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6th International Archean Symposium
6th International Archean Symposium

Convergent margin metallogenic cycling during the Archean: insights from the Abitibi and other Archean terranesConvergent margin metallogenic cycling during the Archean: insights from the Abitibi and other Archean terranes

Keynote

Talk Description

The Abitibi greenstone belt (AGB), in the Superior Craton of Canada, is one of the best exposed, preserved, and mineralized Archean greenstone belts on Earth. It has a very well constrained metallogenic history with world-class VHMS and orogenic gold deposits, along with many other deposit types. The AGB consists of east-trending successions of volcanic, sedimentary, and intervening intrusive rocks. The volcanic construction, comprising seven assemblages, started at 2790 Ma and ended at 2695 Ma. Each assemblage consists of varying abundance of mafic (±ultramafic), intermediate and felsic submarine volcanic and volcaniclastic rocks. Tonalite-trondhjemite-granodiorite (TTG-type) magmatism at ~2750–2695 Ma accompanied volcanism. These rocks may have formed via plume, volcanic plateau, and arc/back-arc style magmatic activity. Turbiditic deep-marine sedimentation at ca ≤2700‒2685 Ma followed the main phases of volcanic construction, coeval with a transition from TTG-type magmatism to sanukitoïd-style magmatism. This deep-marine sedimentation is associated with early thin-skin folding and thrusting (imbrication) and uplift (D1 and D2: ~2685‒2679 Ma in the southern Abitibi). This was followed by extension and uplift, and by basin deepening and syn-orogenic sedimentation over an angular unconformity (ca ≤2679‒≤2669 Ma Timiskaming Group alluvial-fluviatile deposits) and early- to syn-Timiskaming, sub-alkalic to alkalic/shoshonitic magmatism at ~2683–2670 Ma in the southern part of the belt. A switch to compression (D3, thick-skin thrusting) followed at <2670 Ma and is responsible for the development of the east-trending, crustal-scale and/or first-order structures. Regional metamorphism is coeval with D3 and bracketed at ~2660–2640 Ma. Metallogenesis in the AGB began with VHMS deposits at ~2760 Ma, with most deposits formed between 2730 and 2695 Ma. Komatiite- and intrusion-hosted Ni-Cu-(PGE) deposits at ~2740–2700 Ma overlapped VHMS formation as did porphyry-like Au±Cu and Cu±Mo-Au deposits hosted by polyphase TTG intrusive complexes. This was followed by major gold mineralization, starting with early (pre-Timiskaming) intrusion-associated deposits at ~2700–2686 Ma associated with early D1-D2 deformation. A few syn-Timiskaming intrusion-associated gold and porphyry-style Cu-Mo-Au systems formed at ~2683–2670 Ma, coeval with calc-alkalic to alkalic intrusive rocks. Gold metallogenesis gradually transitioned to the classic, and dominant, syn-D3 orogenic (quartz-carbonate vein-style) gold systems at ~2670 Ma along the major east-west fault zones, peaking at ~2660–2640 Ma in the southern Abitibi. Late- to post-tectonic S-type intrusive complexes host ~2650–2745 Ma Mo-Bi deposits and ~2639–2637 Ma LCT-type Li pegmatites. A similar progression from VHMS, Ni-Cu-(PGE) and calc-alkalic porphyry Cu-Au deposits to orogenic gold to granite-related and pegmatite deposits is present in other Archean terranes such as the Slave and Yilgarn. This reasonably consistent temporal pattern is also characteristic, albeit longer-lived, of younger convergent margins and defines what is now referred to as the convergent margin metallogenic cycle (CMMC: cf. Doublier et al., “Convergent margin metallogenesis constrains onset of subduction on Earth”, this volume). The AGB represents one of the oldest CMMC, with a comparatively short, single cycle that lasted ≤125 Myr, typical of shorter-lived metallogenic histories during the Archean in response to secular changes in tectonic processes and the onset of subduction (shallow break-off subduction).

Speakers