Its great to see this topic ‘live’. Having moved into a different branch of 
applied geoscience in recent years, I still find the Witwatersrand debate a 
lively and interesting place to visit.  

Personally, I tend to agree with Steve Micklethwaite’s messages.  It seems 
there's a weight of evidence in favour of a hydrothermal origin for gold in 
the basin’s famous goldfields. The syn-kinematic fluid transport and 
precipitation mechanisms - and the nature of syn-kinematic, dynamic 
permeability have now been documented in some detail.  Nick Fox also made a 
convincing argument about the distance of hydrothermal gold transport 
exceeding the known width of goldfields... There is of course still much to 
learn, but if I may make a few comments from the perspective of our 
particular group of researchers - principally RDR, Leeds and Anglo 
(Barnicoat et al and Jolley et al authored papers below):

We looked at the seismic-to-mesocale structural context, microstructural 
siting and paragenesis of gold and associated features and minerals.  Gold 
particles were seen to occupy a variety of structural and structural-
paragenetic sites, related to thrust-fracture opening, that simply were not 
present at the time of sediment deposition.  Hydrocarbon (pyrobitumen), 
uraninite and gold can be shown to have been emplaced and to have 
precipitated along with other mineral phases syn-kinematically into this 
low strain thrust-fracture network.  The thrust-fracture networks are 
related to metre-scale thrust imbricates, which are in turn related to deco-
metre to seismically imageable thrusts and long wavelength flexures.  The 
meso-scale thrusts and related gold-bearing fractures tend to be nucleated 
within mechanically coupled fracture-prone lithologies, and along 
sedimentological features and other mechanically significant planes in the 
host rocks.  For example, in the ‘Ventersdorp Contact Reef’, economic gold 
grades tend to be concentrated into a 1-2 m thick band immediately beneath 
the base Ventersdop lavas contact (a gently angular unconformity at the top 
of the Witwatersrand) – where fracture-prone sediments are developed – 
regardless of the depositional age of the sediments which sub-crop against 
the lavas.  Thus the gold mineralization follows the mechanical layering 
and the thrust fracture network that developed in response to it – to 
produce a slab-like orebody which crosses depositional surfaces and ‘time 
zones’ beneath the unconformity.

We were able to differentiate several phases of thrusting and syn-kinematic 
hydrothermal gold emplacement during the Witwatersrand to early 
Klipriviersberg period.  We found evidence for regional cross-printing 
between several structural assemblages, and found that some gold-bearing 
structures and ore-zones were truncated at major unconformities in the 
Witwatersrand sequence.  However, whilst we felt the deformation and syn-
kinematic mineralization process was broadly progressive, we were unable to 
establish how much time had elapsed between ‘pulses’ or when the first or 
last pulse might have been.  That some of the hydrothermal gold particles 
might have been liberated from these ores and re-deposited between syn-
kinematic hydrothermal ‘pulses’ - as discrete detrital particles or as 
grains contained inside derived lithic fragments (pebbles) - would seem 
logical.  However, it seems that finding unequivocally detrital grains has 
proven to be extremely difficult.  SEM and Cathodoluminescence studies 
reveal the full detail of the textural relationships and mineral 
compositions of the microstructural siting of gold grains, and show these 
to have secondary hydrothermal characteristics.  Optical microscopy does 
not reveal much of the detail.  It follows that the technique of liberating 
grains from the rock by dissolving the sample and selecting grains that 
look ‘peened’ only uses a small percentage of the data - and clearly 
removes all textural information about the context and origins of the gold 
grains chosen from the total population.

Underground observation shows that the thin hydrocarbon (pyrobitumen) seams 
that have been discussed here, are in fact hosted by low strain thrust-
fracture planes, which gently transgress depositional layering.  They are 
seen to mostly follow bedding – but also to clearly cross-cut bedding, 
pebbles and sedimentological features.  Gray et al (1998) describe the 
morphology and origins of these ‘seams’ and related ‘flyspeck’ pyrobitumen –
 and show that they have a mesophase hydrocarbon structure composed of 
filament-like ‘spindles’ which span the fracture width.  The pyrobitumen 
contains uraninite grains (from uranyl ions in oxidizing fluids, with oil 
acting as a reducing agent to foster precipitation) – with a radial 
mesophase structure around the uraninite grains showing preferred 
orientation of minimum reflectance.  This indicates contemporary mesophase 
structure development and uraninite precipitation within a NW-SE orientated 
deviatoric stress (consistent with local thrust transport vectors).  These 
features are indicative of hydrocarbon emplacement and rapid heating under 
overpressured conditions during fracture propagation, not passive slow 
burial-related baking of algal mats.

best regards to all,

Steve Jolley
Snr Structural Geologist

Shell UK Ltd, 1 Altens Farm Road, Nigg, Aberdeen, AB12 3FY, UK.
Tel: +44 1224882000 1269  Email: [log in to unmask]


Barnicoat, A.C., Henderson, I.H.C., Knipe, R.J.et al. 1997. Hydrothermal 
gold in the Witwatersrand Basin. Nature 386, 820–824.

Barnicoat, A.C., Yardley, B.W.D., Henderson, I.H.C., Fox, N.P.C., 1999. 
Discussion of ‘Detrital origin of hydrothermal gold’ by H.E. Frimmel. Terra 
Nova 10, 347–349.

Barnicoat, A.C., Phillips, G.M., Law, J.D.M. et al. 2000. Refuting the 
irrefutable: a new look at a well-known sample of Witwatersrand gold 
mineralisation. Economic Geology Research Unit James Cook University, 
Contribution 59, 16–17.

Fox, N.P.C., 2002. Exploration for Witwatersrand deposits and analogues. 
In: Cooke, D.R., Pongratz, J. (Eds.), Giant Ore Deposits: Characteristics, 
Genesis and Exploration. CODES Special Publication 4, 243–269.

Gray, G.J., Lawrence, S.R., Kenyon, K., Cornford, C., 1998. Nature and 
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of the Geological Society, London 155, 39–59.

Jolley, S. J., Henderson, I. H. C., Barnicoat, A. C. & Fox, N. P. C. 1999. 
Thrust-fracture network and hydrothermal gold mineralization: Witwatersrand 
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(eds) Fractures,Fluid Flow and Mineralization. Geological Society, London, 
Special Publications, 155, 153–165.

Jolley, S. J., Freeman, S. R., Barnicoat, A. C., et al. 2004. Structural 
controls on Witwatersrand gold mineralisation. Journal of Structural 
Geology, 26, 1067–1086.

Jolley, S.J., Stuart, G.W., Freeman, S.R. et al. 2007. Progressive 
evolution of a late-orogenic thrust system, from duplex development to 
extensional reactivation and disruption: Witwatersrand Basin, South Africa. 
In: Ries, A.C., Butler, R.W.H. & Graham, R.H. (eds) Deformation of the 
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