Page 140 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 119

unoxidised zones with arsenical pyrites (mispickel), which occurs in masses of
`beautiful' stellate or radiating crystals known to the miners as `Prince of Wales
Feathers'. The gold content of the mispickel ranges from 50 to 500 oz./t (Kenny,
1928).
Differences in the vertical continuity of gold-bearing quartz vein systems are
commonly reflected in the greater extent and richness of placers derived from
mesothermal orebodies than from epithermal vein systems. Whereas epithermal
ore bodies appear to be restricted to depths of about 600 m, mesothermal gold
veins are characterised by their vertical and longitudinal continuity. They
include the Braline deposit in British Columbia, which extends to depths of at
least 2 km, and the Champian Reef mesothermal quartz-vein system, Kolar
Goldfield, India which is continuous to a depth of 3.2 km with little change in
mineralogy (Hamilton and Hodgson, 1986). Deeply eroded mesothermal ore
bodies in Australia, American Rockies and the Urals of Asia have sourced most
of the major gold placers worldwide. In most of these deposits the veins are
discontinuous and a typical ore shoot structure consists of interspersed zones of
high and sub-ore grade vein material.

2.4.3 Intrusion-related deposits
Most intrusion-related gold deposits are generated above zones of active sub-
duction at convergent plate margins. The margins range from primitive, through
mature island arcs to continental margins. Sillitoe (1993) lists a number of
important types: gold-rich copper and gold-only porphyry deposits, intrusion-
hosted stockwork/disseminated deposits, deposits in carbonate rocks, carbonate
replacement deposits, stockwork disseminated and replacement deposits in non-
carbonate rocks, breccia-hosted deposits and vein-type deposits. He notes that
intrusion-related deposits commonly occur in juxtaposition and are locally
transitional, one to the other.
Magmatic fluids commonly enhance porphyry type concentrations to ore
grade (Hemley and Hunt, 1992). Contributing factors are water, metals
(including gold), ligands (e.g. S and Cl) and other components of mesothermal
intrusion-related porphyry copper deposits. Depths of formation vary generally
from 2 to 5 km according to lithostatic pressures. Porphyry copper deposits tend
to form in clusters and exploration potential may exist for several kilometres
from known deposits. Gold in intrusion-related deposits clearly has the potential
to precipitate both within and at various distances from a progenitor intrusion.
According to Heald et al. (1987) the hydrothermal zoning is a function of
temperature/concentration relationships. Zoning reversals are thereby suggested
as the result of differences in the relative concentration of metals.
Gold-only porphyries, such as at Fort Knox, Alaska appear to occur at higher
levels than Cu-Au porphyry deposits, which are themselves at higher levels than
Cu-Mo porphyritic intrusives as, described schematically in Fig. 2.24. The fluids