HYDROTHERMAL ALTERATION AND CO_2 METASOMATISM (NATURAL CARBON SEQUESTRATION) OF KOMATIITES IN THE SOUTH-WESTERN ABITIBI GREENSTONE BELT

摘要

Secondary mineral assemblages in komatiites of the south-western Abitibi greenstone belt reflect the tectonic evolution of the region. Changes in mineralogy were accompanied by changes in fluid chemistry over time. Hot, plume-generated komatiites (2719-2704 Ma) were hydrated by hot fluids (200-300 ℃), replacing the primary olivine and pyroxenes by serpentine + magnetite. Accretion and thrusting between neighbouring subprovinces was followed by pulses of granitic magmatism between 2692 Ma and 2677 Ma and by the subsequent devolatilization of the crust. Large volumes of oxidized, CO_2-bearing fluids (250-300 ℃) discharging through faults gave rise to two regionally widespread carbonate-bearing assemblages. Quartz + carbonate rocks formed from fluids having a high CO_2:H_2O ratio and talc + carbonate rocks from fluids of lower CO_2:H_2O ratio. The quartz + carbonate assemblage commonly hosts gold deposits and probably represents the site of CO_2 discharge where fluid build-up and hydraulic fracturing promoted the development of mineralized crack-seal veins. The quartz + carbonate and talc + carbonate assemblages along the Destor Porcupine fault are by-products of natural carbon sequestration of ultramafic rocks where the carbonates provided permanent storage for CO_2. Experimental studies on carbon sequestration support earlier conclusions, based on fluid inclusion microthermometry, that the quartz + carbonate formed from fluids with high CO_2:H_2O ratios. The crystallization of the mineral assemblage was accompanied by volume increase, loss of porosity, and reduced permeability. These effects were ultimately reversed as the build-up of CO_2 gave way to fracturing in the rocks, thereby increasing porosity and permeability. The periodically fractured rocks (crack-seal veins) would have been ideal hosts for mineralizing fluids. CO_2 metasomatism of ultramafic rocks in the Abitibi belt was followed by partial replacement of the carbonate-rich assemblages by chromian muscovite ± chlorite ± rutile at 2633-2617 Ma. The localized low pH metamorphic fluids were associated with folding and the development and re-activation of shear zones in the region.