Base metal metallogeny of the Onaman-Tashota greenstone belt, Ontario, Canada

Abstract

Archean greenstone belts are a significant source of base and precious metals. The metal endowment of greenstone belts varies by several orders of magnitude, a phenomenon that remains enigmatic despite decades of research on greenstone belt ore deposits. This dissertation describes the metallogeny and evolution of the lesser-endowed Neo- to Mesoarchean Onaman-Tashota greenstone belt and demonstrates that prolonged crustal development and a lack of mantle plume activity are critical controls on metal endowment at the belt-scale. Well-endowed greenstone belts, exemplified by the Neoarchean Eastern Abitibi, are dominated by juvenile, tholeiitic volcanic successions hosting komatiite-hosted Ni-Cu, volcanogenic massive sulfide (VMS), intrusion-related gold, and orogenic gold deposits. These settings and deposits reflect extensive, plume-driven rifting and syn-volcanic transcrustal structure development. The Onaman-Tashota belt, in contrast, is characterized by isotopically mature, calc-alkaline-dominant, volcanic successions. The metallogeny of the Onaman-Tashota belt is typified by magmatic Ni-Cu±PGE, polymetallic epithermal-like, porphyry Cu-Au, and hybrid volcanogenic massive sulfide deposits, reflecting a thicker crustal setting, limited transcrustal connectivity, and increased magmatic volatile inputs into mineral systems. Mesoarchean volcanic crust is present within and underlying the Onaman-Tashota belt. In contrast, Mesoarchean crust is absent within the well-endowed Abitibi greenstone belt. Within the Onaman-Tashota greenstone belt this older crustal material led to a thickened Neoarchean crustal setting, and controlled endowment by: limiting rifting and transcrustal structure development; increasing crustal melting and contamination; generating subduction-like volcanic sequences; and leading to an increase in continental freeboard, leading to locally shallower water and subaerial volcanic successions. These results have implications for mineral exploration related to regional area selection, greenstone belt fertility, and exploration strategies. Specific implications include: 1) greenstone belts with evidence of a thickened crust are of an overall lower rank for traditional VMS endowment; 2) FI-dominant felsic signatures, high felsic-mafic ratios, and a lack of komatiites are indicative of thick crust settings, lowering the komatiite-hosted Ni-Cu and traditional VMS potential; and 3) greenstone belts with Mesoarchean (thickened) basement may contain a different metallogeny, necessitating the use of different ore deposit models, such as the porphyry, epithermal, and hybrid VMS models.