Hydrothermal gold mineralization in the Witwatersrand basin

Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis

“…Large δ 34 S and Δ 33 S isotope heterogeneities in rounded pyrites of single samples as well as the presence of a NMDF signal in many grains cannot be explained by post-depositional pyritisation of detrital Fe-oxide minerals by epigenetic hydrothermal or metamorphic fluids (e.g. Barnicoat et al, 1997). Our data add to the long list of arguments in favour of a detrital origin for rounded pyrite, such as the presence of abrasion features, sedimentological control on pyrite distribution, geochemical heterogeneity, and UPb and Re-Os ages greater than the depositional age (Ramdohr, 1958;Saager, 1970;Hallbauer, 1986;Barton and Hallbauer, 1996;Kirk et al, 2001;England et al, 2002;Minter, 2006).…”

“…Although these positive fractionation factors are opposite to the kinetic isotope fractionation during FeS precipitation, either from Fe(II) aq solutions at room temperature (Butler et al, 2005) or from silicate melt at magmatic temperatures (Schuessler et al, 2007), it may explain the occurrence of positive A different interpretation involves sulphidisation of Fe-oxide minerals for the origin of these grains, as positive δ 56 Fe values have so far been described mainly from BIFs in the Archaean rock record Dauphas et al, 2004;Rouxel et al, 2005;Whitehouse and Fedo, 2007). Evidence for sulphidisation of Fe and Fe-Ti oxides, such as magnetite and ilmenite, collectively referred as "black sands", as well as BIF clasts has been observed in previous studies (Ramdohr, 1958) and resulted in a long-standing debate on the timing of sulphidisation, with the "placerists" arguing for pre-depositional sulphidisation (Ramdohr, 1958;Reimer and Mossman, 1990) and the "hydrothermalists" arguing for postdepositional sulphidisation of "black sands" (Barnicoat et al, 1997;Law and Phillips, 2006). (Yamaguchi et al, 2005).…”

“…Fe-oxide) detrital grains during hydrothermal alteration (e.g. Barnicoat et al, 1997;Phillips and Law, 2000). These workers also regard the gold to be entirely secondary and argue that rounded pyrites cannot be used to support the model of a reducing Archean atmosphere (Law and Phillips, 2006).…”

The coupled geochemistry of Au and As in pyrite from hydrothermal ore deposits

“…The preservation of compositional zoning suggests limited mobilization of trace elements in the pyrite structure, including Au and As, during greenschist facies metamorphism of the sediments, which took place at temperatures of 350 ± 50 o C and pressure ≤ 3 kbar and interaction with hydrothermal fluids (Barnicoat et al, 1997;Philips and Law, 2000). This summary confirms that pyrite is a widespread in a broad range of temperature settings, from sedimentary to magmatic hydrothermal (Craig et al, 1998).…”

When did oxygenic photosynthesis evolve?

“…Banded iron formations could have been produced by photoferrotrophic bacteria ( Widdel et al 1993) or ultraviolet photochemistry (Braterman et al 1986), without requiring environmental oxygen. Uraniferous conglomerates may have been produced by post-depositional hydrothermal activity rather than by detrital sedimentation in anoxic water bodies (Barnicoat et al 1997). Few Archaean palaeosols show conclusive evidence of deposition under reducing conditions (Ohmoto 1996).…”