Wall-rock metasomatism of carbonaceous terrigenous rocks in the Lena gold district

The Lena gold district is situated in the fold-and-shear belt of the southern framework of the Siberian Platform. The gold deposits are hosted in the Riphean-Vendian Khomolkho and Aunakit formations, revealing the strict control of ore mineralization by folding and shearing. The microstructure of metasomatically altered ore-bearing carbonaceous sedimentary rocks at the Sukhoi Log, Golets Vysochaishy, and Verninsky deposits (the latter includes the Pervenets vein zone) testifies to parallelism in the development of shearing, foliation, and ore-forming metasomatism. The local pressure gradients are marked by removal of silica from pressured zones into opened cleavage fractures and pockets. Two metasomatic stages are recognized: (1) early sodic metasomatism, which is characterized by the assemblage of magnesian siderite and paragonite, and (2) late potassic metasomatism, with formation of muscovite in association with sideroplesite and ankerite. The rocks altered at the early stage are distinguished by elevated Ni, Cr, and probably PGE contents. The second stage, close in age to the emplacement of Hercynian granitic plutons, was accompanied by the gain of chalcophile metals and deposition of the bulk of gold. In mineral composition, the metasomatic rocks are close to beresites, but the alteration differed in somewhat elevated alkalinity, so that microveinlets of albite and potassium feldspar occur in the ore zone together with muscovite. The ratio of modal muscovite to paragonite contents in orebodies is substantially higher than in the surrounding metasomatized rocks. This ratio directly depends on the degree of rock permeability and the intensity of the flow of ore-forming solutions. Carbonaceous matter (CM) in the ore zone underwent reworking and redeposition. CM is graphitized to a lesser extent than in the rocks affected by regional metamorphism. The spatial distribution of CM containing nitro and amino groups indicates more oxidizing conditions in the zone of ore deposition than at a distance from this zone. The temperature of metasomatic processes estimated from the muscovite, muscovite-paragonite, and chlorite mineral thermometers and fluid inclusions in quartz was 300–350°C at a pressure of about 1 kbar. The S, O, and C isotopic compositions of ore-forming fluids that pertain to the second stage of metasomatism (δ³⁴S= +8.5‰, δ¹⁸O = +10‰, and δ¹³C= ?11 to ?18‰) indicate their crustal origin. The generally similar conditions and products of the ore-forming metasomatic process at the giant Sukhoi Log deposit and at the small Golets Vysochaishy deposit are combined with some differences. The formation of the described deposits was related to the deep convection of fluids along shear zones followed by more local flows of postmagmatic solutions derived from the emplaced granitic magma.