Vertical and lateral variation of mineralogy and chemistry in the Early Jurassic Mt. Milligan alkalic porphyry Au-Cu deposit, British Columbia, Canada

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.

Alteration mineral assemblage, S isotope (ranging from δ³⁴S –5‰ relative to Canon Diablo Troilite in the core to δ³⁴S +0.5‰ in the periphery), and limited fluid inclusion data suggest mineralization at the Au-Cu alkalic porphyry system was derived from an oxidized, CO₂-bearing magmatic fluid that rose upward along the margins and through the Magnetite Breccia stock. Laterally from the Au-Cu mineralized potassic core, magmatic fluid evolved through water-rock interaction, mixing with an external fluid as shown by a shift in calculated ⁸⁷Sr/⁸⁶Sr₀ for alteration minerals to values higher than magmatic values, declining temperature, or some combination of all. Variations in trace element concentrations of epidote (V, Mn, Sb, Zr, As, and Bi) and pyrite (Mn, As, Zr, Pb, and Bi) across the deposit show a local high degree of variability, but general increases or decreases in overall trends in their median values are inferred to reflect the hydrothermal evolution of the system. Pistachite ratios of epidote show an outward decrease in ferric iron as recorded in the pistachite ratio changing from P_S36 to P_S25, suggesting less oxidizing conditions on the system periphery. Additionally, light rare-earth elements in epidote fractionate toward the core of the deposit, and the height of positive Eu anomalies also appears to have a similar spatial trend. In pyrite, there is a general increase in trace element concentration toward the epidote-pyrite–rich outer propylitic assemblage forming the system periphery.

Alteration mineralogy and trace element signatures indicate the southeastern portion of the deposit, the 66 zone, is a down-dropped segment from higher in the paleohydrothermal system. S isotope signatures of sulfides within and surrounding the stratiform Upper Trachyte unit in the 66 zone indicate structural channeling of oxidizing fluids that likely were the distal and cooler expression of more oxidizing magmatic-derived hydrothermal fluids responsible for potassic alteration and Au-Cu in the subjacent Magnetite Breccia zone, the main orebody.