Hydrothermal alteration in the Witwatersrand goldfields

Phillips, Neil; Powell, Roger

doi:10.1016/j.oregeorev.2014.09.031

Cited by 34 publications

(32 citation statements)

References 53 publications

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“…Mathur et al, 2013) and hydrothermal (e.g. Phillips and Powell, 2015). Gold is highly susceptible to remobilisation and recrystallization during weathering due to its soft nature (Stewart et al, 2017).…”

Section: Accepted Manuscriptmentioning

confidence: 99%

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

et al. 2019

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The Quadrilátero Ferrífero in the São Francisco Craton, Brazil is known for its world-class gold deposits hosted by the Rio das Velhas greenstone belt (RdV). As in other greenstone belts, gold is also concentrated in late Archean/early Paleoproterozoic supracrustal units; in this case within the Moeda Formation. This unit has been compared to the Witwatersrand basin where there has been a long-term debate over the nature of the gold and whether it is detrital (placer), hydrothermal or hybrid (modified placer). Presently, links between gold in the Moeda Formation and RdV are tenuous. To enlighten this area of ambiguity, a new approach using chemical, isotopic and mineral inclusion data from detrital rutile found in the gold-bearing horizons of the Moeda Formation is presented. Grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile, related to As-Fe-Cu-Sb-Pb-W rich mineralising fluids. Fe, Nb and V indicate that the likely source of the gold-related rutiles is the Archean banded iron formations within the RdV, where most of the lode-gold occurs. Average Nb/Ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and/or competing mineral phases. U-Pb data yield primary crystallisation ages older than deposition ages (> 2.58 Ga), supporting the detrital nature of these rutiles. Rutiles record

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Section: Accepted Manuscriptmentioning

confidence: 99%

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

et al. 2019

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“…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.…”

Section: Resultsmentioning

confidence: 53%

“…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).…”

Section: Introductionmentioning

confidence: 99%

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

Nwaila

Manzi

Kirk

et al. 2019

The Journal of Geology

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A B S T R A C TThe source of gold in the ca. 2.66 Ga Black Reef Formation (BRF) has been investigated and constrained through petrographic, mineralogical, geochemical, and high-resolution three-dimensional reflection seismic data combined with drill core and underground geological mapping. The BRF is a strong seismic marker and consists of carbonaceous shale, quartz arenite, and conglomerate. Gold grade in the BRF is primarily controlled by the nature of the host conglomerates. Most of the gold in the BRF conglomerate occurs in native form, and its morphology is highly heterogeneous. Gold was initially introduced through mechanical recycling of underlying Witwatersrand reefs, followed by short-range (millimeter-to centimeter-scale) postdepositional alteration/remobilization associated with the Bushveld Complex and the Vredefort meteorite impact. Although the BRF was subjected to high postdepositional fluid circulation facilitated by high fracture density, the volume of dissolved gold was probably too small to form a large gold deposit, except in areas around the Black Reef/Witwatersrand reefs subcrop positions. Findings from this study demonstrate the importance of both sedimentological controls and impact-related structures in the formation of paleoplacer gold deposits during Neoarchean times.

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“…This remains a possibility based on the age data presented here. However, an interpretation of gold source in the Witwatersrand is entirely dependent on the conceptual model for controls on Witwatersrand mineralisation as a modified syngenetic placer (Frimmel & Minter, 2002;Frimmel et al, 2005;McCarthy, 2006) or as later structure-and rheologycontrolled hydrothermal deposits (Jolley et al 2004;Phillips & Powell 2015). The…”

Section: A Potential Source Of Witwatersrand Gold?mentioning

confidence: 99%

Age of the Archaean Murchison Belt and mineralisation, South Africa

Vearncombe¹,

McNaughton

Porter

et al. 2020

South African Journal of Geology

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The east-northeast-trending Murchison-Thabazimbi Lineament in northern South Africa is one of the world’s most important structures for its control on world-class mineral deposits, Proterozoic sedimentary basins and giant igneous intrusions. The deepest exposed Archaean parts of the lineament are the Murchison Belt. Bounded by granitoids, the belt comprises greenschist to amphibolite facies volcano-sedimentary strata with isoclinal folds and the 7 km thick meta-igneous Rooiwater Complex. The Rooiwater Complex is intruded by a northern regional granitoid dated at 2 929 ± 7 Ma by SHRIMP U-Pb on zircons. Using field relationships, published isotopic age data and new SHRIMP zircon dates we confirm the age Rooiwater Complex at 2 965 Ma, showing it to be contemporaneous with the Archaean volcanic and sedimentary formations, the meta-igneous Complex being the lower sequence in a ~2 980 to 2 960 Ma island arc. Despite being implicated as a source of gold for the world’s largest natural accumulation of gold in the Witwatersrand Basin, the absolute age of Sb-Au mineralisation in the Murchison Belt is poorly constrained. We have utilised SHRIMP U-Pb geochronology to date monazites from a Sb-Au ore sample from the granitoid-hosted Malati Pump orebody and determine ages for two different generations of monazite, both associated with ore minerals. The older age of 2 832 ± 23 Ma is from a minority of grains and is interpreted to date the primary Sb-Au mineralisation, about 120 Ma after belt formation. This age predates, or is possibly synchronous with, sedimentation of the upper-Witwatersrand Central Rand Group. The younger age of 1 968 ± 17 Ma from a majority of monazite grains is unrelated in time to known events and interpreted here as a cryptic hydrothermal reworking of the Sb-Au ores in this deposit.

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Hydrothermal alteration in the Witwatersrand goldfields

Cited by 34 publications

References 53 publications

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

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

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

Age of the Archaean Murchison Belt and mineralisation, South Africa

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