The characteristics and origin of the big lake mafic-ultramafic-hosted volcanogenic massive sulfide occurrence, Marathon, Ontario, Canada

The Big Lake volcanogenic massive sulfide (VMS) occurrence, located in the Schreiber-Hemlo belt in the Wawa subprovince of the Superior province, is hosted in an Archean mafic-ultramafic metavolcanic sequence lacking felsic volcanic or volcaniclastic rocks. Discovered in 2006, the occurrence is a subeconomic assemblage of pyrrhotite, chalcopyrite, sphalerite, cobaltite, and galena currently defined over an area of 0.5 × 0.5 km in plan view. The mineralized zone is less than 10 m thick, with the highest grade intersection containing 4.0 m of 7.5% Cu, 2.2% Zn, 138.0 gpt Ag, and 9.2 gpt Au.

Mineralization is hosted in metavolcanic rocks of basaltic to komatiitic basalt compositions. The sulfides and spatially associated hydrothermal alteration (regionally metamorphosed to chlorite, actinolite, hornblende, biotite-phlogopite, and anthophyllite) are located 5 to 50 m structurally below a thick ultramafic cumulate complex. The host sequence to the occurrence is overturned, with VMS sulfides stratigraphically overlying the ultramafic cumulates. Rocks stratigraphically below the VMS mineralization are transitional to alkaline basalts and komatiitic basalts, with convex-upward LREE and fractionated HREE when normalized to primitive mantle. Basalts stratigraphically overlying the VMS mineralization have flat normalized REE patterns (La/Ybcn = 1-2) typical of tholeiitic plateau basalts found elsewhere in the Wawa belt. The lithostratigraphic assemblage at Big Lake is consistent with an oceanic plateau setting and is interpreted to have formed by eruption of plateau basalts and thick ultramafic flows from a mantle plume.

Unlike VMS systems that formed above subvolcanic intrusions >0.5 km below sea floor, the Big Lake VMS occurrence may have formed as a result of cooling of the stratigraphically underlying ultramafic cumulates and demonstrates that similar styles of ponded flow-driven VMS systems, albeit small ones, may be preserved elsewhere in the Superior province or in other Archean cratons.