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

- Many ancient and ore-bearing orogenic belts show sub-vertical crustal stretching.
- Kinematics involve pop-downs of upper crust along steep deformation zones.
- Ore concentrations occur along these steep deformation zones.
- Pop-down tectonics accounts for localised fluid channelling and ore concentrations.
- Pop-down tectonics is not restricted to heavy greenstone-bearing Archaean belts.

Many Archaean and Paleoproterozoic deformation zones, often rich in ore resources, show particular structural patterns in particular marked 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 units within an underlying weak crust submitted to horizontal regional shortening. Here we present three complementary examples from two Archaean greenstone belts (Abitibi sub-Province, Quebec, and Murchison 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 associated with 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.