Search

Almost all economic manganese ores are ultimately or directly derived from hydrothermal vents associated with intermediate volcanic rocks. This source is in contrast to deep-sea nodules, which likely have a larger component of sediment-derived manganese and whose volcanic sources are more mafic. Manganese deposits can be divided into sedimentary rock-hosted, volcanic rock-hosted and karst-hosted, in order of predominance. Two genetic types of sedimentary rock-hosted deposits can also be identified: those with Mn derived via upwelling from oxygen-minimum zones and those formed on the margins of euxinic basins. Most of the large tonnage deposits appear to form by the euxinic mechanism. Manganese ores, like those of Fe, show a strong concentration of deposits in the Paleoproterozoic and a lesser occurrence in the Neoproterozoic, but, unlike Fe, there is an additional strong peak in the Oligocene. Therefore, Mn is not controlled entirely by the level of oxygen in the Earth’s atmosphere. At each peak of Mn deposition, the associated ore deposits are concentrated in a few districts, suggesting a more local than global control on manganese metallogenesis. Age trends can, however, be discerned in some chemical properties of manganese deposits. Overall, there is a trend to progressive increases in chemical diversity from the Archean to the Recent, with a particularly steep increase in the Neoproterozoic-Early Cambrian, corresponding in time to the radiation of metazoans. Also beginning in the Cambrian is the development of upwelling-linked deposits. There is another sharp increase in chemical diversity at the Jurassic-Cretaceous boundary, which includes increased SiO 2 /Al 2 O 3 ratios and corresponds to the radiation of diatoms. There is a conspicuous gap in sedimentary rock-hosted Mn deposits between 1800 and 800 Ma that may correspond to a monotonous, low-oxygen ocean, but one without sulfidic deep water. Alternatively, Mn may have been precipitated entirely in the deep ocean, beneath a sulfidic oxygen minimum layer. The positive Eu anomalies, which in iron formations are equated to vent-sourced metals, are not seen in most Mn deposits, although they are found in Mn-rich iron formations. By contrast, Fe deposits interbedded with major Mn ores lack the usual Eu signal. Therefore, mechanisms of transport between hydrothermal vents and the sites of deposition differed for Fe and for Mn deposits in the Archean-Paleoproterozoic. The dominant pattern in the time trend of Mn deposition is increasing chemical diversity, which reflects an increasing compartmentalization of the Earth’s depositional environments. This compartmentalization was a response to, but also provided a spur to, the diversification of life forms.

It is common to construct a subsidence model for a sedimentary basin when exploring the basin for oil and gas, but subsidence models rarely are used in the search for mineral deposits. To demonstrate the potential of this technique for mineral exploration, we examined geochronologic data from basins with exceptionally large mineral deposits hosted by sedimentary rocks. We chose as examples three basins: one thought to be dominated by foreland thrust loading, one by rifting, and one by strike-slip motion. As a foreland example, we chose the Witwatersrand basin of South Africa, host to the world's largest Au accumulation and regarded by many workers as a rift that subsequently evolved into a ret-roarc foreland basin. Application of a tectonic subsidence model, however, shows that the flexural subsidence expected in the later stages of evolution of a foreland basin did not occur; instead, the basin is dominated by the initial mechanical subsidence of the rift, followed after a long break by renewed rapid subsidence suggestive of a pull-apart basin. The end of this second subsidence episode saw very slow tectonic subsidence and the deposition of the Au-bearing conglomerates. As a rift basin example, we chose the Keweenaw basin, host to the large Cu deposit at White Pine, Michigan. The subsidence model shows two episodes of early rapid subsidence accompanied by great thicknesses of basalt. This volcanic phase was followed by sedimentary infilling of the basin that we equate to the thermal subsidence phase. The model output shows interruption of the thermal phase by two episodes of enhanced subsidence rate, each associated with epigenetic introduction of Cu. The first event was dominantly extensional, the second dominantly compressional. As a basin in a strike-slip terrane, we examined the Huayacocotla basin of eastern Mexico, host to the giant syngenetic Mn deposit at Molango. The model output shows highly episodic subsidence, consistent with deposition in a basin marginal to a major zone of strike-slip activity. Mineralization occurred when sea-level rise and basement subsidence combined to allow Mn-bearing, reducing bottom waters from the Gulf of Mexico to enter the basin...

Bedded barite deposits can be divided into two types based on stratigraphic succession, tectonic setting, and geochemistry. The first, herein termed the "continental margin type," has abundant chert in the host sediments, is deposited in small ocean basins lying between a subduction zone and a passive continental margin, and has a geochemical signature that suggests an appreciable influence of oceanic crust. The barite is deposited on or adjacent to the passive margin side of the basin. Subduction leads to consumption of the ocean floor of the basin, collision of the volcanic arc with the passive margin, and thrusting of the barite-bearing sedimentary sequence onto the stable platform. This group of deposits lacks economic accumulations of Pb and Zn. Examples are the Fancy Hill and Chamberlain Creek deposits in the Ouachita Mountains of Arkansas and the East Northumberland Canyon deposit in central Nevada. The second type, for which we propose the term "cratonic rift type," is hosted by coarse clastics of turbidity current origin and is deposited in intracratonic rifts with strong local relief; the host rocks have a chemistry suggesting entirely continental affinities. Volcanic rocks, where present, tend to be alkalic basalts. Most barite deposits of this type are accompanied by economic accumulations of Pb and Zn. Examples include the Meggen and Rammelsberg deposits in Germany and the Anvil and Howard's Pass deposits in the Selwyn basin of western Canada.The barite deposits of south China can be assigned tentatively to these two types. Those of the Qinling district were deposited in a chert-dominated section on oceanic crust in a closing, remnant-ocean basin, placing them in the continental margin type. Those of the Jiangnan region are chemically more like the cratonic rift-type deposits and seem to reflect a similar tectonic setting. If this assignment is correct, the Jiangnan area should be considered prospective for Pb and Zn mineralization near the barite deposits.

Brice Cairncross and Nicolas J. Beukes. 2013. Random House Struik. Capetown. 394 pages. ISBN 978 1 92057 288 4. US $85. (To order: http://www.uj.ac.za/EN/Faculties/science/newsandevents/News/Documents/Kalahari%20Manganese%20Field%20Book%20Brochure.pdf). This is a stunningly beautiful book, one that is also packed with valuable information about manganese metallogenesis, and modestly priced. It’s a great value. The volume is, in one sense, a second edition of The Manganese Adventure , which came out in 1997, but the focus is entirely on the Kalahari deposits, whereas the earlier version had an in-depth treatment of the Postmasburg mineralization as well. Most of the mineral photographs are new images of new specimens, many collected since 1997, and the drawings are all updated. Therefore, the book is better regarded as a second volume in a series, rather than as a replacement for the earlier one. Serious mineral collectors will want to have both. …

Oolitic iron ores are made up of silicate, oxide, and carbonate minerals that are part of solid solution series. The compositions of the minerals can depart from pure end members and show variation among deposits. This variability can be quantified using electron microprobe techniques. Iron silicates are represented by chamositic clays, both berthierine and chamosite. Glauconite was not seen in any of these samples. Berthierine predominates in younger deposits, whereas the Paleozoic ores studied contained only chamosite. Compositions of the chamositic clays, however, were independent of age. Iron oxides are mostly goethite in younger deposits or hematite in older deposits. Both minerals lack appreciable Al in solid solution. Most analyses show small amounts of Al but usually balanced by Si in the same proportion as in chamosite, indicating a slight admixture of clay. Iron carbonates are siderite with appreciable substitution of Ca and Mg but not of Mn. Apatite is common, but no iron phosphates were found. The compositions of the silicates and carbonates are close to those reported for early diagenetic minerals from other types of sedimentary rocks, indicating that ironstone formation does not require unusual chemical conditions other than a large supply of iron.The analytical results can be used to argue against several hypotheses of ironstone genesis. The lack of Al in the goethitic and hematitic ooliths is inconsistent with an origin either by redeposition of soil ooliths or by oxidation and leaching of earlier chamositic ooliths. The presence of high Al contents in the chamositic ooliths shows that they cannot have formed from earlier aragonitic ooliths, and the presence of small amounts of chamositic clay and apatite as discrete laminae in goethitic and hematitic ooliths suggests that this type of oolith could also not have formed by replacement of aragonite.Times of ironstone deposition show a correlation with climate and sea-level stands that suggests a mechanism for generating locally high iron concentrations in seawater. A combination of a warm, humid climate and low average continental relief would intensify lateritic weathering and the formation of iron-enriched detritus. Coincidence of this increased detrital iron supply with intensification and shallowing of the oxygen minimum layer in the ocean could lead to mobilization of iron from offshore shelf clastics, to be redeposited near the shoreline in intertidal to shallow subtidal environments.