TECTONIC SETTING OF LATE PLUTONISM AND ARCHEAN LODE GOLD DEPOSITS IN THE SUPERIOR PROVINCE

The main stage of gold introduction in the Superior Province is temporally associated with the emplacement of late mantle-derived plutons (LMDPs) that represent the terminal stage of juvenile crust formation. Consequently, the nature of LMDPs has implications for the tectonic setting of Archean lode gold deposits. Gold introduction and LMDPs young diachronously to the south, suggesting that both are related to accretionary growth and stabilization of the Superior Province.

LMDPs are emplaced over a brief (5 - 10 Ma) interval in any given part of the Superior Province and are sourced from long-term depleted mantle that was metasomatized >100 Ma prior to their generation. Based on detailed and reconnaissance investigations, Superior Province LMDPs display a characteristic timing and relationship with respect to tectonic, metamorphic and magmatic development of greenstone belts. LMDPs post-date youngest high-Al TTG plutons and related volcanism by 5-15 Ma and slightly post-date most sedimentation but are broadly coeval with late, unconformable sedimentary-volcanic sequences. The LDMPs post-date early deformational fabrics but are broadly synchronous with regional folding and late, transpressive deformation. LMDPs are broadly synchronous with regional high T- low P metamorphism but post-date an earlier, higher-pressure metamorphic event. Al in hornblende barometry indicates that their emplacement occurred during an interval of rapid uplift.

Rapid uplift associated with extensional orogenic collapse related to slab break-off or roll-back following terrane accretion is proposed as the probable tectonic setting of Archean lode gold deposit formation. High T – low P metamorphism and generation of the LMDPs is interpreted to be a consequence of associated upwelling of hot asthenospheric mantle. This hypothesis rationalizes the character and temporal association of intermediate to felsic magmatism, regional metamorphism and deformation and their relationship to gold mineralization. By analogy with interpretations advanced for the Apennines (Ghisetti and Vezzani, 2002), this process may lead to crustal-scale fluid flow, which, together with emplacement of mantle-derived intermediate to felsic magmas, may facilitate a critical linkage between processes occurring in the upper mantle and mid-crust.