MAGMATIC EVOLUTION OF A NEOARCHEAN GREENSTONE BELT: THE EXAMPLE OF THE CHIBOUGAMAU AREA, ABITIBI SUBPROVINCE

In Archean granitoid-greenstone belts, the relationship between magmatism and mineralizing processes remains controversial. In the Neoarchean (>2.79 to ~2.65 Ga) Abitibi greenstone belt of the Superior craton, Canada, Cu-Au porphyry systems are associated with synvolcanic tonalite–trondhjemite–diorite (TTD) suites rather than the more common tonalite–trondhjemite–granodiorite (TTG) compositions. Syntectonic intrusions, in contrast, may have a petrogenetic connection to intrusion-related gold systems (IRGS). The importance of TTG versus TTD suites in the evolution of greenstone belts, the late onset of K-bearing magmatism and the importance of related mineralization has to date been poorly constrained. This contribution presents preliminary results that describe the chemistry and distribution of tonalite-dominated magmatism in the Abitibi greenstone belt, and provides new insights into physico-chemical parameters such as pressure, temperature, and oxygen fugacity (ƒO₂). Comprehensive lithogeochemical data across the belt are used to evaluate the petrological evolution of TTG and TTD suites, revealing two coeval and differentiated trends of Neoarchean magmas: 1) a HREE-depleted tonalitic magma from high pressure melting of a hydrated basalt source (TTG suite); and 2) a hybrid HREE-undepleted magma that may be a mixture of mantle-derived and tonalitic melts (TTD suite). Synvolcanic, tonalite-dominated magmatism in the Abitibi greenstone belt comprises crustal melts as well as a significant proportion of mantle-derived magma (i.e., diorite) which may have been essential for establishing Cu-Au magmatic-hydrothermal mineralizing systems. In synvolcanic and syntectonic plutons, apatite and zircon chemistry preserve a marked increase in magmatic ƒO₂ and provide preliminary insights into the S and Cl content of Neoarchean magmas. The increase in magmatic ƒO₂ and K-content recorded in later, syntectonic plutonism may indicate a metasomatised mantle source that could be critical for IRGS metallogeny.