Hydrothermal alteration of carbonaceous mudstones hosting the Eskay Creek Au deposit, British Columbia

The Jurassic Eskay Creek deposit in northwestern British Columbia represents an unusual volcanic-hosted massive sulfide deposit that is characterized by a high precious metal content, enrichment of the epithermal-style suite of elements, a complex sulfide and sulfosalt ore mineralogy, and a relatively low temperature of ore deposition. The stratiform ore lenses of the deposit are hosted by carbonaceous mudstones that only show cryptic alteration. An integrated approach consisting of mineralogical and geochemical analysis, multivariate statistical data reduction, mass transfer analysis, and equilibrium geochemical modeling was adopted to identify alteration vectors to ore that can be used to identify synvolcanic precious and base metal deposits hosted by fine-grained, carbonaceous mud-stones. Despite the fairly extensive previous research carried out on the Eskay Creek deposit, the nature of hydrothermal alteration of the carbonaceous mudstone host has not been previously investigated. The present thesis addresses this critical knowledge gap. The research provides new critical insights into the nature and evolution of the hydrothermal fluids involved in formation of the Eskay Creek deposit and the development of the alteration halo surrounding the deposit. The results indicate that the integration of field observations, detailed micro-analysis, multivariate data reduction and geochemical modeling is an effective, integrated and innovative approach for studying petrographically challenging geologic materials.;Textural evidence suggests that the carbonaceous mudstone is a complex rock type. The mineralogical composition of this fine-grained rock can be related to primary processes including deposition, diagenetic modification, hydrothermal alteration, and low-grade metamorphic recrystallization. Hydrothermal alteration patterns in the mudstones include a silicified core with peripheral chlorite and white mica formation and albite destruction, as well as extensive carbonate alteration. Ankerite represents the most common hydrothermal carbonate mineral and is frequently associated with kaolinite. Locally, potassium feldspar alteration of the mudstone is strongly developed. Major element mass transfer can be related to the changes in mudstone mineralogy. Additional vectors to ore include increases in the iron, magnesium, and manganese contents in carbonate minerals proximal to ore. Iron enrichment in chlorite occurs distally in the hanging-wall, while proximal chlorite is enriched in magnesium. Hydrothermally formed pyrite is arsenic enriched. Base metal enrichments in proximal samples are associated with sulfide minerals, while distal samples may contain anomalous base metal contents related to the presence of organic material.;Mineral stability constraints suggest that the observed alteration of the host mudstone must have occurred from slightly acidic to alkaline fluids that have been highly equilibrated with the host rocks. Modeling suggests that the base metal sulfides, precious metal-bearing phases, and hydrothermal clay and carbonate minerals likely do not represent co-precipitates, but must have formed at different physicochemical conditions during the evolution of the hydrothermal system. The primary controls on the distribution of mineral phases in the deposit are CO2 fugacity (which controls the acidity and ionic strength of solutions), temperature, and protolith wall-rock chemistry. Seawater must have contributed the magnesium to proximal carbonates and chlorite alteration, while the wall rock, in particular feldspars and detrital clays, represent the likely source for the calcium required for carbonate formation. The results of the present study demonstrate that low-temperature (<200°C) hydrothermal alteration, diagenesis, and a low-grade metamorphic overprint resulted in the formation of broadly comparable mineral assemblages. This finding has important implications to mineral exploration as minerals of the dolomite-ankerite solid solution represent the only rock-forming minerals directly indicative for a hydrothermal overprint of the carbonaceous mudstone that can be identified tens to hundreds of meters from ore.