The Relationship of Mineral Deposits to Early Crustal Evolution
Authors: Anhaeusser C. R. 
Published: May 2021

The Archean accounts for approximately half the recorded history of the earth, comprising over 2,000 m.y. of geologic time. As the first ~800 m.y. are virtually lost from the record, events on the surface of the planet can only be speculated over. It is probable that this formative episode was magmatically turbulent and culminated with the final stages of meteorite bombardment ~3,900 m.y. ago. The next events, for which there is geologic evidence, involved the regions now represented by the high-grade gneiss terranes in which the first mineralization was developed. Ore deposits are not well represented in these regions for one or a combination of the following reasons: (1) mineral concentrations may have been recycled or destroyed during the initial turbulent episode of protocontinental development; (2) mineralization may have been destroyed by later tectono-thermal events that characterize the high-grade mobile belt regimes; or (3) mineralizing events or conditions may not have evolved sufficiently for metal concentrations to have formed in significant amounts.

The period of the Archean, beginning ~3,500 m.y. ago and terminating ~2,500 m.y. ago, witnessed the development of the low-grade granite-greenstone terranes of the shield areas. An examination of the principal components of these regions is undertaken following an existing model whereby greenstone sequences are subdivided into a basal unit comprising mainly mafic-ultramafic rock types, followed by an upper mafic to felsic volcanogenic succession, the latter succeeded by an essentially sedimentary group of rocks.

The nature and distribution of Archean mineralization associated with the granite-greenstone terranes is described, and it is shown that there is a strong genetic link between mineral types and rock compositions. Evolutionary trends in the nature and geochemistry of magma types are discussed, and these are considered to reflect changing conditions of heat flow, changes in the nature of the atmosphere and hydrosphere, and progressive modification of the earth’s crust resulting from protocontinental nucleation and development. Magma types ranging from komatiitic basalts and peridotites to calc-alkaline series basalts-dacites-rhyodacites-rhyolites suggest that geotectonic conditions have changed not only within individual greenstone belts but also over the time span of the entire Archean. Available information suggests that the island-arc or trench-back arc-marginal basin system provides the closest modern analogue to the ancient greenstone sequences. However, sufficient differences exist to caution against direct correlation with plate tectonic mechanisms operating at present, and it is suggested that the early Archean greenstone sequences may have been involved more with vertical tectonics in response to gravitational differences resulting from sinking volcanic piles and diapirically rising granitoid complexes. In later Archean times conditions might have approached those currently favored for Phanerozoic orogenesis.

Finally, it is suggested that Archean ore deposits are essentially secondary in origin, having formed mainly as a result of superimposed processes of igneous intrusion, metamorphism, structural disturbance, and chemical alteration acting on suitable host rocks during or after their formation and deposition.