The history of the formation of the stratigraphic base of the Precambrian rocks of the Gannivske iron ore deposit of Kryvyi Rih Basin and the reasons for its modernization.
Authors: EVTEKHOV Volodymyr , BEREZOVSKY Anatoliy 
Published: December 2023

The Gannivske deposit of ferruginous quartzites occupies the northern part of the Kryvyi Rih iron ore basin (Krivbass). This deposit is part of the Northern Ore District of Kryvbas and in structural terms is a continuation in the northern direction of the Central Ore District. There is no doubt about the belonging of the ferruginous rocks of the Gannivske deposit to the Proterozoic Sakxagan Formation of the stratigraphic scheme of Precambrian sediments of Kryvbas. However, so far, the comparison of separate stratigraphic horizons of the Northern and Central Ore District causes great difficulties. The Saksagan Formation of the Northern Ore District has a thickness of 350-600 m and is well divided into two sub-formations – lower and upper. The lower sub-formation consists of biotite-amphibolite and amphibole-biotite schists, which often include garnet. In the central part of this sub-formation there is a bed of magnetite-amphibolite quartzite, which includes interlayers of schists. It was found out that this magnetite-amphibolite quartzite bed within the Pervomayske deposit is divided into two iron-ore beds, which belong to the first and second iron-ore horizons. Thus, it would be logical to name the magnetite-amphibolite quartzite bed of the lower sub-formation of the Saksagan Formation of the Gannivske deposit as the first-second iron-ore horizon, the schists underlying it to correlate with the first schist horizon, and the schists overlying it to correlate with the third-fifth schist horizon. However, according to the stratigraphic scheme used at the Gannivske deposit, this magnetite-amphibolite quartzite bed is referred to the zero iron horizon, the schists underlying it – to the zero schist horizon, and the schists overlying them – to the first schist horizon. The upper iron-ore sub-formation of the Saksagan Formation of the Gannivske deposit is divided into two parts: lower and upper. The lower part of the upper sub-formation (the first ferruginous horizon) is the main productive stratum of the Gannivske deposit. It is composed of various magnetite quartzites, which can be easily compared with the fifth and sixth ferruginous horizons of the Central Ore District. However, at the Gannivske deposit this stratum belongs to the two-membered first ferruginous horizon. The lower part of this stratum is distinguished as an overlying pack, the upper part – as a hanging pack of the first ferruginous horizon. The upper part of the upper sub-formation consists of magnetite-silicate quartzites with almost no layer-by-layer concentrations of magnetite. Such composition of the rocks of the ferruginous horizon at the Pervomayske deposit is typical only for the rocks of the seventh schist and seventh ferruginous horizons. However, it is impossible to assign this part of the upper sub-formation of the Gannivske deposit to the seventh schist and seventh ferruginous horizons, as it contains four quartzite strata (from bottom to top) rather than two strata, as in the Pervomayske deposit: magnetite-amphibole-amphibole quartzite (they are referred to I strata), biotite-amphibole-magnetite quartzite (II strata), magnetite-amphibole-amphibole quartzite (III strata) and magnetite-amphibole-biotite quartzite (IV strata). In the northern part of the Gannivske deposit there are layers of ferruginous quartzites and schists, the analogues of which have not been reliably established in the Central Ore District. However, according to the stratigraphic scheme of the Gannivske deposit they are referred to the second schist, second ferruginous, third schist, third ferruginous and fourth schist horizons. For geologists of the Northern Ore District of Kryvbas the most actual, from all problematic issues of the Gannivske deposit, is the problem of identification of ferruginous and schists horizons of the Saksagan Formation, as well as their correlation with horizons of the Saksagan Formation of the Pervomayske deposit. The current ideas about their correlation are now more and more questioned. Therefore, the question of modernisation of the stratigraphic scheme of the Saksagan Formation of the Gannivske deposit on the basis of generalisation of all available data has now become urgent.