Abstract:

The Hidden and Louis formations comprise a composite, dominantly basaltic volcanic edifice that was erupted and grew within a large, subsidence structure that developed within a Paleoproterozoic, rifted arc environment. This subsidence structure hosts the Flin Flon, Callinan and Triple 7 VMS deposits at Flin Flon, Manitoba and the Hidden and Louis formations constitute the immediate hanging wall to these deposits. <br/>The Hidden formation comprises, from oldest to youngest, the 1920 unit, the<br/>Stockwell member, and the Reservoir member. The distribution of the 1920 unit, an icelandite, crudely defines a fault-bounded subsidence structure into which it was emplaced and largely restricted to. The 1920 unit shares many features typical of basaltic flows; however, the presence of peperite along its upper contact, and lense-shaped inclusions of tuff along its basal contact, suggests that it was emplaced as a cryptoflow. <br/>The dominantly basaltic Reservoir member conformably overlies the areally restricted Stockwell member or the 1920 unit where the Stockwell member is absent. Where neither the Stockwell nor 1920 unit exist the Reservoir member conformably overlies the Millrock member of the Flin Flon formation.<br/>The Louis formation conformably overlies the Hidden formation and consists of the Tower and Icehouse members as well as undivided basaltic flows. The Tower member occurs at the base of the Louis formation and consists of an areally restricted coherent and volcaniclastic rhyodacite unit, overlain and locally underlain by mafic tuff. The ponded basaltic flow of the Icehouse member conformably overlies the Tower member and is conformably overlain by a succession of undivided basaltic flows.<br/>Major, trace element, and Nd-, Hf- and Pb-isotopic data indicate that the Hidden and Louis formations were emplaced in a juvenile island arc environment. The rocks are variably contaminated (< 3.5%) by crustal sources with data indicating an increasing amount of crustal contamination from the base to the top of the hanging wall succession. Geochemical variations also record a change from a depleted mantle source to more enriched mantle source during emplacement of the Hidden formation and a return to a more depleted mantle source during emplacement of the Louis formation.<br/>Volcanic textures, facies and synvolcanic structures indicate that these formations are a product of resurgent volcanism and subsidence following a hiatus in effusive volcanism marked by ore formation. The Hidden formation is interpreted to represent a small shield volcano and the Louis formation a resurgence in basaltic volcanism and minor localized subsidence, resulting in the growth of a new lava shield on the flank of the Hidden volcano.<br/>Synvolcanic structures in the Hidden and Louis volcanic edifices define structural corridors that when projected downward encompass massive sulphide deposits at Flin Flon indicating their control on the formation of these ore deposits and their longevity as magma and fluid pathways during emplacement of the hanging wall succession. As such they provide a useful tool for targeting massive sulphide mineralization at depth.