Post-Ore Basite Dykes of the Vorontsovka Gold Deposit (Northern Urals, Russia)—Early Carboniferous Impulse of Post-Collisional Magmatism

Two dyke complexes, pre-ore and post-ore, are identified in the large Vorontsovka gold deposit of finely disseminated gold (Carlin type), and age estimates are obtained for the latter complex. The deposit is located in the near-contact area of the Auerbakh polyphase granitoid pluton, in the North-Uralian segment of the Tagil zone. The ore field contains numerous pre-ore and post-ore mafic dykes from picrobasalts and gabbros to quartz diorites and monzonites. The enrichment in large–ion lithophile elements (Cs, Rb, Ba, Pb, Sr) compared to high-charge and rare-earth elements, as well as Ta–Nb-minimum and Pb-maximum in the spider plots indicate the intrusion of dykes in a suprasubduction setting in a sufficiently thick continental crust. The late dyke group mainly includes high-potassium mafic rocks somewhat enriched in phosphorus (K2O > 2 wt %, P2O5 up to ~1 wt %); they correspond to the shoshonite series of basic rocks and comply mainly with the riftogenic and intraplate regimes in the petrogenetic diagrams. The isotopic composition of lead in dyke rocks is characterized by a linear trend: the most radiogenic lead is characteristic of the granodiorites; lamprophyres have the least radiogenic lead. Lead isotopic compositions of igneous rocks and ores form a single trend on Pb-Pb diagrams, corresponding to a mixed mantle and crustal substance, these indicates the relationship of the gold mineralization with deep magmas. As a result of the studies of K–Ar and Sm–Nd isotopic systems, data on the Carboniferous age of post-ore basite dykes at 346 ± 4 Ma (40Ar/39Ar method) and coincidental estimate, within error, of 340 ± 35 Ma (Sm–Nd method) were obtained for the first time. Rather high positive εNd values (from 4.4 to 5.3) indicate the mantle source of their magmatic melts. The petrogeochemical features of the rocks of the late dyke series, as well as their Early Carboniferous age (346–340 Ma) suggest a post-collisional rift-related regime of their intrusion.