Early Neoproterozoic Metapicrite–Basalt Association of the Angara Region, Yenisei Ridge: Petrogeochemical Composition, Tectonic Settings, and Pb–Zn Mineralization

Abstract

In the Angara part of the Yenisei Ridge, a sequential section of the metapicritic–basalt sequence and overlying carbonate rocks of the Gorevskaya Formation is established. This section is considered as a single volcanic–sedimentary complex formed at the Late Mesoproterozoic–Early Neoproterozoic boundary in the riftogenic marginal–continental paleobasin in the southwestern Siberian Craton. It is proposed to introduce the metapicrite–basaltic sequence into the stratigraphic legend of the Shirokaya Group of the western Yenisei Ridge under the name “Nizhnerechinskaya Sequence”. In the studied section, metamorphosed picrobasalts and picrites predominate in the lower part of the Nizhnerechinskaya Sequence, while the middle and upper parts consist mainly of basalts containing Pb–Zn mineralization (Stepanovsky ore occurrence). At the contact with carbonate deposits of the Gorevskaya Formation, an alternation of volcanogenic rocks and terrigenous–carbonate sediments and metabasaltic sills are observed. The petrochemical and trace-element composition of metavolcanic rocks has much in common, which indicates the paragenetic relationship of rocks of this high-Mg rock association. All these rocks are enriched in Fe and Ti, while metabasalts and metagabbros show elevated alkalinity and P2O5 content. In terms of total alkalinity, metabasalt–picrobasalts deviate from the normal petrochemical series towards the subalkaline series, which correlates with their elevated Ti content. Basites enriched in alkalis, especially, in potassium and corresponding to trachybasalts are observed among them. The contents of incompatible trace elements clearly increase from picrites to basalts and reach maximum in trachybasalts, which show 1.5–3 times enrichment in high-field strength elements compared to the basalts. The paragenetic nature of the considered association is supported by the similar REE distribution patterns: (La/Yb)n = 6–7 at Eu/Eu* = 0.9–1. The Late Mesoproterozoic–Early Neoproterozoic rifting of the Earth’s crust in the Angara region led to the opening of the marginal–continental sea basin, where basalt-picrite volcanism occurred simultaneously with accumulation of terrigenous–carbonate sediments, including ore–bearing (Pb–Zn) rocks of the Gorevskaya Formation, which host the Gorevskoye Pb–Zn deposit. The basin under consideration is interpreted as a relict rift-related paleobasin on the western margin of the Siberian craton, while the volcanic–sedimentary Nizhnerechinskaya Sequence is considered as the lower potential stratigraphic level of the base-metal mineralization in the Gorevsky ore field.