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Placer gold microchemical characterization and shape analysis applied as an exploration tool in western Yukon Wrighton, Timothy Michael


Western Yukon is underlain by metamorphic, extrusive and intrusive rocks that are associated with a variety of orogenic, epithermal and porphyry-style mineral occurrences. This region is unglaciated, exhibits poor bedrock exposure and contains abundant and, in some cases, unusually rich residual placer gold deposits. The methodology employed here involves the analysis of gold alloy composition and gold inclusion mineralogy using SEM-based techniques (microchemical characterization), in addition to quantitative morphological studies that utilize image analysis. Integration of this information with data from conventional exploration techniques provide: (1) better characterization of the diversity of gold types derived from bedrock sources (multiple sources vs. variation within a single source); and (2) information on source location. Compositional analyses have been collected for 4,100 placer gold grains, of which 2,989 grains have been analyzed for both morphological and compositional data. Placer gold samples were collected from 33 localities from five major drainages: Henderson, Tenmile and Thistle creeks (White Gold District) and Mechanic, Revenue and Seymour creeks (Dawson Range). This study applies the shape evolution model of Crawford (2007) in several drainages in western Yukon to generate statistical predictions of source locations. Furthermore, through refinements made during this study, computational processes involved in this analysis are now automated through the development of new computer software. Correlation of the nature and extent of compositional variation within in situ gold from the White Deposit area with that of the associated placers, has allowed the confident prediction that similar mineralization is present in the Thistle Creek drainage. Regionally, most orogenic placer gold may be characterized as either As-bearing (e.g. Tenmile and Sixtymile creeks) or Te-bearing (e.g. Thistle Creek). Gold from Eureka Creek has been reclassified as orogenic as a consequence of characterizing the range of regional orogenic gold signatures. Mechanic and Revenue creeks gold exhibit a diagnostic Pb-Bi sulphide, bismuthinite (Bi₂S₃) and tetradymite assemblage, a mineral association observed within the Nucleus-Revenue Au-Cu porphyry deposit located in their headwaters. This gold type is observed surrounding additional Au-Cu porphyry systems (Casino and Sonora Gulch) that belong to a common metallogenic event in early Late Cretaceous time.

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