Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation — Archean to present: a review: R. Kerrich & K. F. Cassidy, Ore Geology Reviews, 9(4), 1994, pp 263–310

Abstract

Abstract Late Archean (ca. 2.7 Ga) lode gold mineralization in the Superior Province of Canada and Yilgarn Craton of Australia was late kinematic, and syn to late peak metamorphic, and intimately related in space and time to accretionary tectonic processes. Gold mineralization in the Superior Province was coeval with late-kinematic diachronous accretion of allochthonous subprovinces from north to south over ca. 2710 to 2670 Ma. Timing of gold genesis is constrained by field relationships, and rigorous lower ages of cross-cutting dikes. Gold vein formation is an integral part of the thermal and rheological history of deposit host rocks, such that, in general, deposits in greenschist facies rocks have greenschist facies assemblages and occur in brittle-ductile structures, whereas counterparts in amphibolite facies rocks feature amphibolite facies alteration assemblages and generally occur in ductile structures. Gold mineralization in the southeastern Yilgarn Craton is also temporally related to craton-scale compressional events. There is abundant evidence for thermal, structural and hydrothermal disturbance post orogeny in the Yilgarn Craton, with most ArAr, RbSr and reset UPb zircon ages corresponding to known periods of tectonism in adjacent terranes. In addition, gold-related sulphides typically have reset model Pb ages recorded by secondary isochrons. Accordingly, anomalously young UPb ages on rutile and titanite and ArAr ages on hydrothermal silicates in deposits that post-date peak metamorphism by 100 to 240 m.y. are interpreted to reflect multiple secondary hydrothermal events that also locally remobilized primary ore metals. These variably young, reset ages are also recorded in lithological units of undisputed primary age, including VMS deposits, syn-tectonic granitoids and komatiites. These variably young ages provide an exact parallel for reset secondary ages in gold deposits. Mineralization interpreted to have formed 100 to 240 m.y. after terrane accretion based on scattered ages from non-robust isotopic systems or minerals is not compatible with the above relationships, but rather reflects intermittent secondary reactivation of the primary ore hosting structures. The southern Superior Province has experienced thermal, structural and hydrothermal disturbances since termination of S-type magmatism at ca. 2645–2611 Ma, reflected in KAr, RbSr and SmNd ages 100 to 240 m.y. younger than UPb zircon ages on the same lithological units, and reversed apparent blocking sequences. Eight different models, with distinct time connotations, exist for lode gold deposits in the late Archean terranes. The TTG model linking mineralization in the southern Abitibi Subprovince to synvolcanic tonalite-trondhjemite granodiorite batholiths (2695–2685 Ma) can be ruled out on geochronological grounds, as the deposits are most closely associated in space and time to late-kinematic, calc-alkaline and shoshonitic rocks (2685–2670 Ma). Two “late gold” models based on the non-concordant young ages, involving either delayed thermal rebound or young events in Kapuskasing-like mid-crust can be reinterpreted as late kinematic primary mineralization with multistage isotopic resetting. Both young gold models are based on geochronology alone, and ignore tectonic, structural and metamorphic relationships that provide evidence of deposits forming in thermal and rheological equilibrium with their host terranes. Five models (“early gold” models) for lode gold deposits in late-Archean terranes, namely granulization, cratonization, two gold-shoshonite schemes and the crustal continuum model, explicitly relate mineralization to processes resulting from terrane accretion. All five are consistent with rigorously constrained ages of some gold deposits, including primary mineralization at Val d'Or as given by hydrothermal zircons. Lode gold deposits in the Proterozoic Trans-Hudson orogenic belt (ca. 2.0 to 1.7 Ga) also closely follow terrane collision and metamorphism. Similarly, lode gold deposits in late Paleozoic orogenic belts of Appalachia and the Lachlan fold belt of southeastern Australia show temporal relationships of mineralization to collisional tectonics, metamorphism, magmatism and deformation. The temporal relationships of mineralization to collisional tectonics is also repeated in the Mesozoic-Cenozoic Cordilleran belt of the North American continent and the Cenozoic Alpine-Himalayan chain. Giant lode gold metallogenic provinces formed at three times in Earth history, in the late-Archean (2.7-2.6 Ga), late Paleozoic (450–340 Ma) and Mesozoic-Cenozoic. These times correspond to accretionary tectonic assembly of a supercontinent, in external orogens. Minor gold deposits also formed in space and time with collisional tectonics in internal orogenic belts of the supercontinent cycle. Consequently, the local temporal relationships of lode gold mineralization to collisional tectonics and post-peak metamorphism is also part of the secular development of a larger global geodynamic cycle.