Sediment-Hosted Pb-Zn-Ba Mineralisation at Macmillan Pass, Yukon (Canada) – Hydrothermal Fluid Chemistry and Mineralising Processes
Creators: Magnall, Joseph M
Publisher: name: University of Alberta Library
Year: 2015

<p>Sediment-hosted massive sulphide (SHMS) deposits form huge enrichments of reduced sulphur in sedimentary basins, typically in the form of pyrite, galena and sphalerite. In the Selwyn Basin (Canada), this type of metallogenesis occurs as stratiform and stratabound (bedded) sulphides within carbonaceous mudstones (clastic-dominated; CD-type). Previous models have interpreted this relationship to be the direct result of a stagnant water column, enriched in reduced sulphur (euxinic), trapping metal rich hydrothermal fluids and forming sedimentary exhalative (SEDEX) style mineralisation. However, there are limited constraints on vent fluid geochemistry in CD-type systems and there are periodic debates concerning the environment of mineralisation (i.e. whether the ore forms in the water column vs. sub seafloor). <br>
In this thesis, work has focused on two Late Devonian SEDEX-style deposits at Macmillan Pass. The complete deposit architecture is preserved at these localities – samples of the vent complex, which represents the primary conduit of fluid up-flow, overlying bedded sulphides, and surrounding host rock provide the opportunity to evaluate how hydrothermal fluids evolved geochemically in these settings. Particular focus has been directed towards evaluating the extent to which carbonaceous mudstones impact metal solubility, and determining the relative importance of euxinic conditions, and diagenetic processes operating beneath the seafloor. This has been attempted using a variety of techniques, including: petrography and thermodynamic modeling; bulk rock geochemistry; isotopic microanalysis (δ34S values in pyrite and barite); analysis of sulphide mineral separates for δ34S values; in situ (LA-ICP-MS) rare earth element analysis in carbonates; strontium isotope analysis of carbonates; fluid inclusion microthermometry and gas analysis. <br>
Combined, the results presented in this thesis provide evidence of steep thermal and chemical gradients that developed spatially and temporally during the evolution of hydrothermal activity at Macmillan Pass. Importantly, this has led to the development of a new model to explain mineralisation at this locality, which has implications for the interpretation of similar systems in the Selwyn Basin as well as in other sedimentary basins from the geologic record. The radiogenic composition of strontium isotopes in carbonate from the vent complex is evidence that metals were sourced from deep within the basin – likely from clastics derived from the underlying crystalline basement. Importantly, the tectonic activity that promoted fluid up-flow from deep within the basin also led to enhanced circulation of fluids at shallower levels of the basin. Specifically, reducing, methane-bearing diagenetic fluids – highly effective at mobilising barium from organic rich lithologies – precipitated barite upon interaction with sulphate in a shallow diagenetic setting. In this diagenetic environment, both barite precipitation and sulphate reduction associated with methane oxidation served to enrich partially lithified sediments in sulphur. The metal rich hydrothermal fluid, which was hot (&gt;275°C), entered this environment and precipitated sulphides in a sub-seafloor setting, with barite dissolution providing an important source of sulphur for metal deposition. High biological productivity, host rock organic matter content, fluid mobility in partially lithified sediments, deep basin penetrating faults, hot fluids (metal transport) and concentration of sulphur in the host rock were the most important factors for the development of CD-type systems at Macmillan Pass. <br>
In a broader context, the lower concentrations of seawater sulphate, relative to modern day oceans, is an important factor to consider in terms of the mass balance of SHMS systems in the geologic record. This specifically relates to sulphate availability, pathways of sulphate reduction and the formation of an effective metal trap. The work in this thesis provides evidence that a combination of pathways were more effective than single step processes operating in a euxinic water column.</p>