Microstructures and Trace Element Signatures of Orogenic Quartz Veins in the Klondike District, Yukon Territory, Canada. Wolff, W. R. Gareth
The rich placer gold deposits of the Klondike District in the Yukon Territory are derived from orogenic gold-bearing quartz veins, associated with the metamorphism of the Klondike Schist basement rock. Further understanding of the structural context of these veins may be essential for exploration in the region. Petrographic descriptions were made of 12 polished thin sections of Klondike vein samples, observing vein and host rock mineralogy and microtextures. Two broad textural categories were identified: blocky veins produced by a single fracturing event with dilation rate exceeding the rate of quartz growth into open space; and elongate-blocky to fibrous veins with the average rate of opening equal to the average rate of quartz growth. The structural interpretation of this variation is of an early stage of slow vein growth, producing fibrous quartz grains, as well as gold and sulphides. Later rapid fracturing led to the growth of blocky quartz. This variation in vein textures can be attributed to structural changes, and the progression through the brittle-ductile transition in the crust. Polished blocks of texturally complex samples from the Nugget and Sheba veins were analyzed by laser ablation ICP-MS, producing trace element concentrations for the different quartz textures. Aluminium was the most abundant trace element, and orogenic quartz was found to have low concentrations and variability of trace elements when compared with higher temperature magmatic-hydrothermal systems. No significant compositional variations were found, indicating that despite the broad textural differences, there were no significant changes in the physical and chemical conditions of quartz growth, or in the fluid in equilibrium with the host rock.
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