Investigating pyrite genesis and relation to gold at the Lone Star Deposit, YT

Abstract

The Klondike region of the Yukon Territory, Canada, is famous for extensive placer deposits, recovering over 20 million oz since discovery, but lacks any major defined bedrock resources. As a result of a surge in exploration activity, recent drilling efforts have delineated several new bedrock targets, including the Lone Star deposit near Dawson City, Yukon. The Lone Star deposit is hosted by a suite of Late Permian plutonic, volcanic, and sedimentary units known as the Klondike Assemblage. This assemblage formed because of subduction-related arc-magmatism, followed by Late Permian-Early Triassic regional greenschist-amphibolite facies metamorphism during accretion onto Laurentia as part of the Yukon-Tanana Terrane. Gold deposition is thought to be mid-late Jurassic, mainly occurring within discordant quartz veins with common pyrite mineralization but overall low sulfidation (galena, sphalerite, chalcopyrite, etc. only trace). This study adds to the overall understanding of the Lone Star deposit by establishing relations between pyrite paragenesis and gold mineralization. This is done through detailed examination of a suite of samples, selected based on differences in pyrite occurrence. Observations by reflected light and SEM have delineated at least 3 major pyrite types from a textural perspective: syn-tectonic, post-tectonic and vein. Samples have been analyzed by LA-ICPMS using individual spot analyses on pyrite core and rim, and trace element mapping on a subset of samples. Of the trace element suite measured, we found consistently detectable levels of Co, Ni, Cu, As, Sb, Pb, Te and Bi. Spot analyses have revealed both discrete core-rim trace element concentrations and grains that are relatively homogeneous, providing evidence for pyrite growth events involving different trace element sources. Trace element maps reveal these relations in more detail with grains displaying internal chemical structure. Some elements (Co, Ni, As, in some cases Se) are present as a dissolved component, but others (Cu, Sb, Pb, Te, and Bi) are typically found as inclusions along with Au and Ag in small amounts. Cores tend to be enriched in Co, Ni, or As, with rims depleted relatively. Gold concentrations are below detection in pyrite grains and is present as sparse inclusions of an Au or Au-Te phase. The occurrence of Au as a discrete phase, and not a dissolved component, suggest that a disconnect exists between the processes that precipitated gold and those that formed pyrite in the Lone Star system. iv Keywords: orogenic gold, pyrite, trace elements, LA-ICPMS mapping