Orogenic gold and geologic time: a global synthesis

Abstract

Orogenic gold deposits have formed over more than 3 billion years of Earth's history, episodically during the Middle Archean to younger Precambrian, and continuously throughout the Phanerozoic. This class of gold deposit is characteristically associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustal structures. A consistent spatial and temporal association with granitoids of a variety of compositions indicates that melts and fluids were both inherent products of thermal events during orogenesis. Including placer accumulations, which are commonly intimately associated with this mineral deposit type, recognized production and resources from economic Phanerozoic orogenic-gold deposits are estimated at just over one billion ounces gold. Exclusive of the still-controversial Witwatersrand ores, known Precambrian gold concentrations are about half this amount. The recent increased applicability of global paleo-reconstructions, coupled with improved geochronology from most of the world's major gold camps, allows for an improved understanding of the distribution pattern of orogenic gold in space and time. There are few well-preserved blocks of Middle Archean mid-crustal rocks with gold-favorable, high-strain shear zones in generally low-strain belts. The exception is the Kaapvaal craton where a number of orogenic gold deposits are scattered through the Barberton greenstone belt. A few >3.0 Ga crustal fragments also contain smaller gold systems in the Ukrainian shield and the Pilbara craton. If the placer model is correct for the Witwatersrand goldfields, then it is possible that an exceptional Middle Archean orogenic-gold lode-system existed in the Kaapvaal craton at one time. The latter half of the Late Archean (ca. 2.8–2.55 Ga) was an extremely favorable period for orogenic gold-vein formation, and resulting ores preserved in mid-crustal rocks contain a high percentage of the world's gold resource. Preserved major goldfields occur in greenstone belts of the Yilgarn craton (e.g., Kalgoorlie), Superior province (e.g., Timmins), Dharwar craton (e.g., Kolar), Zimbabwe craton (e.g., Kwekwe), Slave craton (e.g., Yellowknife), Sao Francisco craton (e.g., Quadrilatero Ferrifero), and Tanzania craton (e.g., Bulyanhulu), with smaller deposits exposed in the Wyoming craton and Fennoscandian shield. Some workers also suggest that the Witwatersrand ores were formed from hydrothermal fluids in this period. The third global episode of orogenic gold-vein formation occurred at ca. 2.1–1.8 Ga, as supracrustal sedimentary rock sequences became as significant hosts as greenstones for the gold ores. Greenstone–sedimentary rock sequences now exposed in interior Australia, northwestern Africa/northern South America, Svecofennia, and the Canadian shield were the focus of gold veining prior to final Paleoproterozoic cratonization. Many of these areas also contain passive margin sequences in which BIFs provided favorable chemical traps for later gold ores. Widespread gold-forming events included those of the Eburnean orogen in West Africa (e.g., Ashanti); Ubendian orogen in southwest Tanzania; Transamazonian orogen in the Rio Itapicuru greenstone belt of the Sao Francisco craton, west Congo craton, and Guyana shield (e.g., Las Cristinas); Tapajos–Parima orogen on the western side of the Amazonian shield; Trans-Hudson orogen in North America (e.g., Homestake); Ketalidian orogen in Greenland; and Svecofennian orogen on the southwestern side of the Karelian craton. Where Paleoproterozoic tectonism included deformation of older, intracratonic basins, the resulting ore fluids were anomalously saline and orogenic lodes are notably, in some cases, base metal-rich. Examples include ore-hosting strata of the Transvaal basin in the Kaapvaal craton and the Arunta, Tennant Creek, and Pine Creek inliers of northern Australia. The Mesoproterozoic through Neoproterozoic (1.6 Ga–570 Ma) records almost 1 b.y. of Earth history that lacks unequivocal evidence of significant gold-vein formation. To a large extent, the preserved geological record of this time indicates that this was a period of worldwide major extension, intracontinental rifting, and associated anorogenic magmatism. Some juvenile crust was, nevertheless, added to cratonic margins in this period, particularly during the growth of the Rodinian supercontinent at ca. 1.3–1.0 Ga. Some early Neoproterozoic dates are reported for important orogenic gold ores within the older mobile belts around the southern Siberian platform (e.g., Sukhoi Log), but it is uncertain whether these dates are correct or, in many cases, are ages of country rocks to the main lodes that may have formed later. Late Neoproterozoic collisions, which define the initial phases of Gondwana formation, mark the onset of the relatively continuous, orogenic gold-vein formation in accretionary terranes that has continued to the Tertiary and probably to the present day. Ore formation first occurred during Pan-African events in the Arabian–Nubian shield, within the Trans-Saharan orogen of western Africa and extending into Brazil's Atlantic shield, within the Brasilia fold belt on the western side of the Sao Francisco craton, and within the Paterson orogen of northwestern Australia. Paleozoic gold formation, accompanying the evolution of Pangea, occurred along the margins of Gondwana and of the continental masses around the closing Paleo-Tethys Ocean. In the former example, orogenic lodes extend from the Tasman orogenic system of Australia (e.g., Bendigo–Ballarat), to Westland in New Zealand, through Victoria Land in Antarctica, and into southern South America. Early Paleozoic gold-forming Caledonian events in the latter example include those associated with amalgamation of the Kazakstania microcontinent (e.g., Vasil'kovsk) and closure of the Iapetus Ocean between Baltica, Laurentia and Avalonia (e.g., Meguma). Variscan orogenic gold-forming events in the middle to late Paleozoic correlate with subduction-related tectonics along the western length of the Paleo-Tethys Ocean. Resulting gold ores extend from southern Europe (e.g., in the Iberian Massif, Massif Central, Bohemian Massif), through central Asia (e.g., Muruntau, Kumtor), and into northwest China (e.g., Wulashan). The simultaneous Kazakstania–Euamerica collision led to gold vein emplacement within the Uralian orogen (e.g., Berezosk). Mesozoic break-up of Pangea and development of the Pacific Ocean basin included the establishment of a vast series of circum-Pacific subduction systems. Within terranes on the eastern side of the basin, the subsequent Cordilleran orogen comprised a series of Middle Jurassic to mid-Cretaceous orogenic gold systems extending along the length of the continent (e.g., Mother Lode belt, Bridge River, Klondike, Fairbanks, Nome). A similar convergent tectonic regime across the basin was responsible for immense gold resources in the orogens of the Russian Far East, mainly during the Early Cretaceous (e.g., Natalka, Nezhdaninskoe). Simultaneously, important orogenic gold systems developed within uplifted basement blocks of the northern (e.g., Dongping deposit), eastern (e.g., Jiaodong Peninsula), and southern (e.g., Qinling belt) margins of the Precambrian North China craton. Orogenic gold veining continued in the Alaskan part of the Cordilleran orogen (e.g., Juneau gold belt) through the early Tertiary, and was also associated with Alpine uplift in southern Europe, and strike–slip events during Indo-Asian collision in southeastern Asia, through the middle, and into the late, Tertiary. The important periods of Precambrian orogenic gold-deposit formation, at ca. 2.8–2.55 and 2.1–1.8 Ga, correlate well with episodes of growth of juvenile continental crust. Similar characteristics of the Precambrian orogenic gold ores to those of Phanerozoic age have led to arguments that “Cordilleran-style” plate tectonics were also ultimately responsible for the older lodes. However, the episodic nature of ore formation prior to ca. 650 Ma also suggests significant differences in overall tectonic controls. The two broad episodes of Precambrian continental growth, and associated orogenic gold-veining, are presently most commonly explained by major mantle overturning in the hotter early Earth, with associated plumes causing extreme heating at the base of the crust. This subsequently led to massive melting, granitoid emplacement, depleted lower crust and resultant extensive buoyant continental crust. The resulting Late Archean and Paleoproterozoic crustal blocks are large and relatively equi-dimensional stable continental masses. Importantly for mineral resources, such blocks are thermally and geometrically most suitable for the long-term preservation of auriferous mid-crustal orogens, particularly distal to their margins. More than 50% of the exposed Precambrian crust formed between 1.8 and 0.6 Ga, yet these rocks contain few orogenic gold deposits, therefore indicating that more than volume of preserved crust controls the distribution of these ores. Despite much of this appearing to have been a time of worldwide extension and anorogenic magmatism in cratonic interiors, significant continental growth was still occurring along cratonic margins (e.g., Albany–Fraser and Musgravian orogens in Australia, growth of North America on southern side of Hudsonian craton, collisions on southwestern margin of Amazonian craton, etc.), culminating with the formation of Rodinia by ca. 1.0 Ga. Beginning at the end of the Paleoproterozoic, however, there was a change in crustal growth patterns, such that juvenile crust began to be added as long narrow microcontinents and accretionary complexes around the margins of older cratons. This probably reflects the gradual change from strongly plume-influenced plate tectonics to a less-episodic, more-continuous present-day style of slab subduction and plate tectonics as a more homogeneous, less layered mantle convection evolved. The long and narrow strips of juvenile crust younger than 1.8 Ga would have been relatively susceptible to continual reactivation and reworking during Mesoproterozoic through Phanerozoic collisions, and the high metamorphic-grade of most 1.8–0.6 Ga crustal sequences indicates unroofing of core zones to the orogens. These schist and gneiss sequences would have been beneath the levels of most-productive orogenic gold-vein formation within most orogens. The distribution of orogenic gold ores formed during the last 650 m.y. of Earth history is well-correlated with exposures of the greenschist-facies mobile belts surrounding 1.8 Ga cratonic masses. Reworking of cratonic margins has eroded away most indications of orogenic gold older than ca. 650 Ma in these crustal belts, whereas younger lode systems are especially well preserved from the last 450 m.y. The immense circum-Pacific placer goldfields collectively suggest a short lifespan for many of the lode systems; veins are apparently recycled into the sedimentary rock reservoir within ≤100–150 m.y. of their initial emplacement if continental margins remain active. Where continent–continent collisions preserved Phanerozoic orogens in a “craton-like” stable continental block (e.g., central Asia) during supercontinent growth, gold lodes (e.g., Muruntau) could be better preserved. The lack of any exposed, large orogenic gold-systems younger than about 55 Ma indicates that, typically, at least 50 m.y. are required before these mid-crustal ores are unroofed and exposed at the Earth's surface.