POP-DOWN TECTONICS, FLUID CHANNELLING AND ORE DEPOSITS WITHIN ANCIENT HOT OROGENS

Many Archean and Paleoproterozoic deformation zones have remarkable structural specificities. Large strains marked by steeply dipping fabrics bearing steeply plunging stretching lineations are thus often ubiquitous. These regions contain deformation bands showing large strains despite lacks of major metamorphic jumps, and some host important ore concentrations.

Here we describe three representative examples: the Southernmost Abitibi Greenstone Belt (Abitibi Sub-Province, Quebec), the Murchison Greenstone Belt (South Africa), and the Thompson Nickel Belt (Canada). These belts show comparable relationships between structural patterns and ore concentrations, despite their differences in age (Archaean and Paleoproterozoic), in metamorphic grade (from sub-greenschist facies to upper amphibolite facies), in metal contents (gold, antimony, nickel), in metal sources, transfers and concentration histories.

We propose a general model of pop-down tectonics, fluid flow and mineralization interactions at crustal scale, based on field observations and analogue models of shortening of weak lithospheres. Syntectonic upper-crustal deposits concentrate within spaces provided at fault footwalls during first stages of pop-down motions of upper crust pieces tens of kilometers wide. During their downward motion, pop-downs are deformed in the ductile field. Meanwhile, fluids are produced through compaction, diagenesis, and metamorphic and melting reactions, steep foliations and shear planes favoring efficient upward circulation. As a positive feedback, fluid-induced softening may favor strain localization in areas of pop-down piling-up.

We show that similar structural patterns do occur within greenstone-bearing Archaean belts and within greenstone-lacking Paleoproterozoic belts, and thus irrespective of the presence of dense upper-crustals that might favor gravity-driven sagduction. Consequently, we propose that pop-down tectonics is the first-order process that concentrates upper-crustal deposits, strains, fluid transfers, and metal transport and deposition along steep deformation zones that develop during shortening of weak lithospheres. Our model provides new bases for future works dedicated to tectonics and ore deposits within ancient cratons.