MANTLE-CRUSTAL ORE-FORMING SYSTEMS IN THE UKRAINIAN AND BALTIC SHIELDS - KIROVOGRAD AND PECHENGA ORE DISTRICTS

The paper is based on a comparison between two unique Early Proterozoi c ore districts. The Kirovograd ore district is situated in the Ukrain ian Shield and combines a series of metasomatic uranium deposits in al bitites. The Pechenga ore district, located in the Baltic Shield, is w ell known for sulfide copper-nickel deposits. The paper presents a new , broader interpretation of ore district geological boundaries and dea ls with the deep structure of both districts using data from superdeep continental drilling and regional seismic sounding. The Kirovograd or e district was formed between 2.1 and 1.8 Ga ago due to the protoactiv ization of the previously consolidated continental earth crust. Within the district, the granitization processes, the ductile deformations o f cooling granitoids, retrogressive cataclastic metamorphism, and uran iferous sodic metasomatism occurred at the abyssal level of the Earth' s crust and were followed by the intrusion of anorthosites and rapakiv i granites. These phenomena were associated with the development of a mantle diapir as indicated by an uplift of the Moho discontinuity, the spatial arrangement of deep faults, and the highly reducing nature of fluids. The metallogeny of the Pechenga ore district was dominated by the endogenous processes of the Karelian cycle. These processes proce eded for 600-700 million years, both within and on the surface of the Earth's crust, on the background of changing geotectonic regimes. The prerift stage was marked by the intrusion of PGE-and Ni-bearing layere d plutons. Later on, the crystalline basement was deeply eroded and ex perienced rifting, a prolonged (more than 300 Ma) period of subsidence , sedimentation, and basic-intermediate volcanism. This rift stage con cluded with the emplacement of gabbro-wehrlite sills and the formation of copper-nickel deposits. The partial remobilization of sulfide copp er-nickel ores took place at the collision stage. The postcollision st age was characterized by the intrusion of granitoids and the formation of uraniferous sodic metasomatites. In the course of metallogenic evo lution, the role of mantle chambers decreased and that of crustal cham bers increased. The established Moho relief and gravity field anomalie s coincide with the recently exposed structural elements of the Pechen ga ore district. These facts testify to the close relationship of vari ous ore-bearing structures with each other and with the upper mantle. Despite the different duration and trends of metallogenic evolution in the Kirovograd and Pechenga ore districts, the ore-forming processes were most intensive in both areas at a time boundary of 2.0-1.8 Ga. Th is is further proof of the influence of mantle processes on the endoge nous ore formation in the ancient continental crust.