Orogenic gold and geologic time: a global synthesis / Орогенные месторождения золота и геологическое время: глобальный синтез

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 rustal 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 Ž . 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 Ž . 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 Africarnorthern 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 Ž . orogen in the Rio Itapicuru greenstone belt of the Sao Francisco craton, west Congo craton, and Guyana shield e.g., Las Ž Cristinas ; Tapajos–Parima orogen on the western side of the Amazonian shield; Trans-Hudson orogen in North America . Ž . 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 <...>