University of Tasmania
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The geochemistry of metalliferous black shales : understanding primary enrichments, metamorphic processes, and the role of metal-rich black shales in archiving earth evolution

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posted on 2023-05-27, 11:14 authored by Johnson, SC
Black shales offer an important context to study the evolution of the oceans and atmosphere. As an archive of past climatic, evolutionary and geodynamic events in Earth history they offer the opportunity to further understand the nature of such events. Various redox sensitive and bio-essential trace metals can become enriched in black shales as a function of the nature of the environment in which they were deposited. Of particular interest are shales with high metal enrichment, as they may represent an important geochemical flux. In addition to archiving paleo-ocean and climatic conditions, metal-rich black shales offer important economic targets for hydrocarbon and mineral exploration. Previous work has shown that many important trace metals in black shales are hosted by diagenetic pyrite. Furthermore, metal-rich pyrite has been argued as a potential source for orogenic gold deposits where the metamorphic breakdown of pyrite to pyrrhotite releases gold and other trace metals. This thesis presents a series of studies on aspects of black shale enrichment and mineralogy and offers greater insight as to the evolution of our oceans and atmosphere and how this may relate to secular metal enrichments. Each adopts a multi-proxy, geochemical approach, augmenting pyrite trace element chemistry with isotopic, and whole-rock geochemical or mineralogical data. In addition to addressing the nature of the depositional environment of black shales, mechanisms of ore genesis via the metamorphism of metal-rich shale sequences are also presented and evaluated. Four main aspects of the geochemistry of black shales are investigated in this thesis: 1) primary metal enrichment as a result of the changing nature of the water column during climatic perturbation, 2) the application of pyrite trace element chemistry along with other geochemical techniques to understand, and differentiate, primary metal enrichments and metamorphic overprint, 3) mechanisms by which pyrite may convert to pyrrhotite in black shale and what implications this may have for metal mobility, and 4) understanding of metal enrichments in black shale through geological time in the context of geodynamic and geochemical evolution. The understanding of primary metal enrichments in black shales is critical in order to interpret the nature of the depositional environment. Chapters 2, 3 and 4 offer reviews of metal enrichment in sediments through time, as well as whole rock and pyrite trace element chemistry, as a means to record the chemical nature of the water column at the time of deposition. This is then applied in Chapter 5 to a metalliferous black shale section in NW Estonia of Cambrian to Ordovician age. This work contains a new detailed sedimentological study of the section to establish a depositional context. This is augmented by a high-resolution multi-proxy data and isotope geochemistry suggests that the section archives a major carbon isotope excursion (the Steptoean Positive Isotope Excursion; SPICE), not previously reported in these rocks, and allows the section to be placed in a global context. The high phosphorous content of the biograinstones deposited prior to the carbon isotope excursion suggests that phosphogenesis may have led to increased bio-productivity, affecting the carbon cycle. The high pyrite content and the geochemistry of this mineral in the lower portion of the black shale, which sits above the isotope excursion, suggests that the SPICE event may have led to the onset of low-oxygen conditions across the Baltica shallow shelf, and the deposition of metal-rich black shales. In Chapter 6, the geodynamic and biochemical processes associated with metal enrichments in the formation metal-rich shales are reviewed and discussed together with a geochemical and mineralogical study of the Talvivaara deposit in Finland. As the world's largest black-shale poly-metallic deposit it offers a context to evaluate the economic aspects of black shale enrichment. The Paleoproterozoic age of this deposit and the fact that there are no known analogues that match its size, suggests that there may have been unique environmental factors that played a role in its metal enrichment. Much like the Estonian example, Talvivaara was deposited in the immediate wake of a large, global carbon isotope excursion. The excursion (the Lomagundi- Jatuli event, ~2.1 Ga) is the largest carbon isotope excursion preserved in the rock record and is followed by the first, and largest, mass burial of organic carbon in Earth history. This suggests that there may be analogous processes operating, but on different scales, in order to facilitate such enrichments. The deposit has been metamorphosed to amphibolite facies with only some primary pyrite, zircon and bituminous organic matter preserved. In-situ ˜í¬•\\(^{34}\\)S-isotopic values of the sulfide phases and LA-ICP-MS analyses of iron sulfide phases, shows that the earliest pyrite (interpreted to be of syn-sedimentary to early-diagenetic in origin) is trace metal rich. The ˜í¬•\\(^{34}\\)S values in these pyrites suggest bacterial sulfate reduction from an open sulfate reservoir. The earliest pyrite is the most metal-rich, suggesting that the original depositional environment facilitated high metal concentrations. The shale package was then metamorphosed releasing trace elements and sulfur that then went on to form their own phases. Chapter 7 presents new data and insights as to the nature of metamorphic reduction of pyrite to pyrrhotite in black shales. Often this transformation is only considered as proceeding at greenschist facies metamorphism (~300¬¨‚àûC, and above). An experimental study, focusing on the magnetic character of pyrite, suggests that the transformation begins at ~177¬¨‚àûC. The lower temperature proposed is supported by LA-ICP-MS studies of pyrite and pyrrhotite in Estonian oil shale and diagenetic to catagenetic sulfide concretions in sub-greenschist facies black shales from NW Russia. A model is presented suggesting that the degradation of organic matter during catagenesis is sufficient in producing enough reducing potential to facilitate the initial stages of the transformation to pyrrhotite. Chapter 8 presents a synthesis of the geochemistry of metal-rich black shales. Some of the most metal-rich black shales worldwide are classified and compared to 'normal' black shales from throughout Earth history. In doing so, a new model is presented which suggests that during the Phanerozoic, plate collisional events led to increased detrital material into basins, leading to increased bio-productivity and atmospheric O\\(_2\\). With increasing O\\(_2\\) in the atmosphere, the processes of weathering and erosion became more vigorous leading to further influx of detritus and blooms of bio-productivity promoting the development of water column anoxia. Eventually, the cycle breaks when uptake by organisms exceeds input from the continents, leading to the return of 'normal' black shales and increasing carbon release to the atmosphere. This process is likely to have been the dominant process during the Phanerozoic, where supercontinent cycles are more dynamic and cover greater surface area. However, the Proterozoic also has some of the largest metalliferous, and organic-rich, sediments in the rock record and do not directly correlate to periods of supercontinent amalgamation. Instead, a discussion is presented that addresses the influence of widespread glaciations as a process to effectively erode, whilst simultaneously release oxygen, leading to increased oxidative weather and bio-productivity in their aftermath.


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