Page 58 - Introduction to Mineral Exploration

3: MINERAL DEPOSIT GEOLOGY AND MODELS 41

100 Glacial till
lb –1 ) 90 Other types of deposits Sandstone ● ● ● ● ● ● ● ● Unconformity ●
Volcanic hosted
massive sulfide deposits
C1 production cost 1994 (US/¢ 70 hosted deposits Porphyry ● ● ● ● ● ● ● ● ● ● ● ● ● Limit of ●
●
●
●
●
80
●
●
Sedimentary
●
60
50
40
copper deposits
alteration
30
20 hydrothermal
0 1 2 3 4 5 6 7 8 9 10 Basement
Cumulative production (billion lb) rocks
Carbon-bearing rocks,
FIG. 3.10 Cost curves for copper production. schists, marbles, etc.
The C1 cost is the cash operating cost. Note the
predominance of porphyry copper production. Fault or shear zone
(After Moore 1998.)
Massive ore
0 100 m
Fracture
producing from porphyry copper deposits and controlled ore Approximate scale
the economies of scale from large production
relative to sedimentary copper and volcanic-
associated massive sulfide deposits. FIG. 3.11 Generalized diagram of an unconformity-
To understand how geological models are associated uranium deposit. (After Clark et al. 1982.)
constructed we will examine the deposit model
for unconformity-related uranium deposits.
These deposits are currently the main source of Table 3.1 shows the elements that were
high grade uranium in the Western world and used by Eckstrand and Cox and Singer in
mainly occur in two basins; the Athabasca their unconformity-related deposit models.
Basin of Northern Saskatchewan and the Both accounts agree that the deposits occur at
Alligator River area of the Northern Territory or near the unconformable contact between
in Australia. Good accounts of most deposit regionally metamorphosed Archaean to Lower
models can be found in two publications by Proterozoic basement and Lower to Middle
North American Geological Surveys, Cox and Proterozoic (1900–1200 Ma) continental clastic
Singer (1986) and Eckstrand (1984), later re- sediments, as shown in Fig. 3.11. The details
vised as Eckstrand et al. (1995). Cox and Singer of the controls on mineralisation are more sub-
adopt a strictly descriptive approach, based on jective; the Canadian deposits occur both in the
their classification of mineral deposits (see sec- overlying sandstones and basement whereas
tion 3.6), but also give grade-tonnage curves the Australian deposits are almost exclusively
and geophysical signatures whereas Eckstrand in the basement. Most deposits in Canada are
is more succinct but gives brief genetic models. spatially associated very closely with graphitic
Some major deposit types are also dealt with in schists whereas the Australian deposits are
depth by Roberts and Sheahan (1988), Kirkham often in carbonates, although many are car-
et al. (1993), and a special edition of the Aus- bonaceous. Both models agree that the key
tralian Geological Survey Organisation journal alteration is chloritization together with seri-
(AGSO 1998). Internet versions of the USGS citization, kaolinization, and hematitization
deposit models and a more extensive classifica- along the intersection of faults and the un-
tion by the British Columbia Geological Survey conformity. Generalization on the mineralogy
are available at their web sites (BCGS 2004, of the deposits is difficult but the key mineral
USGS 2004). is pitchblende with lesser amounts of the