The trace element composition of sedimentary pyrite : factors effecting uptake and uses of the data for determining paleo-ocean conditions

The trace element content of pyrite has received much research and is important to the fields of environmental geochemistry, ore deposit genesis and paleo-ocean reconstructions. Four aspects of the geochemistry of pyrite are investigated in this thesis: 1) the incorporation of trace elements into pyrite in a modern setting, 2) the variability of trace elements in sedimentary pyrite in ancient marine settings, 3) the use of trace elements in pyrite to interpret palaeo-ocean chemical conditions, and 4) modification of trace elements in sedimentary pyrite associated with later metamorphic and hydrothermal processes. To facilitate these investigations, detailed LA-ICPMS analyses were performed on pyrite from 49 different shale and unconsolidated sediment formations. These include detailed studies of the Derwent and Huon Estuaries that serve as modern high and low metal environments of sedimentary pyrite formation; the Hamersley and Fortescue Basins sediments which provide low metamorphic grade Archean aged examples of sedimentary pyrite; the Kapai Slate which is a high metamorphic grade, hydrothermally overprinted example of Archean aged pyrite and a series of 44 other shale units that are used to create a database of average trace element content of sedimentary pyrite. The examination of trace element contents of sedimentary pyrite in modern settings is an important component of understanding how trace elements are incorporated into pyrite. Recent studies have provided data on the trace element content of pyrite from modern settings with current oceanic trace element contents. However, as the trace element content of the oceans varies throughout geologic time it is important to examine the incorporation of trace metals in pyrite at higher trace element concentrations than present. The Derwent Estuary provides an ideal site to examine the trace element uptake by pyrite in a high metal environment. Detailed mineralogical and geochemical analyses demonstrated the degree to which trace elements were available to be incorporated into pyrite, and LA-ICPMS analyses show the amount of each element incorporated into the pyrite. The data showed that not only is it important to have significant quantities of trace elements available to be incorporated but also, if the concentrations of certain trace elements available for incorporation are high, they can prevent other trace elements from being incorporated into the pyrite. The data have also shown that even though some trace elements can be incorporated into pyrite in contaminated estuaries, the amount of pyrite formed, and the amount of the trace elements within the pyrite, is not sufficient for pyrite to act as a major sink for the contaminants. To demonstrate how trace element contents in sedimentary pyrite have varied according to geologic time, depositional setting and pyrite texture, a data base of pyrite chemistry from 45 different shale units was produced using LA-ICPMS. Some trace elements decrease with Earth evolution due to a decrease in Earth degassing and mafic magmatism. However, some elements, such as Mo, increase as a result of enhanced oxygenation that promoted the breakdown of sulfides and contributed additional trace elements to the oceans. Depositional setting of the shales in which the pyrite forms, is also an important control on the trace element content of sedimentary pyrite, with high trace element supply-settings having the highest trace element content. Pyrite texture differs based on diagenetic history and content of Fe and S present at the time of pyrite formation. Generally coarser grained pyrite forms later in digenesis and finer grained pyrite forms earlier (though fine euhedral pyrite is an exception to this). While no systematic differences were noted across the entire dataset with pyrite texture, in individual samples the early formed pyrite had the highest trace element content. The dataset also provides geochemical criteria to characterize sedimentary pyrite and can be used to support inferences made by mineral texture alone. Changes in pyrite from individual basins compared to the averages provided by the dataset can also be used to explain changes in ocean chemistry syn-pyrite deposition, similarly to how traditional analyses of whole rock are compared to average shale chemistry. The variations in pyrite chemistry, as determined by LA-ICPMS, in the shales from the Hamersley and Fortescue basins, Western Australia, were investigated to provide an alternative method for paleo-ocean reconstructions. Pyrite is a sink for several trace elements in the marine environment and the amount of trace elements captured by sedimentary pyrite is essentially a record of the trace element content of the ocean and/or sediments at the time of deposition. Therefore, the content of trace elements in pyrite can be used to determine the chemical conditions of the ocean basin at the time of deposition of the sediments. In the Hamersley and Fortescue basins, trace elements form cycles that generally increase up-section and then decrease abruptly at the end of each cycle. These cycles are interpreted to be reflective of a series of pulses of atmospheric oxygen which result in the decomposition of continental sulfides and thus an increase of metal flux to the ocean. Periodic drops in trace element content reflect influxes of volcanically sourced S and/or Fe (as suggested by positive ˜ívÆ33S shifts) which causes elevated pyrite production and a drawdown of trace elements. Whole rock analyses show similar relationships and support the use of pyrite analyses to determine paleo-oceanic conditions. One of the main benefits of using LA-ICPMS to determine trace element content of sedimentary pyrite is that it is a selective, in situ technique, unlike traditional bulk sample or sequential extraction techniques. This allows the determination of sedimentary pyrite trace element content in areas that have undergone significant (greenschist facies) metamorphism and/or hydrothermal overprint. The Kapai Slate from the St Ives gold district is an ideal area to demonstrate this capacity. The sedimentary pyrite from the Kapai Slate fit all the geochemical criteria developed for sedimentary pyrite. These analyses were used to determine that the Kapai Slate was deposited in an anoxic to euxinic basin with relatively low iron content and biological productivity. The analyses of the pyrite from the Kapai Slate also showed that it is not likely to be a major source of gold for the St Ives gold district, though the contained pyrite is elevated in gold. Preliminary investigations suggest that ratios of trace elements in pyrite may be a potential tool in mineral exploration, but further research is required for pyrite chemistry to be used as a robust technique with confidence. By analyzing pyrite from a number of different settings across geologic time this study has greatly increased the understanding of the controls on the trace element contents in pyrite. The analyses of pyrite from the metal enriched Derwent Estuary have demonstrated the effects of high trace element abundance on trace element content in pyrite. The data base of trace element content of pyrite has shown how the trace elements available for incorporation have varied throughout earth's history and how that is reflected in the trace element content of pyrite. A more detailed study of how this element content of pyrite varies within individual basins was provided by the Hamersley and Fortescue basin study which also demonstrated how the pyrite data can be used to determine paleo-oceanic conditions. Finally the Kapai Slate analyses provide an example of how metamorphism and hydrothermal over print effect the trace element content of sedimentary pyrite and how, using LA-ICPMS, the sedimentary pyrite from these basins can still be used to determine paleo-oceanic conditions.