Genetic diversity of manganese deposition in the service Email alerting

Terrestrial manganese deposits formed by hydrothermal, sedimentary and supergene processes. Ancient analogues of modern oceanic hydrothermal deposits formed in spreading centre and subduction-related settings and those deposited from terrestrial hot springs are discussed. Sedimentary Mn oxide deposits formed in shallow water at the margins of stratified oceans above the redoxcline during sea-level changes. Mn carbonate deposits probably formed by diagenesis through Mn oxyhydroxide reduction coupled with organic matter oxidation. Climatic variation, and basin water stratification, responsible for Mn concentration, were manifestations of atmospheric COz content prompted by tectonism. Supergene manganese enrichment in continental weathering profiles was mainly dictated by climate, topography and drainage systems. Manganese deposits of diverse genetic types occur in the terrestrial geological record (Roy 1981). These were produced by direct hydrothermal activity, sedimentary processes, and continental weathering. Although the processes may be interrelated, each involves distinct mechanisms that place the deposits into specific genetic types. Some of these processes are best understood in preset-day depositional sites. The ancient environments were determined by the intensity and style of tectonism, volcanism and hydrothermal activity, the composition of the atmosphere and the hydrosphere, and the development of the biosphere, which all varied with time. It is my intention to unfold the total panorama of the environmental evolution during different time envelopes that produced a variety of manganese deposits in the past. Only major deposits and those providing distinctive evidence of their environment of deposition will be considered here. Hydrothermal manganese deposits Hydrothermal concentration of manganese as generally small deposits is fairly common in the geological record particularly during the Phanerozoic con. These deposits are often stratabound, but may also occur as irregular bodies and epithermal veins in a large variety of host rocks. In many places, these ancient stratabound orebodies display characteristic tectonic, rockassociation, mineralogical and chemical signatures that permit their correlation to deposits that are now being generated hydrothermally in the marine realm at or near spreading centres, mid-plate seamounts, and subduction-related island-arc settings. Emission of hydrothermal solution in shallow continental basins (e.g. lakes) has also produced stratabound manganese deposits (Nicholson 1990). Vein-type hydrothermal deposits are hosted mainly in volcanics of wide-ranging compositions as well as in a variety of sedimentary rocks of different ages. A number of deposits from active plate margins such as those of the northern Apennine assemblage (Bonatti et al. 1976), Pindos and Othris zones (Robertson & Varnavas 1993), and the Olympic Peninsula deposit (Park 1946) demonstrate their derivation from mid-ocean ridge settings (Table 1) with the volcanic rocks showing only MORB-type chemistry. Similar settings for hydrothermal manganese deposits have been reported from the Austrian Alps, Ethiopia and Japan. Subduction-related stratabound and vein-type hydrothermal manganese deposits have been assigned different palaeoteetonic settings such as fore-arc terraces, small inter-arc basins, shallow marginal basins and trenches adjacent to continental plate margins, and back-arc basins. The deposits are mostly hosted in sedimentary rocks (radiolarian chert, volcanielastic rocks and hemipelagic rocks) overlying island-arc type basalts, andesite, dacite, and rhyolite. Different models have been expounded to explain the occurrences of these deposits in the given settings such as upward Mn-bearing fluid expulsion (at temperatures <100°C) by compaction and dewatering of subducting sediments in oceanic island-arcs (for discussions see Glasby 1988), and subduction of a mid-ocean ridge at a continental margin. However, determining the precise palaeotectonic setting of an ancient hydrothermal manganese deposit is a difficult task and a holistic assessment of all From Nicholson, K. , Hein, J. R. , Biihn, B. & Dasgupta, S. (eds), 1997, Manganese Mineralization: Geochemistry and Mineralogy of Terrestrial and Marine Deposits, Geological Society Special Publication No. 119, pp. 5-27.