Volcanogenic massive sulphide deposits: essential attributes for discovery in Abitibi
James M. Franklin (Franklin Geosciences)

Abitibi VMS deposits are all in the bimodal mafic-dominated group. Variations in form, composition, district tonnage and alteration assemblages are great but predictable. Volcanological attributes determine their composition and form. Copper-rich, lead-poor deposits formed in flow-dominated deep water regimes (>2 000 m). Zinc(±silver)-rich deposits formed in volcaniclastic-dominated shallower water settings (1 500-500 m). Noranda and Matagami Lake exemplify the former, whereas the Barville, Val-d’Or, Kidd Creek and possibly Bousquet districts exemplify the latter. District sizes and deposit distributions are controlled by the heat input into the upper 5 km of the crust, manifest by the size, composition and attitude of subvolcanic intrusions. Petrochemical data reflect temperature conditions of crustal melting (felsic melts) and melt contamination (andesite abundance), all related to the heat input needed to “drive” convective fluid flow and metal leaching within a hydrothermal reservoir.

District-scale lower “reaction” zones (albite-epidote-actinolite-quartz) are all similar, regardless of setting. The upper alteration assemblages depend on the regional volcanological characteristics. Flow-dominated systems have distinctive semiconformable reservoir caps consisting of silicified, locally albitized and Mg-chloritized strata, above which the rocks are relatively unaltered (e.g., Noranda, Matagami). Volcaniclastic-dominated regimes have widely distributed carbonate-impregnated strata in their upper 500 m, and less well-defined “caps”. These relatively permeable strata underwent massive volumes of convective fluid circulation, causing stratigraphically extensive Na depletion, with the formation of aluminous assemblages (andalusite, chloritoid, pyrophyllite: Barville).

Alteration pipes in flow regimes are vertically extensive, aerially confined to the deposit footprint, and dominated by Mg-chlorite and sericite. Those in volcaniclastic regimes are laterally extensive zones (2-3x the footprint width) of silicified and sericitized strata with less chlorite.

The ore zones in flow regimes consist of sulphide mounds formed on the seafloor (e.g., Millenbach) although some may have formed within an earlier-formed hydrothermally generated stratum (talc mounds, cherty exhalite: Matagami). Those in volcaniclastic regimes formed sub-seafloor by replacement of clasts (Kidd).

Gold enrichment formed either by oxidative destruction of bisulphide-transported Au, requiring boiling conditions (shallow water) and probably an oxidative ocean, by secondary circulation of heated seawater through a pre-existing sulphide mound (oxidative precipitation?) or a second source of gold, such as a magmatic system (Bousquet, Horne?).

Determination of the volcanological attributes of various parts of the Abitibi is the most critical step in establishing additional VMS potential, as well as determining the key field characteristics to be measured by explorationists.