The Early Proterozoic Primorskii complex of rapakivi granites (western Cisbaikalia): geochemistry, crystallization conditions, and ore potential

The Primorskii complex in western Cisbaikalia, which formed in the Early Proterozoic at the postcollisional stage in the Siberian craton evolution, comprises rapakivi granites, equigranular biotitic and leucocratic granites, and alaskites. It is a K-rich granitoid assemblage with a medium and high alkalinity, whose F, Ba, Pb, REE, Zr, Th, and Zn contents exceed the clarkes. The complex consists of three plutons: Bugul’deika–Anga, Ulan-Khan, and Trekhgolovyi, which formed in two intrusive stages. The evolution of the main-stage composition was marked by an increase in silica content, with a similtaneous increase in agpaite and Fe contents and a decrease in Na2O/K2O. The Bugul’deika–Anga and Trekhgolovyi plutons are the most contrasting in composition and crystallization conditions. The former originated from a weakly differentiated water-undersaturated melt, which crystallized at medium depths (Ptot = 3–4 kbar). The crystallization was unaccompanied by considerable accumulation of granitophile elements (the concentration index (CI) of granitophile elements is ~3) in the leucogranites, except the alaskites, which crystallized in the upper part of the magma chamber (CI = 5). The Trekhgolovyi pluton originated from a leucogranitic melt enriched in Cs, Li, Rb, and Sn, which crystallized at a low Ptot (~2 kbar). The average contents of some elements in the leucogranites are higher than their clarkes in Ca-poor granites: by a factor of 4 for Sn, 3.8 for Th, 2.7 for Rb, 2.5 for Cs, and 2 for F (CI ≈ 9). The final-stage granites in the Trekhgolovyi pluton are associated with quartz-muscovitic (±topaz, fluorite) greisens, which contain cassiterite, columbite, ilmenorutile, wolframite, bismuthinite, and other minerals. The data suggest that the Trekhgolovyi pluton has a Sn potential.