The process of formation of manganese carbonate deposits hosted in black shale series

Abstract

The two most important types of manganese deposits in China are sedimentary and supergene, which account for 71 and 15 percent of the total manganese reserves, respectively. According to the host-rock associations, sedimentary manganese deposits can be classified into (1) a mud-rock type, (2) a black shale series type, (3) a carbonate rock type, and (4) a chert-limestone-mudstone type. The black shale series-type manganese deposits account for 48 percent of the total manganese reserves and are large- or medium-sized deposits.The black shale series or dark-colored sedimentary association consists of dark-colored mudrock-chert-carbonate rocks rich in sulfides and organic matter (C organic > or =1%). The term, black shale series-type manganese deposits, is a general name for all manganese deposits hosted in black shale, black chert, black siliceous shale, dark gray limestone, etc.Black shale series-type manganese deposits are distributed in 13 stratigraphic horizons from middle Proteozoic to Early Triassic. They can be divided into three subtypes: (1) a black shale subtype, (2) a black chert-siliceous shale subtype, and (3) a dark gray limestone subtype. Subtype 1 consists mainly of primary Mn carbonate ores; subtypes 2 and 3 consist mainly of Mn-containing limestones. Weathering and leaching processes can change the Mn limestones into Mn oxide ores which form large and high-quality supergene deposits, for example, the Mugui and Dongxiangqiao deposits.Manganese carbonate ores hosted in the black shale series consist mainly of rhodochrosite, calcio-rhodochrosite, kutnohorite, manganocalcite, Mn-containing calcite, Mn-containing dolomite, etc. The ores are black, dark gray, light gray, yellowish, and pink in color and have banded, laminated, massive, oolitic, spotted, and brecciated structures.Commonly, changes in the ore-bearing sequences show, from bottom to top, (1) black shale series --> (2) Mn carbonate ore bed --> (3) light-colored rocks. This indicates that ore beds formed in the transition from reduced to slightly oxidized and from weakly alkaline to alkaline conditions. The evolution of the ore-bearing sequence indicates that manganese mineralization occurred during the early stage of regression.The important Mn carbonate deposits hosted in the black shale series resulted from three main processes during formation: concentration in seawater, precipitation from seawater, and enrichment in the sediments. Anoxic conditions are a basic factor for manganese concentration in seawater; a weak oxic and alkaline environment is favorable for Mn carbonate precipitation, and microbiological activities play an important role for early diagenetic enrichment of manganese in the deposits.