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 corresponding outward decrease in mafic minerals, but did not significantly affect the distribution of orthoclase or plagioclase. The circular zonation of the high specific gravity mafic minerals gave rise to a similar zonation of specific gravities.
This thesis is available online at http://data.geology.gov.yk.ca/Reference/68112. A copy of this thesis is available at the EMR library – QE195 S26.
