Pop-down tectonics, fluid channelling and ore deposits within ancient hot orogens

Abstract : Many Archaean and Paleoproterozoic deformation zones, often rich in ore resources, show particular structural patterns in particularmarked by regional vertical stretch. These zones are not restricted to greenstone-bearing Archaean domains that may have suffered gravity-driven sagduction of heavy supra-crustals, as extensively discussed since the last twenties. Structures are actually best explained by pop-down tectonics of upper-crustal unitswithin an underlyingweak crust submitted to horizontal regional shortening.Herewe present three complementary examples fromtwo Archaean greenstone belts (Abitibi sub-Province, Quebec, andMurchison belt, South Africa) and one greenstone-lacking Paleoproterozoic belt (Thompson belt,Manitoba). In the three examples, ore is concentrated along steeply dipping deformation zones, rich in syntectonic deposits and marked by substantial sub-vertical crustal stretch. On the other hand, the three regions show differences in age, in metamorphic grade (from sub-greenschist facies to upper amphibolite facies), in metal contents (gold, antimony, nickel), in metal sources, transfers and concentration histories. Our compared analysis emphasizes that pop-down tectonics associatedwith horizontal shortening of weak lithospheres may account for observed geometric patterns and provide a new and promising frame for the analysis of relationships between structural patterns and ore concentrations within old cratons.