Mal'javr – the first gold prospect in the Archean conglomerates, the Kola region

Abstract

A new type of prospects of gold mineralization in the Kola region is considered in the paper. The Mal'javr prospect is located in the south-western flank of the Uragubsky greenstone belt in biotite gneiss – the metamorphosed sedimentary rocks with interlayering of polymictic conglomerate, gravelite, and sandstone. Gold-bearing sulfide mineralization was found in altered rocks, which form a series of lens-shaped bodies, concentrated along a shear zone of NNE strike. The bodies of altered rocks are zonal: the central zone makes 50–80 % of the lens volume, it consists of garnet and quartz, the intermediate zone is of garnet-biotite mineral composition, and in the outer zone, which is often reduced, the main minerals are hedenbergite, hornblende, and grunerite. The metasomatic alteration is connected with an increase of iron content, decrease of Al, Si, and alkaline metals Na and K, redistribution of calcium to the outer zone of metasomatic lenses; and magnesium is inert. If compared to the unaltered gneiss, the altered rocks are depleted in Rb, Cs, Sr, Ba, Zr, Nb; and content of Cu, Zn, Mn, As, Ag, Te, W, Bi (all these elements are known to associate with gold) increases. All zones of altered rocks are rich in sulfide mineralization. Pyrrhotite and arsenopyrite are the main sulfide minerals, minor minerals are chalcopyrite, pentlandite, magnetite, tochilinite, native gold, and late pyrite and marcasite. Rock alteration and formation of sulfide mineralization happened at a high temperature > 600 °С, and the late alteration processes at ~400 °C. The arsenopyrite-gold mineralization probably formed with As and Au mobilized from the host biotite gneiss during regional metamorphism or due to pegmatite vein intrusion.