Behavior of alkaline and ore elements during formation of epithermal silver-polymetal mineralization (Northeastern Russia)

A comparative analysis helped to elucidate the behavior of alkaline and ore elements in hydrothermally metasomatized rocks and endogenous aureoles of epithermal silver-polymetal deposits in northeastern Russia. Studies at thr ee deposits (Mechta, Tidit, Gol'tsovoe) revealed a stable typomorphic compo sition of endogenous aureoles and similar compositions of near-ore metasoma tic associations. The major ore- and aureole-forming elements are Ag, Pb, A s, Sb, Zn, Mn, B, and Hg. Such elements as Ag, Pb, As, and Sb form highly c ontrasting aureoles, whereas Zn, Mn, B, and Hg - lowly contrasting ones. We have established the following metasomatic associations: albite-chlorite-s ericite-quartz (Ab-Chl-Ser-Qz) in outer zones and hydromica-sericite-orthoc lase-quartz (Hm-Ser-Ort-Qz), hydromica-sericite-quartz (Hm-Ser-Qz), and hyd romica-quartz (Hm-Qz) in inner zones. The consanguinity of the ore and metasomatic associations is distinctly see n from their zonal distribution (in the direction of the flow of ore-formin g solutions): Ab-Chl-Ser-Hm (lowly contrasting Ag, Ph, As, and Zn aureoles) --> --> Hm-Ser-Ort-Qz (moderately contrasting Ag, Pb, As, Zn, and Mn aureo les) -, Hm-Ser-Qz (contrasting Ag, Pb, As, and Sb aureoles, with B) -,Hm-Qz (highly contrasting Ag, Pb, As, and Sb aureoles, with B and Hg). The chemi cal composition of the host rocks changes in the same sequence: The content s of volatiles, SiO2, FeOtot, K2O, MnO increase, and those of Na2O, CaO, Mg O, and Al2O3 decrease, with the intensity of the rock metasomatism growing. The concentrations of alkaline elements - K, Na, Li, Rb, and Ca - depend di rectly on the composition of metasomatic parageneses. In all inner zones, n ear-ore metasomatites have high contents of K, Rb, Cs, and, to a lesser ext ent, Li and low contents of Na. Deposition of Li and, later, Rb from ore-fo rming solutions occurs on the background of a decrease in K concentrations. The boundaries of zones of intense deposition of Cs are still more shifted up the ore zones. Alkalies show a zonal distribution from bottom to top of the ore zone: Na -, K -, Li -, --> Rb --> Cs. In searches for Ag-Pb mineralization, it is recommended to lean upon concen trations and zonal distribution of both ore and alkaline elements, particul arly on K/Rb and K/Na. These ratios are stable in the revealed metasomatic associations and regularly change up the ore zones. They are especially eff ective as geochemical criteria in estimating the depth of erosion of the zo nes and predicting the depth of the mineralization.