Abstract

Silver isotopes are a potential tracer for understanding the geochemical enrichment of gold, which is monoisotopic but occurs in metallic form with silver as a stable alloy component. This study applies high-precision measurements of silver isotope ratios to two of the oldest world-class hydrothermal gold vein deposits from the Archean (3.55–3.10 Ga) Barberton greenstone belt of the Kaapvaal craton in South Africa. Published structural-geologic and geochronological results show that the hydrothermal gold deposits formed late in the tectono-metamorphic history of the greenstone belt, well after major crustal deformation and regional metamorphism, probably postdating even postorogenic granitic intrusions. Gold is concentrated in extensional brittle-ductile structures, indicating a common extensional stress field during mineralization. Orebodies occur close to the stratigraphic contact between older (ultra-)mafic submarine lavas (the Onverwacht Group) and overlying carbonaceous siltstones and graywackes (the Fig Tree Group), marked by a variably tectonized former chert. Observations at New Consort (higher temperature, close to granitoid contact), Fairview, and Sheba (lower temperature in the interior of the greenstone belt) confirm the structurally late introduction of gold into veins and sulfide replacement orebodies.

Chemical and isotopic analyses were performed on native gold from Barberton, on two other lode gold deposits and two carbon leader-type samples from the Witwatersrand, and on native silver from various epithermal silver deposits. All native gold samples contain significant (1–12 wt %) Ag but only trace levels of other metals such as Cu or Hg. Stepwise leaching experiments show that native gold is the main carrier of silver even in sulfide-rich ores. Different gold samples from Barberton yield ε109Ag values (differences in 109Ag/107Ag relative to NIST SRM 978a in parts per 10,000) from −4.2 ± 0.4 to 3.6 ± 0.4, encompassing almost the entire range of reported terrestrial Ag isotope variations and all hydrothermal silver samples. Gold samples from New Consort have negative ε109Ag values, whereas samples from Sheba have variably positive ε109Ag values.

Isotopically different metal sources of the structurally coeval deposits at Barberton are less likely than a process of isotopic fractionation during transport and ore deposition. The observed isotopic difference can be explained by reduction of dissolved Ag+ complexes in the ore fluid to native Ag0 in the gold alloy. Assuming that the oxidized Ag+(aq) is enriched in the heavy isotope compared with the reduced metallic form, as in other heavy metal systems, the variation in ε109Ag between the two deposits can be explained by fractional precipitation of precious metals from a fluid that enters the greenstone belt from an external source, in line with published geologic arguments. Following a Rayleigh distillation model, a first fraction of silver that precipitated close to the contact of the greenstones against surrounding granitoids is isotopically light, as observed at New Consort. Residual silver reaching the interior of the greenstone belt eventually precipitates with a high ε109Ag, as seen at Sheba.

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