Precambrian subduction onset: a porphyry copper perspective

Porphyry Cu ± Mo ± Au deposits of Phanerozoic age formed during either active subduction or subduction-induced collision. Although porphyry deposits of Precambrian age are relatively uncommon, 23 authenticated and well-dated examples are reviewed to determine what they may reveal about the timing of subduction onset. Several Archean porphyry deposits and prospects, including the oldest at 3.3 Ga, are associated with tonalite–trondhjemite–granodiorite intrusions, which are commonly, but by no means unanimously, assigned to a single lid tectonic regime predating modern-style subduction. Most Precambrian porphyry Cu deposits and prospects formed in three time periods (2.8–2.5, 2.2–1.7 and 0.9–0.6 Ga) when, based on the redox-sensitive trace element contents of sedimentary pyrite, atmospheric O 2 and marine SO 4 2− contents were elevated. This could have been because subducted slab-derived fluids containing SO 4 2− are required to generate the relatively oxidized magmas essential for porphyry Cu formation. Porphyry Cu deposits indistinguishable from Phanerozoic examples, and therefore very likely subduction-related, first appeared at c. 2.2 Ga. A notable gap in porphyry Cu formation from 1.73 to 0.88 Ga, except for a single occurrence linked to intraplate mantle plume activity at 1.1 Ga, broadly coincides with the Earth's middle age when marine SO 4 2− contents may have again been depressed.