Page 294 - Handbook of Gold Exploration and Evaluation

260 Handbook of gold exploration and evaluation

critical entrainment velocity. Particles much smaller than 0.01 mm, once
entrained, do not settle freely. Dust-sized particles are swept up to very great
heights and may be transported for hundreds or thousands of kilometres before
being washed out of the air by raindrops, perhaps to be deposited as beds of
loess. King (1966) notes that the mean particle diameter of thick loess deposits
as in China, is about 0.05 mm, i.e. coarse silt in the Wentworth scale of sediment
size classification.
The depth limitation of the weathering profile of recently exposed source
rock in hot dry conditions is a few metres at most. The volume of gold-bearing
detritus is typically small and its value rests mainly as a pointer to the possible
size and value of the primary orebody and/or to the possible presence and
whereabouts of palaeo-placer deposits formed previously under more humid
climatic conditions. For example, although evidence of gold mineralisation is
widespread in Saudi Arabia, a regolith of shifting sands that covers most of the
landscape has a masking effect on the geochemical indication of primary gold
and fluvio-aeolean palaeochannel development on a regional scale. Possibly
because of this, only small-scale gold mining activities appear to have been
carried out in much of the western part of the Kingdom in pre-Islamic days. In
the Murayjib area, gold placer workings in Wadi Haradah can be traced for a
distance of about 7 km from source to larger workings at Efshaigh adjacent to
Wadi Kohr but this type of occurrence is rare. Recent checking has shown that
the ancients were very thorough in their treatment of the surface materials and
shallow channels, but a great deal more remains to be done in locating major
alluvial and primary gold mineralisation.

Fluvio-Aeolean settings
Fluvial-aeolean gold placers are formed in semi-arid environments as the result
of heavily concentrated, though ephemeral, stream flow over short and inter-
mittent periods of time. Although rainfall rates are low and sporadic in desert
regions, running water is essential for the accumulation of commercially viable
gold placer concentrations. The general absence of plant cover over exposed
rock formations in deserts provides for high rates of run-off over the stony desert
surface and fans grow in stages as sites of deposition change from one side of the
fan to the other. Sedimentation is cyclic and climatically controlled and placers
exhibit definite sediment patterns of sorting, rounding and particle distribution
according to weight, rate of flow and channel gradient.
An elevated area of source rocks intermittently eroded by short ephemeral
streams is envisaged by Prudden (1990) in the construction of a conceptual
geological model of fluvio-aeolean placer formation (Fig. 4.37) from the gradual
release of gold from the weathering of exposed source rock. Tributaries draining
down from these deposits join together at lower levels to form a larger integrated
channel system into which the intermittently flowing streams discharge their