Magmatic oxidation in the syenitic Murdock Creek Intrusion, Kirkland Lake, Ontario: evidence from the ferromagnesian silicates

The syenitic Murdock Creek Intrusion, of late Archean age, lies within one of the major lode gold camps of the world, at Kirkland Lake, Ontario. There is a strong spatial association between syenitic intrusions, gold mineralization, and carbonatization. Hydrothermal fluids that carried the gold had an unusually high oxygen fugacity (f_O2). Chemical compositions within the intrusion indicate a deficiency of Fe²⁺ in the magma during crystallization, interpreted to result from an intrinsically high magmatic f_O2 that remained constant during pluton evolution (f_O2 = 10^-12 bars). Mineral compositions, proportions of crystallizing mineral phases, and textural features indicate that the pluton formed from a single pulse of already-evolved syenitic magma that fractionally crystallized in situ under low water pressure at mid-crustal depths. An oxidized source region is implied such as metasomatized upper mantle where partial melting occurred. Co₂-rich fluid or melt was the likely agent for metasomatism. Dissolution and transport of gold is favored by high f_O2 in the fluid phase. The high f_O2 of the syenitic magma suggests a common genetic link between magma generation, CO₂ represented by carbonatization, and gold-bearing fluids. -from Authors