Alunite-containing mineral associations of the Nikopol manganese deposit’s ore stratum and their genetic significance

Abstract

The geological structure of the Nikopol manganese ore deposit and the ores’ composition have been studied for decades, but some interesting questions of the mineralogy of the region were not given enough attention. For instance, alunite KAl3 [SO4 ]2 (OH)6 was discovered within the ore-bearing stratum of the deposit, yet it is not mentioned in a recently published article summarizing information on this mineral in Ukraine. Meanwhile, analysis of mineral associations of alunite in manganese ores gives additional information about the geological history of the formation of this large deposit. The Nikopol deposit’s manganese ores are divided into three types by mineral composition. The northern part of the deposit, the most elevated above sea level, is composed mainly of manganese oxides and hydroxides. Further to the south is a zone of mixed oxide-carbonate ores. And furthest south lies a zone of carbonate ores. There is not a consensus on the nature of such mineral zoning among specialists. It seems that to solve this problem, it is necessary to clarify the role of sulfuric acid weathering, the typomorphic mineral of which is alunite. The research is based on field observations during geological documentation of ores of the deposit. Laboratory studies of samples were conducted: study under a binocular microscope, diagnostics of minerals using differential thermal analysis and X-ray diffraction. Also, ontogenetic analyses of individual minerals and aggregates were performed. As a result of the laboratory studies the following minerals associated with alunite were identified: aluminum compounds (halloysite, gibbsite), manganese compounds (сryptomelane, vernadite, todorokite) and iron compounds (jarosite, goethite). Morphological signs of age relationships between listed minerals were studied. It is shown in the article that during the Lower Miocene break in sedimentation sulfuric acid weathering took place, which affected the ore horizon and the overlying sandy-clay rocks of the Upper Oligocene age. The conclusion is made that at the beginning of this process the northern part of the deposit was composed of carbonate manganese compounds. Thus new information on the conditions of the Nikopol manganese ores formation was obtained. Understanding the processes of manganese ores deposition is necessary for prospecting this mineral resource.