Two stage of gold-bearing sulfide ores of early Proterozoic gabbroids in the North Ladoga area

Abstract

The article presents a study based on Pb isotopes, which revealed that the formation of goldsulfide mineralization in the Early Proterozoic gabbros of the Northern Ladoga region is associated with different tectonic phases. This conclusion is based on the comparative analysis of isotopic data for feldspars and lead sulfides in two Early Proterozoic intrusions: Velimäki and Alattu-Päkylä. The investigated intrusions have similar geological settings at the junction of the Svecofennian accretionary complex and the Karelian craton. Both massifs are characterized by manifestations of noble metal sulfide mineralization associated with zones of local shear deformations. The formation of the studied ore massifs is attributed to the Svecofennian and Caledonian tectonic stages. Feldspars of Velimäki intrusion have parameters and a model age of Pb, significantly older than the U-Pb age of zircon (1.9 Ga), even though they correspond to the Svecofennian time of magmatic crystallization of gabbro and clinopyroxenites. Sulfide pyrite- pyrrhotite mineralization is associated with gold in these rocks with the Caledonian stage of fluid-thermal processing of the Early Proterozoic magmatic mineral paracenteses. Feldspars and sulfides of the Alattu-Päkylä intrusion have similar lead isotopic parameters and a Pb model age (~2 Ga), also somewhat older than the U-Pb age of zircon, but at the same time indicating the formation of rock-forming and ore sulfide associations with gold during the Svecofennian (Early Proterozoic) plutonic and tectonic-thermal events. The source of lead sulfides of the Velimäki massif is characterized by the parameters of the upper crust with high μ2 = 238U/204Pb, while the μ2 parameter of the minerals of the Alattu-Päkylä massif is slightly lower, which indicates the participation of the mantle-lower crustal reservoir material in the formation of magma.