Page 129 - Handbook of Gold Exploration and Evaluation

108 Handbook of gold exploration and evaluation

Table 2.7 Approximate ranges of permeability of strata

Rock type Permeability
(Darcy units)

Clay, shale or dense rock with tight fractures 0.0001
Dense rock with few tight fractures 0.001
Dense rock, 0.13mm fractures 0.5
Silt or fine sand 1
Dense rock and highly fractured 2
Sand 500
Gravel 1250

world. Wotjik (1984) (Table 2.8) lists 17 fields in which at least one of the above
rocks is represented. Sedimentary interceptors have been recognised in all but
three of the seventeen districts.
Whilst a mix of good physical and chemical properties is clearly desirable,
very low grade source rocks can host major gold deposits given sufficiently
large volumes of flow and time for the reactions to take place. The required
volume of flow to leach and transport specific amounts of gold in solution can
only be approximated because of factors such as dispersion into underground
aquifers, but is clearly of enormous proportions. Some indications of flow rates
have been obtained from measurements of total discharge of fluid from modern
hot spring gold-bearing systems. Estimates of discharge rates approximating
75 kg/s from the Broadlands geothermal system, New Zealand (Heinrich et al.,
1989b) suggest that the system could have transported a minimum of 2.3 million
ounces of gold to the surface during the past 10,000 years. Since only about one-
tenth of large-scale fracturing this can be accounted for, the remainder could be
disseminated in the subsurface 200±300 m of the system or perhaps located in
some fracture zones not tested by drilling.
Until recently, deposition was believed to be a slow process. Radiometric
studies show that millions of years separate the formation of host volcanic rocks
from subsequent mineralisation. For example, host rocks in the Hauraki region
of New Zealand are dated at 17 Ma whereas hydrothermal alteration associated
with the mineralised veins gives an age ranging from 2.5 to 7.0 Ma (Robinson,
1974). Considerable evidence now supports the episodic nature of epithermal
ore deposits thus suggesting that the ore forming fluids responsible for
mineralisation may be present for only short periods in the very long life of a
hydrothermal system. These periods probably correspond with specific tectonic
or hydrothermal events (Hedenquist and Lowenstern, 1994).

2.3.3 Alteration assemblages
Paragenetic relationships also extend to alteration assemblages for which the
main control appears to be the nature of the host rock lithology. Such relation-