Vertical and Lateral Variation of Mineralogy and Chemistry in the Early Jurassic Mt. Milligan Alkalic Porphyry Au-Cu Deposit, British Columbia, Canada*

摘要

The moderately tilted and faulted Early Jurassic Mt. Milligan Au-Cu deposit provides a cross-section view of the hydrothermal alteration, sulfide mineralogy, and geochemical zonation of a silica-saturated alkalic porphyry system over a vertical distance approaching 700 m. Magnetite-bearing potassic alteration and associated Au-Cu form a core to the system in the central monzonitic stock and adjacent basaltic trachyandesite host rock. Lateral to the high-temperature core are sodic-calcic and inner and outer propylitic alteration assemblages. Chalcopyrite dominates the high-temperature potassic core whereas pyrite is the predominant sulfide within and outboard from the sodic-calcic assemblage. A funnel-shaped remnant of carbonate-rich phyllic alteration of the host supracrustal rocks in the fault-bounded 66 zone represents the upper auriferous alteration in the alkalic porphyry Au-Cu system.