UBC Theses and Dissertations
Late Jurassic fault-hosted gold mineralization of the Golden Saddle deposit, White Gold district, Yukon Territory Bailey, Leif Anthony
The Golden Saddle deposit is a lithologically and structurally controlled gold deposit located in west-central Yukon Territory. The deposit is located near the Klondike district, which is best known for its rich history of placer gold mining. The discovery of the Golden Saddle deposit in 2008 was the first significant lode gold discovery in this region. Based upon the geology, structure, and age of the deposit, an amagmatic orogenic model is proposed to describe the genesis of the deposit. The deposit area is underlain by Late Paleozoic amphibolite-grade metasiliciclastic, metavolcanic, metavolcaniclastic and metaplutonic rocks of the Yukon-Tanana terrane. Mineralization occurs at the intersection between a Jurassic east-striking sinistral transpressional fault system and an older north-striking thrust fault. Metavolcanic and metaplutonic rocks occur in the hanging wall of this thrust fault, whereas metasiliciclastic rocks occur in the footwall. Economic gold mineralization is hosted predominantly by a series of subparallel northeast-striking, northwest-dipping faults within the sinistral transpressional fault system. The gold-hosting faults are complex structures comprising early folds and shears that are crosscut by polyphase quartz-carbonate-sulphide veins, fracture zones, and breccias. Gold and related alteration minerals typically occur in brittle, dilational deformation zones. Some gold-bearing brittle deformation zones are oriented parallel to the fold-axial plane of ductile folds. This textural relationship is indicative of gold mineralization occurring near the brittle-ductile transition. Gold mineralization is accompanied by a progression of alteration mineral assemblages, vein mineral assemblages, and vein textures. Pre-gold alteration and veins vary by host rock composition, whereas syn-gold alteration and veins are characterized by quartz-carbonate-illite and gold-bearing pyrite. Mineralization is accompanied by enrichment of Au-Ag-Pb-S-Te, with enrichment of other elements dependent upon host rock composition, and variably including As, Ba, Bi, Cu, Hg, Mo, Sb, Se, and Zn. Values of δ³⁴S for hydrothermal pyrite range from –9.8 to 6.8‰, whereas ratios of ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/ ²⁰⁶Pb range from 2.043-1.933 and 0.828-0.757, respectively. These sulphur and lead isotopic ratios are interpreted to vary in part based upon host rock composition. The age of mineralization is constrained by ¹⁸⁷Re/¹⁸⁷Os model ages of 163–155 Ma for molybdenite in gold-bearing veins within the deposit.
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