Chapter 32: Gold Deposits of the Archean Abitibi Greenstone Belt, Canada

The Neoarchean Abitibi greenstone belt in the southern Superior Province has been one of the world’s major gold-producing regions for almost a century with >6,100 metric tons (t) Au produced and a total endowment, including production, reserves, and resources (measured and indicated), of >9,375 t Au. The Abitibi belt records continuous mafic to felsic submarine volcanism and plutonism from ca. 2740 to 2660 Ma. A significant part of that gold is synvolcanic and/or synmagmatic and was formed during the volcanic construction of the belt between ca. 2740 and 2695 Ma. However, >60% of the gold is hosted in late, orogenic quartz-carbonate vein-style deposits that formed between ca. 2660 and 2640 ± 10 Ma, predominantly along the Larder Lake-Cadillac and Destor-Porcupine fault zones. This ore-forming period coincides with the D3 deformation, a broad north-south main phase of regional shortening that followed a period of extension and associated crustal thinning, alkaline to subalkaline magmatism, and development of orogenic fluvial-alluvial sedimentary basins (ca. <2679–<2669 Ma). These sedimentary rocks are referred to, in the southern Abitibi, as Timiskaming-type. The tectonic inversion from extension to compression is <2669 Ma, the maximum age of the D3-deformed youngest Timiskaming rocks. In addition to the quartz-carbonate vein-style, stockwork-disseminated-replacement-style mineralization is hosted in and/or is associated with ca. 2683 to 2670 Ma, early-to syn-Timiskaming alkaline to subalkaline intrusions along major deformation corridors, especially in southern Abitibi. The bulk of such deposits formed late-to post-alkaline to subalkaline magmatism and the largest deposits are early- to syn-D3 (ca. 2670–2660 Ma), whereas the bulk of the quartz-carbonate vein systems formed syn- to late-D3 and metamorphism. At belt scale, these illustrate a gradual transition, as shortening increases, in ore styles in orogenic deposits throughout the duration of the D3 deformation event along the length of the Larder Lake-Cadillac and Destor-Porcupine faults. The sequence of events, although similar in all camps, was probably not perfectly synchronous at belt scale, but varied/migrated with time and crustal levels along the main deformation corridors and from north to south. The presence of high-level alkaline/shoshonitic intrusions, which are spatially associated with Timiskaming conglomerate and sandstone, large-scale hydrothermal alteration, and numerous gold deposits along the Larder Lake-Cadillac and Destor-Porcupine faults indicates that these structures were deeply rooted and tapped auriferous metamorphic-hydrothermal fluids and melts from the upper mantle and/or lower crust, late in the evolution of the belt. The metamorphic-hydrothermal fluids, rich in H2O, CO2, and H2S were capable of leaching and transporting gold to the upper crust along the major faults and their splays. Although most magmatic activity along the faults predates gold, magmas may have contributed fluids and/or metals to the hydrothermal systems in some cases. This great vertical reach explains why the Larder Lake-Cadillac and Destor-Porcupine fault zones are very fertile structures. The major endowment of the southern part of the Abitibi belt (>8,100 t Au) along the corridor defined by the Larder Lake-Cadillac and Destor-Porcupine faults may also suggest that these faults have tapped particularly fertile upper mantle-lower crust gold reservoirs. The concentration of large synvolcanic and synmagmatic gold deposits along that corridor supports the idea of gold-rich source(s) that may have contributed gold to the ore-forming systems at different times during the evolution of the belt.