Page 29 - Handbook of Gold Exploration and Evaluation

12 Handbook of gold exploration and evaluation

Table 1.5 Reported fineness of gold bars from early days of mining in
Victoria, Australia

District Fineness (carat system) Fineness (1000 series)

Ararat 23.0 5 961
8
Ballarat 23.2 5 969
8
Dunolly 23.1 5 965
8
Kingower 22.3 4 931
8
Dry Gully 20.3 2 847
8
equivalent to 24 K in the carat system used by jewellers. A 50% gold alloy is
equivalent to 12-carat gold; 18-carat gold is 0:75 1000 ˆ 750 fine and with a
deeper colour. Alloys of gold with other metals provide mixtures of
correspondingly lower fineness.
The advantage of the fineness system in mine evaluation studies is its
elimination of fractions and decimal places. While gold of say 94.4% purity is
designated 944 fine, it is an unwieldy 22.656 K in the carat system. The
inconvenience of the carat system is demonstrated clearly in Table 1.5 which
compares analyses of gold bars, reported in Fairfax's Handbook to Australia (in
Smyth, 1869), in the early days of mining in Victoria, with the fineness of the
bars as calculated by the fineness method.

Geochemical significance of fineness
Fineness is a rough and sometimes uncertain indicator of deposit type. For
example, higher fineness Au is typically produced in larger grains by deep, high-
temperature, high-pressure mesothermal solutions than by epithermal solutions,
which tend to produce smaller particles of gold of lower fineness. But as a
general model, this is often too simplistic; in addition to pressure differences at
depths of formation, a number of other factors that may contribute to fineness
include:

· the extent of adsorption of contaminating metals such as Ag and Cu along the
flow path of the hydrothermal fluids and the conditions under which they are
precipitated
· gradual facies changes in the mineral composition of the ores, due to merging
of solutions of different composition
· the multi-stage nature of hydrothermal solutions
· reactions of fluids with the wall rock which vary as functions of the state of
sulphur and oxygen in solution at various depths
· superposition of new mineralisation stages
· formation of veinlets of different ages within the same deposit.