Geology of the Spearhead Mountain Stock, Sheldon Lake Map sheet, Yukon Territory

A two square mile stock of biotite quartz monzonite forms the core of Spearhead Mountain. The stock has intruded Ordovician and Silurian sediments and has thermalIy altered them for a distance of up to one quarter mile. Mineralizing fluids ascended near vertical cooling fractures in the quartz monzonite, creating a porphyry copper deposit of chalcopyrite and pyrrhotite. Trend surfaces from the first to the third order were established from plagioclase, orthoclase, quartz and total mafic minerals modal data and specific gravity values for the intrusive rock. Quartz, total mafic minerals and specific gravity vary systematically while orthoclase and plagioclase have erratic distributions. Contoured modal data and second degree trend surfaces for quartz, specific gravity and total mafic minerals show a circular distribution pattern. Linear trend surfaces indicate northwesterly and southeasterly gradients for quartz, plagioclase, orthoclase and specific gravity. The original homogeneous granitic magma was contaminated by silica assimilation of the intruded, highly siliceous sediments. Incomplete diffusion of the contaminating silica Iead to a circular outward increase in silica and a…

Publisher

Government of Yukon

Resources

3

Catalogue metadata updated

2026-08-26