Ore Apatite-Bearing Mineralization of the Velimyaki Gabbroid Massif in the Raahe–Ladoga Zone of the Northern Ladoga Region: Identification of Formation Conditions and Estimation of Apatite Age

The Early Proterozoic gabbros of the Velimyaki intrusion of the Northern Ladoga region contain titanomagnetite ore, which has been mined as early as the end of the 19th century. Titanomagnetite horizons are enriched in phosphorus in form of apatite reaching 10 vol %. Isotopic Pb–Pb dating indicates that apatite was likely redeposited during superimposed metamorphism that was significantly separated in time from the magmatic stage of gabbros and clinopyroxene–titanomagnetite ores. Mineralogical, petrological, and isotope-geochemical criteria for the superimposed nature of the mineral formation with apatite recrystallization are the relationship of this mineral with the formation of other metamorphic minerals (hornblende, biotite, sodic plagioclase), the isotopic age of apatite (1790 ± 5 Ma), and the lower temperature (620–710°C) of its formation compared to the crystallization temperatures (900–1260°C) of magmatic minerals. The Pb–Pb age of apatite coincides with the age of metamorphic minerals from other rocks of Late Svecofennian region, as well as with the Rb–Sr ages of biotite and amphibole from host supracrustal rocks. Based on the data obtained, it was concluded that recrystallization of apatite and resetting of the U–Pb system occurred during the Late Svecofennian regional metamorphism.