To address these geological problems,they were tackled as part of a PhD project, with the objectives to:
To develop a robust geological model of the Coffee gold deposits that can be tested with predictive targeting exercises, the following objectives are identified:
- Develop a structural model for formation of the Coffee gold deposits. Geometric, textural and kinematic structural information from oriented drill core, outcrops and geophysical datasets will be integrated into a coherent structural model to establish structural targeting criteria.
- Detailed geological maps, sections and a 3D geological model of the metamorphic and igneous host rocks of the Coffee deposits will be integrated with assay information to establish lithological, structural, geochemical and mineralogical associations and to determine the structures and lithologies most favourable for ore formation.
- Determine the mineralogical and geochemical footprint of gold mineralization by creating a detailed alteration model that relates alteration mineralogy and geochemistry with host rock lithology. This will be achieved by the systematic collection of mineralogical information by chemical staining, SWIR/Terraspec, and XRD, and integration/interrogation of this data with bulk rock geochemical data.
- Establish the physicochemical properties of the mineralizing fluid and determine the implications for grade distribution, deposit characteristics and genesis. Fluid properties can be (reverse engineered) from the mineralogy of alteration and ore minerals. Isotopic fingerprinting of sulphide minerals (S, Pb) and gangue minerals (O, H, C) will establish fluid origin and rock-fluid interactions, which will further contribute to the understanding of deposit genesis.
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The geology and genesis of the Coffee gold deposit in west-central Yukon, Canada : implications for the structural, magmatic, and metallogenic evolution of the Dawson Range, and gold deposit models
The Coffee gold deposit, located in the Dawson Range, west-central Yukon, Canada, is an example of structurally-controlled, gold-only mineralization with a global resource of ~4 Moz gold. Establishing the controls on ore distribution enhances exploration potential, and provides implications for the tectonic and metallogenic evolution of the northern Cordillera, and global gold deposit models. The controls on ore distribution were identified by drill core logging, field mapping, petrography, and analytical techniques that included geochemistry, shortwave infrared spectroscopy, and geochronology. The results from these analyses allowed the geological and structural framework, and a hydrothermal model for ore formation to be established. Furthermore, a genetic model for hydrothermal events and exhumation within the Dawson Range was developed. Coffee is hosted in metamorphic rocks of the Yukon-Tanana terrane, and mid-Cretaceous plutonic rocks. Metamorphic rocks include psammitic to semi-pelitic schist and K-feldspar augen-bearing orthogneiss that were metamorphosed to lower amphibolite in the Permian. Plutonic rocks include biotite granite of the ca. 100 Ma Coffee Creek pluton, and coeval intermediate dykes. The east-trending dextral strike-slip Coffee Creek fault exerts an important control on mineralization, which is disseminated in the wall rock of high order faults and fractures, and ~1m wide breccia corridors. Mineralization comprises auriferous sulphides that are associated with an alteration assemblage of quartz-muscovite-illite-kaolinite-carbonate, which sulphidize host rock muscovite and biotite. Age determinations from hydrothermal white mica constrains mineralization to ca. 97 Ma, and the coexistence of alteration and ore minerals constrains the character of the hydrothermal fluid to ~250°C and pH ~5. Isotope data provides evidence for decoupled metal sources, and low temperature thermochronology constrains the depth of mineralization to ~5km. Coffee occurs in close spatial and temporal proximity to orogenic gold mineralization. This spatial association, the ore fluid characteristics, and the mid-Cretaceous tectonic regime argues for ore fluids sourced from metamorphic devolatilization of likely siliciclastic rocks at depth. Coffee is interpreted as an epizonal orogenic gold deposit, the result of a regionally significant gold mineralizing event. This interpretation has implications for the metallogeny and exploration of orogenic gold mineralization in the Dawson Range and northern Cordillera, as well as globally.