GEODYNAMIC SETTING OF OROGENIC GOLD DEPOSITS: A SECULAR TRANSITION FROM ARCHEAN PLUME-ARC OROGENS TO PHANEROZOIC CONVERGENT MARGIN OROGENS

Many greenstone belts in the 3.1 to 2.7 Ga Superior Province are characterized by two principal volcanic associations. A komatiite-tholeiitic basalt association erupted from a mantle plume, representing intraoceanic plateaus, and a bimodal basalt-dacite association, paired to trench turbidites, formed at intraoceanic arcs. Arc retreat may result in capture of the arc, and jamming of the subduction zone, by a contemporaneous plume, followed by subduction step back. In the Abitibi terrane, a 300 km belt of boninite series to low Ti tholeiite volcanic rocks at the plume-arc interface represents subduction initiation. Preservation of the composite, imbricated, plume-arc crust stems from buoyant rise of the refractory residue of plume melting from ~ 150 km, forming the thick, diamondiferous, continental lithospheric mantle (CLM) characteristic of Archean cratons. Orogenic gold deposits form at accretionary boundaries, marking plume-arc sutures. Cs muscovite alteration develops in komatiites.

Phanerozoic orogenic gold provinces develop in accretionary orogenic belts of the external supercontinent cycle, also termed Turkic-type orogenic belts by Sengor. Many of the characteristics of the deposits, and ore-forming fluids, are similar to Archean counterparts. However, Cr muscovite-quartz-carbonate alteration develops in ultramafic units that are alpine-type peridotites, from obducted subarc mantle lithosphere. Examples are the Mother Lode, Bridge River, and Atlin Camps of the North American Cordillera. Differences between Archean and Phanerozoic orogenic gold provinces include prevalent volcanic rocks and deep water iron formation in the former, whereas the latter are dominated by siliciclastic sequences. Proterozoic gold provinces, such as the Birimian terrane, have intermediate characteristics. These observations can be accounted for by a secular decrease in the intensity of mantle plumes. Intense Archean plume activity creates frequent plume-arc interactions, deep ocean basins, and iron formations. Decreased plume intensity results in more composite tectonostratigraphic terrane-terrane, terrane-continent, and arc-continent accretions, and greater freeboard with commensurately larger siliciclastic budget at subduction-accretion margins.