The role of fluids during formation and evolution of the southern Superior Province lithosphere: an overview [Review]

摘要

Models for fluid flow and hydrothermal alteration for the Abitibi greenstone belt are reviewed in the light of Lithoprobe results in the region. In the Abitibi greenstone belt, eruption of volcanic sequences over 2750-2700 Ma was accompanied by pervasive low-temperature hydrothermal alteration at high water/rock ratios, giving systematic O-18-enrichment. Archean ambient ocean water bottom temperatures were likely ca. 30%C, and delta(18)O similar to 0 +/- 1 parts per thousand. Chert-iron formations precipitated from low temperature hydrothermal discharge. Base metal massive sulphide deposits formed at or near the seafloor from focussed discharge of high-temperature (similar to 300-400%C) fluids in convective cells sited above subvolcanic intrusions. The ore fluids were evolved seawater that had undergone compositional and isotopic evolution by high-temperature, low water/rock exchange with the volcanic pile to NaCl (3-7 wt.%) or CaCl2-NaCl (up to 30 wt.%) brines of delta(18)O = 0-8 parts per thousand. These volcanic associated hydrothermal deposits are associated with greenstone belt asemblages in the northern Abitibi subprovince that were emplaced as a series of thrust slices over the Opatica plutonic belt. In the southern Abitibi subprovince the hydrothermal deposits were associated with a series of rift basins (Noranda, Val d'Or, etc.), formed on top of accreted oceanic assemblages comprising primitive arcs and plateaus, or in protoarcs, and associated with oblique convergence. Contemporaneous erosion of emergent arcs and the older cratonic provenance terrane of the Pontiac subprovince by orographic rainfall, and submarine weathering, fed first-cycle vol cano genic sediments to depositional basins in the Abitibi, but siliciclastic sediments of mixed old 3 Ga continent and 2.7 Ga arc provenance in the Pontiac subprovince. Abitibi subprovince turbidites were more weathered and O-18-enriched than Pontiac subprovince equivalents. Subduction-accretion assembly of the Opatica-Abitibi and Pontiac terranes involved allochthonous thrusting of the Abitibi subprovince over the Pontiac subprovince. There were several pulses of granitoid magmatism during accretionary assembly over 2695 to 2674 Ma. Syn- to late-tectonic tonalites were generated by melting of hot young hydrous ocean crust in a shallow-dipping intraoceanic subduction zone. The intrusions exsolved small quantities of magmatic fluids that formed Cu-Zn showings. Late-tectonic shoshonites formed at greater than or equal to 80 km in subarc mantle wedge by slab dehydration-wedge melting. This late-stage of arc development involved transfer of significant quantities of gas-rich alkaline magmas 80+ km through the lithosphere along the accretionary terrane bounding structures, and produced small phosphorus and barite deposits. Synmagmatic metamorphism was of the high-temperature low-pressure type, and occurred in several pulses; water/rock ratios were generally low distal from structures. Tens of thousands of cubic kilometres of fluids generated by dehydration reactions at the base of the subduction-accretion complex, during thermal relaxation following collision and the main granitoid pulses, advected up terrane boundary structures and locally generated lode gold deposits. At the highest structural levels these fluids mixed with Archean meteoric water where delta(18)O < 0. A second metamorphism (M2) occurred over 2645 to 2611 Ma leading to melting of Pontiac sediments and formation of S-type granites. Deposits of Mo, Th, and P were precipitated from magmatic fluids of delta(18)O 8-9 parts per thousand. M2 variably reset radiogenic and stable isotope systems in nonrobust minerals of vol canogenic massive sulphide and lode gold deposits. Hypersaline CaCl2 formation brines evolved in Paleoproterozoic glaciogenic sediments; these penetrated into the Archean basement where they redistributed gold and are pervasively present as low-temperature secondary brine inclusions. The Matachewan (