FINELY DISPERSED ORE MINERALS IN PREMINERAL METASOMATIC ROCKS AT THE DUKAT ORE FIELD, NORTHEASTERN RUSSIA

Abstract

The composition and structure of aggregates of postmagmatic minerals from zones of premineral hydrothermal alterations at the Dukat ore field suggest that the main ore elements of the deposit (Ag, Zn, Pb, and Cu) were concentrated in microminerals (MM), whose grains range from 1 to 100 μm in size, and colloid-sized (<0.1 μm) nanoparticles (NP). The quartz-clinozoisite-chlorite association of PM in the outer zone of the ore field typically contains C-Tl-Pb-bearing todorokite, ilmenite with a high pyrophanite content, pyrite, sphalerite, anatase, and eskolaite; and the quartz-chlorite-phengite zone contains galena, acanthite, Fe sulfates, jarosite, scorodite, and NP of native Ag and of a CuZn alloy. Metastable MM and NP, which are characterized by a high surface energy, are thought to be formed in relation to local heterogeneities at the growth front of the main PM. Their occurrence is regarded as an important indication of the metalliferous character of the early hydrothermal solutions. The low-contrasting geochemical fields of the ore-element concentrations generated by premineral postmagmatic processes over an area of approximately 15 km2 is a characteristic feature of a high-gradient ore-forming system capable of generating high-grade ores.