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The Cover of Mineralium Deposita's Anniversary Volume Uncovered

: Holzing, A.; Frimmel, H.E.; Voland, V.; Dremel, K.; Zabler, S.; Minter, W.E.L.

André-Mayer, A.-S. ; Society for Geology Applied to Mineral Deposits -SGA-:
Mineral resources in a sustainable world : 13th SGA biennial meeting : Nancy – France, 24–27 August 2015. Proceedings
Vandoeuvre-lès-Nancy: ASGA, 2015
ISBN: 978-2-85555-066-4
Biennial Meeting on Mineral Resources in a Sustainable World <13, 2015, Nancy>
Conference Paper
Fraunhofer IIS ()

Non-destructive micro-CT techniques were employed to illustrate in-situ the three-dimensional distribution and morphology of gold particles within a most exceptional specimen of Witwatersrand ore from the 2.87 Ga Basal Reef (lower Central Rand Group), which is depicted on the cover of Mineralium Deposita's 50th Anniversary volume. These images confirm unambiguously the dominance of isolated rounded gold particles which define, as heavy mineral concentrates, three foresets and one bottomset of an overall cross-bedded structure on top of a palaeosurface. Local mobilization of this detrital gold generation by post-depositional fluids is evident from minute, micrometre-scale secondary gold in the immediate vicinity of the, on average, 0.14 mm large micro-nuggets. The study provided not only strong support for the modified palaeoplacer hypothesis for the Basal Reef (and by analogy for most other Witwatersrand ore bodies), but it also revealed that micro-CT is most suitable for unveiling the 3-dimensional distribution and shape of gold particles in ore with a resolution of as good as a few micrometres.