Principais domínios metalogenéticos do ouro: representatividade no tempo geológico

Principais domínios metalogenéticos do ouro: representatividade no tempo geológico, sua distribuição espacial e importância econômica.
The distribution of gold deposits in rocks of the geological column from Archaean to Recent is not uniform but is markedly asymmetrical. During the Earth's historya, the Archaean was the most favourable period for the concentration of gold, due mainly to the multiple combination of tectono-metamorphic and igneous events which gave rise do greenstone belts. Important gold deposits in Archaean greenstone belts are found in almost ali cratonic areas of the planet. Examples in the Superior and Slave Structural Provinces of Canada include the Porcupine, Kirkland-Larder Lake, Noranda Cadillac-Marlartic-Val d'Or, Long Lac, Red Lake districts and the Yellowknife district in the North West Territories. The Homestake mine is an important early Precambrian deposit in the United States. The Kaapvaal and Rhodesian Cratons of Southern Africa contain the deposits of the Barberton, Steynsdorp and Murchison districts as well as many districts in Zimbabwe. The important Kalgoorlie and numerous other gold districts of Western Austrália occur in Archaean greenstone belts of the Yilgarn Block. Finally, the famous Kolar goldfields of lndia occur in Archaean greenstones of the Dharwar system. ln the Proterozoic, gold mineralization are rare, of small size and of low grade (except Witwatersrand), whereas in the Phanerozoic they again become economically important, notably from the Mesozoic onward, with the younger deposits often containing significant silver. An important Mewzoic example is the Mother Lode in Jurassic rocks of Sierra Nevada in California (U.S.A.), where as Tertiary deposits are mainly associated whith porphyry coppers (e.g. Bingham, Utah and several deposits along the Andean Cordilleran), carbonatic rocks (e.g. Carlin, Nevada) and polymetaIlic veins (e.g. Butte, Montana). ln Brazil, similarly to other Archaean terranes, the greenstone belts are the most important source of either primary (e.g. Morro Velho, Raposos, Crixás and Andorinhas) or secondary (e.g. placer deposits of the Eastern Amazon) gold deposits. The Brazilian Proterozoic is represented in terms of auriferous mineralization by the conglomerates of the Jacobina Group, Moeda Formation and Chapada Diamantina as well as by Serra Pelada of the Rio Fresco Formation and the Weber Belt of the Rio Itapicuru Greenstone Belt (BA). ln any metallogenic analysis of an auriferous province whase aim is the understanding af the laws which gaverned the mineralizatians either in time or space, ane must take into accaunt the existence of a multitude of metalIotects, their relationship and intensity of the processes, notably in the case of gold deposits in Archaean greenstone belts.