Page 232 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 203

dimensional shapes (e.g., sphericity and roundness). Sphericity is defined as the
ratio of the surface area of a sphere having the same volume as the particle to the
surface area of the particle. Roundness is the ratio between the radius of
curvature of the particle and that of an inscribed circle and is controlled largely
by the type and extent of weathering, thus being an indication of wear.
Angularity is essentially due to fragmentation. Since spherical particles have the
lowest resistance to transport, sphericity is adopted as the standard against which
irregularly shaped particles can be compared.
Most sizing analyses in gravity concentration plant are classified by sieve
aperture size and not by projected area diameter. British Standard BS3406 (Part
4, 1963) suggests multiplying sieve aperture size by a factor of 1.40 to obtain
projected area diameter, but this factor should be applied with caution if the
particles are of very irregular shape. Two other shape factors that have gained
limited practical acceptance are the Corey and Heywood Shape factors. Of these,
the Heywood factor is used mainly in mineral processing applications and is
dealt with in Chapter 8.

Corey shape factor
The Corey factor S f regards each particle as being represented by an ellipsoid of
the same general proportions as the particle and is defined as:
0:5
S f ˆ T= LB† 4.3
where T is the thickness of the particle, L is the length, and B the breadth. This
factor is determined by direct measurement of the three principal axes. The
sphere L ˆ T ˆ B†, which has a factor of unity, is adopted as the standard for
settling.
By definition, all other shaped particles have factors less than unity with
settling rates decreasing according to their degrees of departure from unity. In
practice, the Corey factor works reasonably well for river gravels and sand
grains that are relatively equant in shape, and for coarse nuggety gold. The
method suffers a number of constraints in the finer sizings, particularly when
related to the settling of fine and flaky gold. Surface area:volume ratios increase
with decreasing size, and viscous drag rather than density becomes the dominant
factor influencing settling below about 100 microns. Borehole sample
measurement is extremely labour intensive and the task of testing a statistical
population of gold grains from most test programmes by the Corey method
would probably not be economically feasible.

Weight-size factor
A different type of shape factor used in parts of Asia is related to average sieve
sizes of gold grains. Figure 4.6 shows the relationship between this shape factor