Geological setting and mineral composition of Famennian manganese ores in the Lemva zone of Pai-Khoi: Evidence from ore occurrences of the Nizhny Silov group

Abstract

The results of geological-mineralogical study of stratificated manganese ores in Famennian rocks of the Lemva facies in Pai-Khoi are presented. Carbonate manganese ores make up conformable stratified and lenticular bodies (up to 0.6 m thick) in the interval between the Gromashor and Silovayakha formations that are composed of jasperoids and carbonate-siliceous rocks. Ores are characterized by fine wavy bedding and development of transverse quartz veinlets. The ores are mainly composed of kutnahorite. Secondary minerals are represented by dolomite, calcite, pyrite, ransayite, and cryptomelane (?). Mn-muscovite, micro-cline, pyrophanite, galena, barite, apatite, and monazite are accessory minerals. The average MnO content is 23.81 wt %. The interval also includes a long (up to 40 m) lens of rhodonite rocks (Silovayakha occurrence) that replace carbonate ores along the strike. The major minerals in these rocks are represented by rhodochrosite, rhodonite, pyroxmangite, and quartz. Secondary minerals are observed as tephroite, alleghanyite, friedelite, caryopilite, neotocite, sussexite, pyrite, and supergene manganese oxides. Spessartine, albandine, barite, and apatite are accessory minerals. Based on the analysis of factual material, we suggest that ore material was derived from hydrothermal paleoceanic systems associated with Devonian volcanism. Ore concentration in the sedimentation zone was related to the stagnant reduced setting of bottom water. Ore deposition was promoted by the delivery of fresh portions of ocean water. Ore matter was accumulated mainly by chemogenic mechanism and partly with the’ participation of bacteria. It is suggested that boundary between the Gromashor and Silovayakha formations should be corrected to unite rocks of the Famennian manganiferous association into a single formation.