Textural analysis and chemical mapping of Paleoarchean sedimentary pyrite to unravel its potential biogenicity, oxidation and transport history
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Unlike in an oxygen-rich setting, pyrite is a persistent mineral in anoxic environments and is therefore capable of recording environmental changes that occurred during its crystal growth phases. Pyrites from the Barberton Greenstone Belt (BGB) in the Kaapval craton in South Africa have an age of 3.55-3.22 Ga, and can therefore give a unique insight in the environmental changes that happened in Paleoarchean era. In this thesis study, the chemistries of pyrite crystals from the BGB, which were sampled from the Mapepe formation at depths of 309.58 meters and 440.30 meters, were studied with the Electron Microprobe Analyser (EMPA). Furthermore, the Raman spectrometer was used to (1) test the biogenicity of the framboidal pyrite cores and (2) to determine the maximum metamorphic temperature, using the carbonaceous material (CM) as a geothermometer. The EMPA provided chemical maps of the pyrites that showed that one grain could contain up to 8 different pyrite generations. The pyrite overgrowths were identified by changes in enrichments in Co, Ni, As or Cu. The pyrites in sample 309.58m contained detrital pyrite cores enriched in Cu, whereas the grains in sample 440.30m were nearly completely depleted from Cu and occasionally contained an As enriched core of detrital origin. The detrital cores from both drill core depths contained overgrowths enriched in both Co and As, and in both, As and Ni. Enrichments in Co + As, Co or Ni are attributed to either hydrothermal (Co + As) or metamorphic fluid (Ni) interactions and the As + Ni enriched pyrites and the depleted pyrites are associated with diagenetic crystal growth. Cobalt, Nickel and Copper are proposed to be sourced from chemically weathered mafic rocks that were subsequently transported and incorporated within the pyrites. The As enrichment is associated with marine sediments from As-rich ancient oceans. Many pyrite overgrowths were attributed to fluid-crystal interactions, implying that hydrothermal and metamorphic fluids played a big role in the BGB and its alteration history. Results from the Raman spectrometer established that the detrital framboidal cores have an abiotic origin, thus do not consist of pyritized bacterial colonies. Furthermore, the determination of the maximum metamorphic temperature included a comparison of multiple geothermometer models in order to obtain the most accurate results for these pyrites. A peak temperature of approximately 340 ˚C occurred during metamorphism.