Page 30 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 13

In fact, high fineness gold may occur in a wide range of rock types ranging
from ultramafic through mafic and dioritic types to the granites, related
porphyries and high sulfidation epithermal deposits. Gold in gold-rich porphyry
deposits is mainly fine grained, less than 60 m in size and is generally present
as high-fineness native metal (Sillitoe, 1993). Of particular importance are high-
fineness gold-bearing quartz veins derived from granitic rocks, often found in
intruded rock (e.g. carbonaceous slates) well away from the parent intrusive.
High fineness gold is also found in carbonate veins, particularly along con-
tinental margins and in gold-only veins in some skarn deposits, e.g. the Suian
District, North Korea (Watanabe, 1943).
Regionally, some authors suggest that the nature of the basement may have
some influence on fineness. Deposits that occur within the cratonic part of
Guatemala and Honduras in the Central American epithermal gold belt are
uniformly rich in silver while those of Costa Rica, Panama and Nicaragua,
which are not underlaid by the craton are uniformly rich in gold. Similarly in
North America, the deposits of Nevada and Colorado, which formed in cratonic
sialic crust are pre-eminently silver-rich (Hutchison, 1985) whereas those lying
outside the craton in California are gold-rich. From this he concludes that `silver
is of continental origin, whilst gold is of oceanic or mantle origin'.

1.1.5 Gold grain morphology

The morphology of a grain of gold is inherited from its primary state and to a
large extent, irregularities of gold grains in source rocks predetermine grain
morphology in an alluvial setting. Gold is one of the last minerals to crystallise
out under hydrothermal conditions of deposition and thus tends to fill cracks and
spaces between other minerals with which it comes into contact. The gold grains
are moulded by the geometry of the opening into aggregates of irregular shape
and size and commonly contain inclusions of quartz and other rock forming
minerals. The Blanch-Barkly nugget (1,743 oz.) found at Kingower, Victoria
Australia contained 2 lb of quartz, clay and iron oxide. In the Oso Peninsular,
Costa Rica, Berrange (1987) describes flakes of gold less than 0.5 mm diameter
containing quartz inclusions. He noted the presence, in both the vuggy interiors
of quartz veins and smoother surfaced particles, of a variety of inclusions of
syngenetic quartz, calcite, epidote and pyrite, together with adventitious Fe-Mg
silicates, spinels and limonite veins picked up and included in the particles
during transport. Micro-inclusions, although mainly quartz, may also be
represented by heavy minerals such as ilmenite and corrundum.
Although physically, gold grains may be crystalline (octahedral, rhombo-
dodecahedral or cubic) they occur more frequently in other forms. These forms
include aggregates of irregular shaped grains in quartz and other hydrothermal
minerals; reticulated dendrite, filiform, spongy massive and scales. Size, shape
and surface texture are strongly influenced by their depositional environment