A Giant Mesoarchean Crustal Gold-Enrichment Episode<subtitle>Possible Causes and Consequences for Exploration</subtitle>

Two billion years of evolution enclosed in hydrothermal rutile: Recycling of the São Francisco Craton Crust and constraints on gold remobilisation processes

“…Witwatersrand, South Africa; Jacobina, Brazil; Huronian, Canada; Sierra Nevada, USA). In 2013, approximately 30% of global gold production (about 52 000 metric tons) was mined from placer districts worldwide (Frimmel, 2014;Lipson, 2014). Placer gold has historically been envisaged as an indicator of significant ore deposits in the basement source (e.g.…”

Recycling of Paleoplacer Gold through Mechanical and Postdepositional Mobilization in the Neoarchean Black Reef Formation, South Africa

“…The major and minor minerals present in the conglomerate samples are quartz, muscovite, chlorite, monazite, pyrite, chromite, galena, sphalerite, gold, uraninite, brannerite, zircon, and pyrobitumen. This mineralogical assemblage is similar to that of the Witwatersrand and Ventersdorp Supergroup rocks (Ramdohr 1958;Minter 1976;Phillips and Powell 2015;Frimmel 2018). Pyrobitumen, which is dispersed through the rock, fills microfractures in quartz pebbles, and occurs disseminated in quartz cement, is a specific feature of the BRF.…”

“…Much of the BRF gold is concentrated in the matrix of oligomictic conglomerates, cross beds, and scour surfaces within fluvial channels that truncates the underlying reefs. The mechanical recycling of Archean gold in sedimentary rocks has been noted in the gold deposits of the Witwatersrand reefs (Frimmel 2018) and in post-Archean gold deposits (i.e., Huronian gold deposits in Canada) because of truncation and reworking of underlying stratigraphically older units (Whymark and Frimmel 2018). It is envisaged that mechanical reworking released gold particles from the underlying reefs, which were then concentrated in the BRF along subcrop positions.…”

“…Progressive depletion of minable gold reserves in the Witwatersrand Basin, South Africa, has triggered a significant amount of research on potential replacements. Several Witwatersrand-type gold occurrences exist elsewhere, such as the Huronian Basin in Canada, the Bababudan Basin in India, the Moeda Formation in Brazil, and the post-Witwatersrand Transvaal Basin in South Africa (Frimmel 2014). Witwatersrandtype gold deposits share certain similarities, such as the style of mineralization, granitoid-greenstone provenance, and the fact that they are almost ex-clusively hosted by conglomerate horizons commonly referred to as reefs.…”

“…According to Sibson and Scott (1998) and Blenkinsop and Doyle (2014), structurally controlled stratabound gold mineralization has three main characteristics: (1) local shear zones control the orientation of the ore body; (2) gold mineralization is governed by fluid flow in a network of fractures and veins formed in response to a regional stress field; and (3) fault zones and their intersection points impart a directional permeability, so that the mineralizing fluids are channeled along dilatant zones. Almost all Precambrian basins have been affected by postdepositional alteration, affecting the chemical composition of the host rocks, and, in some places, the alteration can be linked to coeval gold mineralization and remobilization (Wallmach and Meyer 1990;Frimmel 1994;Phillips and Law 1994;Phillips and Powell 2015;Fuchs et al 2016). In a hydrothermal system, the occurrence of gold mainly depends on tempera-ture, pressure, pH, Cl 2 concentration, and fugacity of H 2 S (Williams-Jones et al 2009).…”