Page 228 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 199

4.2 Phi±micrometre conversion chart. The phi scale converts data that are non-
normal when measured on a simple arithmetic scale to a normal distribution
(after Briggs, 1977).

· Gravels, 2 to 64 mm diameters (ÿ1 ' to ÿ6 ') include fragments of the
parent rock.
· Boulders up to several metres diameter may contain all components of the
original rock.
The sand-gravel class is typical of most gold placers and individual measure-
ments need only be approximated in the field to determine how much oversize
material can be screened out ahead of the gravity concentration plant. Flow-
sheet design depends upon measurement of associated silts and clays. While
fractions of these materials rarely contain economically recoverable gold, they
cause both handling and recovery problems at all stages of mining and
treatment.

Particle size reduction
Particle size reduction is accompanied by increasing specific surface area to
volume relationships. This may be demonstrated by successively reducing a
cube of 1 cm/side into smaller cubes of 0.1, 0.01, 0.001 and 0.0001 cm sides
respectively (Fig. 4.3). Whereas the ratio of volume to surface area for the
1.0 cm/side cube is 1:6, the ratio of volume to surface area of the same cube,