Manganese mineralization in andesites of Brestovačka Banja, Serbia: evidence of sea-floor exhalations in the Timok Magmatic Complex

Abstract

Andesites near Brestovacka Banja belong to the Late Cretaceous Timok Magmatic Complex (TMC), which hosts the world-class Bor metallogenic zone including numerous porphyry-copper and epithermal deposits. Two main volcanic phases are recognized in the TMC. The newly discovered Mn mineralization reported here is associated with the second volcanic phase of Turonian-Campanian age. Manganese mineralization containing 58 % MnO on average, occurs as black veins, lumps and nests filling cracks and cavities within an autoclastic andesite, which was deposited in a subaqueous environment. This rock also contains minor Fe mineralization, which is contemporaneous with the manganese mineralization. Manganese mineralization predominantly consists of Mn-Ca silicates (macfallite, pumpellyite-Mn, orientite, bustamite) and Mn oxides (pyrolusite, manganite). Micrometer-scale mineral intergrowths and locally preserved botryoidal and colloform textures are characteristic features of this uncommon mineral assemblage. The features could indicate that the mineralization was formed by deposition from a primary colloidal assemblage and is of sub-marine hydrothermal origin. Orientite is the only Mn mineral with grain size reaching several tenths of micrometers and showing prismatic crystal habit. Scarce to rare associated minerals are hollandite, crednerite, an unknown REE mineral, powellite, pyrite, barite and galena, in decreasing abundance. Trace element analyses of the Mn mineralization show different element contents and REE patterns compared to those of the volcanic host-rock. Manganese mineralization shows relatively high contents of Cu – 1784 ppm, Mo – 20 ppm and As – 268 ppm. These elements are commonly enriched in the Cu deposits of the Bor zone and their relatively high contents in the studied Mn crusts indicate sea-floor hydrothermal vents as a source of the metals.