Pyrite in the Mesoarchean Witwatersrand Supergroup, South Africa
- Authors: Guy, Bradley Martin
- Date: 2012-08-20
- Subjects: Pyrites , Isotope geology , Paragenesis , Formations (Geology) , Petrology , Witwatersrand Supergroup (South Africa)
- Type: Thesis
- Identifier: uj:2779 , http://hdl.handle.net/10210/6219
- Description: Ph.D. , Petrographic, chemical and multiple sulfur isotope analyses were conducted on pyrite from argillaceous, arenaceous and rudaceous sedimentary rocks from the Mesoarchean Witwatersrand Supergroup. Following detailed petrographic analyses, four paragenetic associations of pyrite were identified. These include: 1) Detrital pyrite (derived from an existing rock via weathering and/or erosion). 2) Syngenetic pyrite (formed at the same time as the surrounding sediment). 3) Diagenetic pyrite (formed in the sediment before lithification and metamorphism). 4) Epigenetic pyrite (formed during metamorphism and hydrothermal alteration). It was found that the distribution of the pyrite varies with respect to the stratigraphic profile of the Witwatersrand Supergroup and depositional facies within the Witwatersrand depository. In this regard, the four paragenetic associations of pyrite are either scarce or absent in marine-dominated depositional environments, which occur in the lower parts of the succession and in geographically distal parts of the depository. Conversely, the four paragenetic associations are well represented in fluvial-dominated depositional environments, which occur in the middle and upper parts of the succession and in geographically proximal parts of the depository. However, it is worth noting that diagenetic pyrite in the West Rand Group occurs as in situ segregations in carbonaceous shale, whereas syngenetic and diagenetic pyrite in the Central Rand Group occurs as reworked and rounded fragments in fluvial quartz-pebble conglomerates. The strong association between fluvial depositional environments and sedimentary pyrite (syngenetic and diagenetic pyrite) infers a continental source of the sulfur (sulfide weathering or volcanic activity), whereas the lack of pyrite in marine depositional environments is consistent with the model of a sulfate-poor Archean ocean. The connection between epigenetic pyrite and the fluvial-dominated depofacies is probably related to the elevated concentrations of precursor sulfides (i.e., remobilization of syngenetic and early diagenetic pyrite) and the presence of organic carbon (conversion of metal-rich early diagenetic pyrite into pyrrhotite and base metal sulfides). In support of the petrographic observations above, it was found that the trace element chemistry of each paragenetic association of pyrite yields a distinctive set of chemical compositions and interelement variations (Co, Ni and As contents). Regarding detrital pyrite, two chemical populations can be distinguished according to grain size: 1) small grains (tens of μm’s) with high levels of metal substitution (up to wt. %) and interelement covariation and iv 2) large grains (>100 μm) with low levels of metal substitution (≤200 ppm). These two populations are thought to represent pyrite derived from sedimentary and metamorphosed source areas, respectively (see below). The trace element chemistry of diagenetic pyrite varies relative to the Fe-content of the host rock. Diagenetic pyrite from Fe-rich host rocks, such as magnetic mudstone and banded iron formation (BIF), generally contain low Ni contents (<500 ppm), moderate As contents (<1500 ppm) and relatively high Co contents (up to a few wt. %). Elevated concentrations of As probably reflect desorption of As from clays and Fe-oxyhydroxides during diagenetic phase transformations, whereas anomalous concentrations of Co are tentatively linked to the reductive dissolution of Mn-oxyhydroxides.
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The Paleo-environmental significance of the iron-formations and iron-rich mudstones of the Mesoarchean Witwatersrand-Mozaan Basin, South Africa
- Authors: Smith, Albertus Johannes Basson
- Date: 2009-04-28T07:17:48Z
- Subjects: Geology , Petrology , Mineralogy , Geochemistry , Iron ores , Formations (Geology)
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/370928 , uj:8312 , http://hdl.handle.net/10210/2440
- Description: M.Sc. , The Mesoarchean Witwatersrand and Pongola Supergroups of South Africa are the oldest, well preserved supracratonic successions worldwide. Various banded iron formation (BIF) and iron-rich mudstone units occur within the West Rand Group of the Witwatersrand Supergroup and the Mozaan Group of the Pongola Supergroup. A granular iron formation (GIF) occurs in a single unit in the Nconga Formation of the Mozaan Group. The Witwatersrand Supergroup and Mozaan Group have been lithostratigraphically correlated and are interpreted to have been part of the same sedimentary basin. The studied BIF units occur in two associations: shale-associated and diamictiteassociated BIF. The GIF seem to have been deposited in shallower environments with greater hydrodynamic activity. The iron-rich mudstone shows a similar stratigraphic setting to that of the shale-associated BIF. The lithostratigraphic setting of the Witwatersrand-Mozaan basin BIFs are similar to what is seen for Superior-type ironformations, with the mudstones and associated BIFs marking marine transgressions. Various mineralogical facies of BIF were identified, including oxide, carbonate and silicate facies BIF, as well as mixed facies between these end members. The GIF is a unique facies and shows abundant petrographic evidence for biological activity. The iron-rich mudstone has been subdivided into iron-silicate rich, magnetite-bearing, carbonate-bearing, magnetite-carbonate-bearing and garnet-bearing subtypes. BIF, GIF and iron-rich mudstone have been subjected to lower greenschist facies metamorphism with some occurences of localized contact metamorphism. The abundance of magnetite shows that oxidation played an important part in BIF deposition, whereas the occurrence of 12C-enriched iron-rich carbonates suggests post depositional reduction of the deposited oxidized iron-rich minerals by organic matter. Al-bearing minerals are rare in the BIFs xxi and abundant in the iron-rich mudstones. Apatite and rare earth element (REE)- phosphates occur throughout. The major element geochemistry shows an inverse proportionality for Fe and Si in all the studied samples. BIFs show slightly higher Fe- and lower Si- and Al-concentrations compared to iron-rich mudstones which show higher Si- and Al- and lower Feconcentrations. The studied BIFs show major element geochemical attributes intermediate to those of Superior- and Algoma-type iron-formations. Provenance studies on some of the iron-rich mudstones illustrate that they were sourced from a mixture of mafic and felsic sources. The rare earth element (REE) geochemistry suggests strong hydrothermal input into the units, and positive correlation with the Fe-concentrations suggests that the Fe was introduced by high temperature hydrothermal fluids. The majority of the REEs are hosted by apatite and the REE-phosphates monazite and xenotime. The REEs were reconcentrated into these phosphates during diagenesis. A comparison of the studied lithostratigraphically correlatable units between the Witwatersrand Supergroup and Mozaan Group makes it possible to construct a depositional model for basin-wide BIF deposition in the Witwatersrand-Mozaan basin. Shale-associated BIF was deposited during the peak of transgression when reduced Ferich hydrothermal bottom waters were introduced into shallow ocean water that was either oxygenated or filled with anoxygenic phototrophic bacteria. Diamictite-associated BIF, in contrast, was deposited during interglacial periods when the melting of glacial ice introduced sunlight, nutrients and oxygen to the reduced, hydrothermally influenced Ferich ocean water. GIF was probably deposited in shallow, above wave base waters cut off from clastic input, and then washed into deeper depositional environments. Iron-rich mudstone was deposited in a similar setting as the shale-associated BIF, but in environments that were not completely cut off from detrital influx. The study shows that it is impossible to construct a general depositional model for Precambrian BIFs, since the lithostratigraphic and depositional settings vary between different examples of BIF.
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