Ferruginous and manganiferous haloes around massive sulphide deposits of the Urals

Proximal brecciform ferruginous and manganiferous rocks related to VMS deposits of the Urals are subdivided into jasperites, gossanites, and umbers, in addition to thin-bedded jaspers and cherts. The coherence of host rock composition and Mn–Fe-fertility of the sediments have been established. Fe-poor pink hematitic and gray sulphidic chert are typical of the felsic class of VMS deposits. In contrast the contents of Fe vary from high to moderate in ferruginous rocks enclosed in basaltic units associate with VMS deposits. Fe- and Mn-rich ferruginous rocks and umbers occur in association with limestones and calcareous sedimentary rocks in both types of volcanic sequences. A common feature of jasperites and umbers is the abundance of replacement textures of hyaloclastites and carbonates by hematite and silica. In addition, replacement of clastic sulphides by hematite and magnetite is a characteristic genetic feature of gossanites. All of these sedimentary rocks are accompanied by pseudomorphs of hematite and quartz formed after bacterial filaments. The abundance of replacement textures are supportive of the halmyrolysis model, in addition to hydrothermal sedimentary and sub-seafloor hydrothermal replacement theories. Study of chemical zonation of altered hyaloclasts shows depletion of their rims, not only in mobile Na, K, Mg, but also in immobile Al, Ti, and REE; whereas Si and Fe are concentrated in situ. The halmyrolysis model presented here, involving organic-rich calcareous hyaloclastic sediments, resolves the problem of subtraction of Al, Ti, REE and other elements, which are commonly immobile under hydrothermal conditions. The evolution of the halmyrolysis process from acidic reducing to alkaline oxidized conditions infers a possible range in transformation from Fe^II–Mg smectites to Fe-silicates and Fe-Si oxides as precursors of brecciform jasperite and thin-bedded jasper. The higher acidic, initial stage, of gossanite formation seems to be required for oxidation of organic matter and/or pyrite. The acidic condition facilitates the temporal preservation of “immobile” elements (Al, Ti, REE) in “immature”chlorite–hematite gossanites. Another peculiarity of the gossanite-forming processes is the likely sorption of P, U and V by iron hydroxides displacing sulphides. The general evolution of all ferruginous sediments results in complete Fe²⁺ oxidation and silicification accompanied by subtraction of other elements. The vertical diagenetic differentiation leads to concentration of Mn-carbonates, silicates and oxyhydroxides into the tops of jasperite and gossanite layers. Mn oxyhydroxides scavenge positively charged hydrated cations like Co and Ni. Near-vent bacterial communities may activate the processes of volcanic glass and sulphide degradation. The proposed processes of halmyrolysis followed by silicification, in situ, may resolve the enigma of silica-rich sediment formation in a silica undersaturated ocean. The discrimination between gossanite and jasperite is useful for elaboration of new criteria for local exploration of VMS- and Mn-deposits. Halo dispersion of gossanites covering an area about two to three times that of the massive sulphide deposit is a good vector for ore body discovery. Proximal gossanites can be differentiated from jasperites by presence of relic sulphide clasts or elevated contents of chalcophile elements (Cu, Fe, Zn, Pb, Bi, Te, As, Sb, Ba), noble metals (Au, Ag) and distinct REE patterns with La and Eu positive anomalies. The development of halmyrolysis and biomineralization models merit further elaboration and testing in on-going research, in order to add or revise theories of iron and manganese deposit formation.