Page 225 - Handbook of Gold Exploration and Evaluation

196 Handbook of gold exploration and evaluation

Table 4.1 General order of resistance to mechanical wear of some common
placer minerals

Economic minerals Non-economic minerals

Gold Pyrite
Monazite Plagioclase
Zircon Orthoclase
Rutile Muscovite
Ilmenite Quartz

intensity and duration of the forces tending to break them down. The less
resistant minerals break down quickly to form clays and silts or, if soluble, are
taken into solution in ground waters. More durable rocks and minerals survive
longer, but are still subject to physical disintegration and chemical decay and
disappear progressively with distance of travel and time. The general order of
increasing resistance to wear of some common gold placer minerals is shown in
Table 4.1.

4.1.1 Size
Of the various sediment properties, size is the most important parameter
determining the hydraulic behaviour of sediments in solids-fluid flow. It is also
the most readily measured and other physical properties of sediments such as
shape and density tend to vary with size in a roughly predictable fashion.
Cobbles, gravel and sand comprise the main constituents of streambeds with
cobbles and gravels represented preferentially in the lower layers. Division
between sand and silt occurs at around 62 microns. This approximates the upper
size limit of a quartz sphere settling in still water in accordance with Stokes Law
but does not describe the behaviour of such particles in turbulent flow, as
discussed in Section 4.4.2.
Sieves probably emerged as a means of sizing when man first commenced to
deal with commodities in bulk. References were made to sieving by the Greeks and
Romans around 150 BC in written descriptions of sieves constructed from planks,
hides punched full of holes, and screens woven from horsehair, reeds or human
hair. Sieving was an established procedure in the Middle Ages, although still in a
relatively crude form as illustrated in a sketch by Agricola (1556) (Fig. 4.1).
A number of sediment size classifications have been proposed, of which the
system developed by Udden (1898) and modified by Wentworth (1922) is the
most generally accepted. The Wentworth scale (Table 4.2) envisages five main
size groupings represented by boulders, gravel, sand, silt and clay. It forms a
geometrical progression with 1 mm as the base (1/8, 1/4, 1/2, 1, 2, 4, 8, etc.)
which makes it convenient for plotting and subsequent mathematical treatment.
Free settling particles can usually be sized into fractions, which exhibit