited from the east by the thrust of the western vergence.

The Gubaidullino ore occurrence is a synform complicated by a series of small folds.  Among them, there are both practically isoclinal structures and more open asymmetric folds of western vergence.

The structure of both sites can be clearly recognized according to the electrical survey data. At the Gubaidullino site, several submeridional elongated folded zones are obvious by the change of the pattern of apparent resistance. On the Mamiliya site, the isoanomals are stretched into a single submeridional zone.

Geochemical data indicates that the deficiency of light lanthanides and the Eu and Ce minima may serve as an indicator of deposits of metalliferous hydrotherms typical for volcanically active regions of the oceans.

 Discussion. Thus, a new model of formation of siliceous strata and associated manganese mineralization can be proposed. These sites represent areas of volcanic unloading of active areas of the ocean floor associated with hydrothermal vents. Most likely, the volcanoes were located to the east of the described ore occurrences, and now they are located under the allochthon composed of the Late Devonian tufopsamite strata. 

Differences in the structure of ore occurrences are probably related to differences in their position within the West-Irendyk thrust pack, which includes these fragments. Thus, the Gubaidullino site is confined to the frontal part of the thrust and the Mamiliya site is located in the rear part of this thrust, which results in its simpler structure.

Jasper formation occurred in a developed island arc environment with an intermittent chain of volcanic structures, and they were already deformed in the Late Paleozoic, during the Ural collision.

Financial support. The study was prepared with partial financial support of the RFBR, grant No. 19-55-26009.