There is a direct link between scientific breakthroughs in gold geology, advances in exploration methods, discovery and the rise of Yilgarn gold production since 1980 (Phillips et al. 2019). The 20,000 or more Archean goldfields worldwide share characteristics which constrain their genesis. A fundamental subdivision is into gold-only deposits with gold as the economic product, and gold-plus with economic base metals. This subdivision is easy to apply, relatively unambiguous, and reflects different ore-forming processes. Gold-only deposits are dominant in Archean cratons and their distribution in provinces reflects the scale of genetic processes. Most Archean greenstone belts contain gold deposits, and some are inordinately rich with linear concentrations of goldfields for 100s km (e.g. Kirkland Lake – Larder Lake fault system, Canada), and districts with multiple large deposits (e.g. Kambalda – St Ives goldfield, Western Australia). The intervening granite batholiths, however, are devoid of gold workings over most of their area except within 0-2 km of greenstone margins. Gold-only deposits represent enrichment of gold by four orders of magnitude, and extreme segregation of base metals from gold. A distinctive feature is the H2O-CO2–H2S fluid recorded from Archean gold-only deposits from most cratons and from gold-only deposits of younger age. Structurally, the gold-only deposits are all later than sedimentation and volcanism (thus synchronous with periods of orogeny), and larger goldfields coincide with areas of structural complexity. The metamorphic devolatilisation model can account for these shared characteristics of Archean gold-only deposits. The model can be conveniently divided into 1) extraction of an auriferous fluid from crustal source rocks, 2) transport of gold in that fluid via structural channel ways, and 3) gold deposition. The source involves greenschist to amphibolite facies metamorphism of mafic or greywacke sequences to generate a metamorphic fluid. The H2O-CO2–H2S fluid reflects the source rock mineralogy and grain-by-grain break down of cubic kilometres of rock mass undergoing metamorphism. This fluid has the capacity to dissolve gold while the fluid forms and provides an effective way to remove gold from large volumes of rock. The transport stage begins on grain boundaries, migrates to shear zones, and may end in faults, breccias, veins or the wall rock alteration halo. Deposition is facilitated by fluid reduction near carbonaceous rocks, or fluid interaction with Fe-bearing wall rocks to precipitate pyrite and gold. The overall appearance of deposits today reflects subsequent modifying events including partial melting and high-grade metamorphism, retrogression, weathering and erosion. These need to be considered before gold genesis can be unravelled.

Reference(s)

Phillips GN, 2022, Formation of Gold Deposits. Springer Nature Singapore. doi.org/10.1007/978-981-16-3081-1. 320 pages 

Phillips, GN, Vearncombe JR, Anand RR, Butt CRM, Eshuys E, Groves DI and Smith RE 2019, The role of scientific breakthroughs in gold exploration success: Yilgarn Craton, Western Australia. Ore Geology Reviews, v. 112. doi.org/10.1016/j.oregeorev.2019.103009