| Abstract: | Granitoid rocks interspersed with greenstone belts together comprise Archaean cratons throughout the world. The greenstone belts contain a wide variety of volcanic rocks which, despite cyclical variations in composition, generally change from ultramafic komatiites toward the stratigraphic base of the successions, upward through tholeiitic basalts and calc-alkaline andesites, to silici-alkalic rhyodacites toward the top. These extrusive rocks are intruded by rocks of a similar wide compositional range, which are probably comagmatic and subvolcanic to the former. The volcanic rocks are also intercalated with, and flanked by, volcaniclastic and distinctive immature sedimentary strata, including turbiditic greywacke and polymictic conglomerate. All are products of the prolonged volcanism that dominated Archaean supracrustal evolution and metallogenesis.Rare element pegmatites are associated with the Archaean granitic intrusions. Four important types of metalliferous ores, iron-manganese, nickel-chrome, gold-silver and copper-zinc occur in the greenstone belts, often co-regionally with one another in the same mining districts. Algoma type iron-formations of oxide, carbonate, silicate and sulfide facies occur throughout the volcano-sedimentary successions from base suggest common genetic processes for these ores. The Algoma type iron-formations are chemical sedimerare chromite deposits are restricted to the stratigraphically lower, ultramafic komatiites. Important gold ores are hosted primarily in the tholeiitic basalts, particularly where these are intercalated with ankeritic-pyritic chemical sedimentary strata, but smaller gold deposits are also known in stratigraphically lower ultramafic and higher felsic volcanic rocks. The largest massive base metal sulfide deposits occur in the stratigraphically higher felsic rhyodacitic members.The close spatial associations between deposits of these metals in Archaean rocks, particularly those of certain nickel, gold and base metal ores with iron-formation, together with their many similar geological characteristics, suggest common genetic processes for these ores. The Algoma type iron-formations are chemical sedimentary precipitates from ferruginous hydrothermal fluids that were periodically discharged on the sea floor during the prolonged Archaean subaqueous volcanism. The massive base metal deposits are of similar origin, essentially Cu-Zn-rich varieties of sulfide-facies iron-formation. The auriferous cherty, ankeritic or pyritic chemical sedimentary strata were also formed by similar sea floor exhalative hydrothermal activity. Although seldom of mineable gold content themselves, these constituted important, pre-enriched source rocks for later metamorphic generation of gold veins. Although many of the nickel sulfide and chromite bodies are of magmatic generation, others closely associated with iron-formation, and themselves delicately interbedded with cherty or talc-carbonate laminae, may be due to similar sea floor hydrothermal discharge that accompanied ultramafic extrusive activity. Considering their close spatial and genetic links, the occurrence of any one of these four types of deposit suggests the possibility of the others wherever the favourable Archaean host rocks are present.Different Archaean cratons however, have differing proportions of these four types of deposit, and of their distinctive host rocks. Greenstone belts in all cratons throughout the world contain the iron-formations and gold deposits. Greenstone belts of southern Africa and Western Austrialia, however, have more abundant ultramafic rocks and more important nickel-chrome deposits. Some of them may be older than comparable belts in Canada which contain more rhyodacitic rocks and more important copper-zinc ores. Some belts of Brazil and West Africa may be still younger, contain more pyroclastic-volcaniclastic rocks, lack both the nickel-chrome and copper-zinc ores, but contain important manganese in their iron-formations. These relations suggest worldwide diachroneity of Archaean greenstone belt generation, late-Archaean granitic orogeny and ensuing Proterozoic sedimentation. |
