Episodic concentration of gold to ore grade through Earth's history

Concentration of gold to economic ore grades has been highly episodic through Earth's history with different sedimentary, magmatic and hydrothermal processes having operated as principal driving forces at different times. Comparison of the various types of gold deposits, their secular and spatial distribution and the amounts of gold contained in them, combined with thermodynamic considerations, reveals that the formation of gold deposits reflects fundamental changes in environmental conditions, biological evolution and styles of large-scale tectonism over the past four billion years. Here I suggest that most of the gold in the Earth's crust was first concentrated at around 2.9 billion years (Ga) ago by early photosynthesising microbes under an oxygen-deficient atmosphere. Sedimentary reworking of auriferous microbial mats provided the source for the richest known gold palaeoplacers between 2.9 and 2.7 Ga and an overall gold-enrichment of coeval marine sediments. Only after modern-style plate tectonics had begun to operate, sediments enriched in gold had been accreted along active plate margins, subducted and subcontinental lithospheric mantle had been correspondingly metasomatised, did the formation of larger orogenic, porphyry-type and epithermal, and to a lesser extent also other hydrothermal gold deposits become possible. Thus the principal secular peak in orogenic gold formation at 2.75-2.55 Ga can be explained.