Magmatogenic manganese ores of the South Minusa Intermontane Trough

Abstract

The first data on the mineral composition and formation conditions of manganese ore at the Chapsordag and Malosyrsky deposits in the Askiz ore district of Khakassia are integrated and systematized. The detailed mineralogical mapping of the deposits has been carried out. The identification of minerals and examination of the ore microstructure were performed with optical microscopy in transmitted and reflected light and using SEM/EDS, EMPA, XRD, IRS, and other methods. It was established that the ore mineralization is spatially and genetically related to the Early Devonian magmatism and accompanying hydrothermal activity and metasomatism. Syngenetic braunite was detected for the first time in elevated amounts reaching an economic level in the devitrified groundmass of volcanic rocks, in cement of lava breccia, and in fragments in pyroclastic rocks. By analogy with iron deposits, this magmatogenic type of manganese mineralization is regarded as ore lavas and tuffs combined with metasomatic and hydrothermal mineral assemblages into a strata-bound orebearing complex and as a source of hydrothermal metasomatic ore. The elevated Mn content in magmatic melts of the Early Devonian trachybasalt-trachyandesite-trachydacite association is caused by assimilation of Riphean and Lower Cambrian high-Mn carbonate sequences in crustal magma chambers. In contours of economic orebodies, the hydrothermal economic ore is recognized as sites of massive, patchy and impregnated, brecciated, stringer-disseminated, and disseminated varieties. High-grade massive ore occurs as stratiform and branching bodies up to 1.5 m thick and a few tens of meters long and as smaller pocketlike bodies. Braunite and pyrolusite (polianite) are major ore minerals varying in size, degree of crystallinity, and character of intergrowths with associating minerals. Gangue minerals include carbonates, sulfates, albite, quartz, chlorite, actinolite, piemontite, and okhotskite, a Mn-pumpellyite identified in Russia for the first time and studied in detail in this paper. The veined hydrothermal ore is classified as a calcite-barite-pyrolusite type. The crystallization temperature of hydrothermal metasomatic ore is estimated at 350–180°C; oxygen fugacity is above the hematite-magnetite buffer. The surface of high-grade ore is encrusted with supergene goethite-hydrogoethite, chalcedony-hematite, and pyrolusite-psilomelane crusts and veinlets (less than 1% of the bulk ore mass). The data obtained facilitate prospecting for high-quality manganese ore at walls of superimposed rifts in fold regions, including large economic manganese concentrations in the form of ore lavas and tuffs as products of solidification of metalliferous melt.