Deposition and alteration of volcaniclastic strata in two large, early Proterozoic iron-formations in Canada

The Gunflint Iron-formation of western Ontario and the Sokoman Iron-formation of the Labrador–Quebec geosyncline both contain interbeds of coarse-grained volcaniclastic detritus. Volcaniclastic beds in the Gunflint are typically less than a metre thick and display normal grading and other physical structures typical of high- and low-density turbidites. Similar volcaniclastic beds are present in the Sokoman, as well as thicker accumulations with structures indicative of deposition from high-density turbidity currents. The volcaniclastic detritus in both iron-formations consists largely of well-sorted vitric ash and lapilli with accessory holocrystalline grains and solitary feldspar crystals. Internal textures of the vitric grains, plus the presence of armored lapilli in the Gunflint, suggest they are products of hydroclastic eruptions. However the clasts in most beds are heterogeneous and well-rounded, indicating they are sedimentary rather than eruptive deposits. Quench textures, coalesced vesicles, and diabasic textures indicate that the volcaniclastics were originally basaltic in composition, but the rocks have been pervasively altered to iron-rich chlorite, calcite, and K-feldspar (Or98 Ab2 An0) with minor quartz and illite. In addition to being pseudomorphs after the original volcaniclastic textures within grains, these minerals also occur as interstitial and vesicle-filling cements. Fibrous rims of chlorite and poikilotopic to blocky calcite are the most abundant cement types. Cementation commenced early, inasmuch as some zones show little evidence of compaction. Patterns of cementation and alteration may indicate that geothermal gradients in such iron-formation basins were steeper than they are in the most closely comparable modern settings, namely passive margins.