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Episodic fluid flow and mass transfer along pre-existing carbonate veins : implications for gold endowment in Carlin-type gold deposits Steiner, Andrew Paul

Abstract

Metal precipitation in several types of hydrothermal ore deposits is a product of fluid-rock interaction. The degree to which fluids can interact with rocks is, therefore, a fundamental control on the metal endowment of such deposits. This dissertation investigates hydrothermal flow paths and fluid-rock interactions along these flow paths in the Osiris cluster of Carlin-type Au deposits, Nadaleen trend, Yukon. The main findings of the dissertation are that steeply-plunging folds in the deposits were rotated to their modern steep geometry during progressive thrusting and flattening against the northern margin of the Selwyn basin, which acted as a rigid backstop. The vein networks that formed during this folding were later preferentially exploited by the ore fluids, which altered the pre-existing veins to Mn-rich, Sr-poor, Fe-poor, Mg-poor, UV-fluorescent (UVF) calcite veins. The chemistry and δ¹³C and δ¹⁸O composition of these UVF veins suggest that they formed during the progressive mobilization of carbonate cations down the flow path from carbonate dissolution zones. Most fluid flow occurred where the pre-existing vein network was closest spaced, such as in rigid carbonate units and steeply-plunging fold closures. This interpretation is supported by lithogeochemical relationships between Mn, Sr, Au, As, δ¹³C and δ¹⁸O that suggest the fluids generally exploited numerous flow paths smaller than the ~3 m lithogeochemical sample interval. Gold grades are higher in parts of the deposit where fluids could percolate through the rock mass rather than being confined to veins. Their ability to partition between fractures or veins and the wall-rock is controlled by the ratio of the fracture network’s bulk permeability to the rock layer’s permeability. Consequently, changes in the pre-existing vein network permeability are equally, if not more important, than changes in host rock permeability for promoting fluid-rock interaction. Variations in time-integrated fluid flux between flow paths also significantly influence fluid-rock interaction along the flow path. Laser ablation U-Pb carbonate geochronology of ore-stage carbonate veins indicates that the deposit likely formed from multiple hydrothermal events over several tens of millions of years and that magmatism was probably not a heat, fluid, or metal source. Supplementary materials available at: http://hdl.handle.net/2429/81829

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Attribution-NonCommercial-NoDerivatives 4.0 International