Abstract:

The Heninga Lake volcanic-associated massive sulphide (VMS) deposit is located within the Neoarchean, Kaminak Rankin-Ennadai greenstone belt (2.7Ga), about 100 km west of Hudson Bay. It consists of three massive sulphide lenses that occur within a synvolcanic fault bounded basin that developed within a large silicic caldera or calderas. Caldera collapse occurred in response to voluminous, subaqueous pyroclastic eruptions that produced pumice-rich deposits with a volume that is estimated to have exceeded 120 km3. The VMS deposits formed following collapse, during a hiatus in volcanic activity. This interval was marked by caldera wide sedimentation and low temperature hydrothermal venting that produced an extensive banded iron formation. <br/>The local stratigraphy is divided into three informal formations. The lower formation is up to 1600 m thick, and consists of felsic lapillistone, lapilli tuff and tuff emplaced predominantly as subaqueous mass flow deposits. These are typically monolithic, and consist of felsic pumice and quartz crystals set in a fine, recrystallized ash-sized matrix. The >80 depositional units within the lower formation are divided into five emplacement units, each representing individual pyroclastic eruption episodes. Depositional and emplacement units display variations in incompatible immobile elements that record crystal sorting during deposition and possibly igneous fractionation processes. This indicates that the deposits are primary, and have not been significantly re-worked. By analogy with modern subaerial pyroclastic eruptions, the volume (>100 km 3) of pyroclastic rock, deposit morphology, and the juvenile constituents are consistent with its generation by catastrophic phreato-plinian or plinian eruptions, associated with caldera collapse. The middle formation consists of tuffs that include both suspension and turbidite deposits, and show minor evidence of sedimentary reworking. Plane bedded felsic tuff of the middle formation attains thicknesses of up to 30 m and occupies paleotopographic depressions, bounded by synvolcanic faults. The upper formation consists of intercalated andesitic and felsic tuffs, banded chert-magnetite iron formation and a distinct, quartz-rich, crystal tuff of mass flow emplacement. The lower and middle formations are interpreted as caldera fill deposits, although mapping has not yet defined the margins of the inferred caldera structure. <br/>The Heninga Lake VMS deposits formed as sub-seafloor replacement bodies within tuffs of the middle formation. Although a shallower water environment may be inferred for part of the middle formation, reconstruction of the volcanic stratigraphy suggests that sulphides formed following subsidence, in a deeper water environment than their host rocks were emplaced.