Page 99 - Introduction to Mineral Exploration

82 C.J. MOON & M.K.G. WHATELEY

the primary ore and sampling of the surface
outcrops does not reflect that of the primary
ore. Near the water table the copper grade
increases, as the result of the conversion of N
chalcopyrite (35% Cu) to chalcocite (80% Cu).
This enrichment, known as supergene enrich- Mineralized boulder
ment, often provides high grade zones in dis- Subcrop of deposit
seminated copper deposits, such as those of the
porphyry type. These high grade zones pro-
vide much extra revenue and may even be the
basis of mines with low primary grades. In con-
trast to the behavior of copper, gold is less
mobile and is usually concentrated in oxide Lake
zones. If these oxide zones represent a signific-
ant amount of leaching over a long period then
the gold grades may be become economic. An
example of this is the porphyry deposit of Ok
Tedi discussed in Box 11.4. These near surface,
high grade zones are especially important as
they provide high revenue during the early 0 1 km
years of a mine and the opportunity to repay
loans at an early stage. These gold-rich gossans
can also be mechanically transported for con- FIG. 5.7 Train of mineralized boulders from the
siderable distances, e.g. the Rio Tinto Mine in Laisvall lead deposit, Sweden.
southern Spain.
Canada, Ireland, and Scandinavia (Fig. 5.7). In
Finland the technique proved so successful
Float mapping
that the government offered monetary rewards
The skill of tracing mineralized boulders or for finding mineralized boulders. Dogs are also
rock fragments is extremely valuable in areas trained to sniff out the sulfide boulders as their
of poor exposure or in mountainous areas. sense of smell is more acute than that of the
In mountainous areas the rock fragments exploration geologist!
have moved downslope under gravity and the
lithology hosting the mineralisation can be
matched with a probable source in a nearby 5.1.7 Physical exploration: pitting and
cliff and a climb attempted. Float mapping and trenching
sampling is often combined with stream sedi- In areas of poor to moderate outcrop a trench
ment sampling and a number of successful (Fig. 5.8) or pit is invaluable in confirming the
surveys have been reported from Papua New bedrock source of an anomaly, be it geological,
Guinea (Lindley 1987). In lowland areas miner- geochemical, or geophysical. The geology of
alized boulders are often disturbed during cul- a trench or pit wall should be described and
tivation and may be moved to nearby walls. illustrated in detail (Fig. 5.9). For further details
In this case it is often difficult to establish a see section 9.2. Trenches and pits also provide
source for the boulders and a soil survey and a large samples for more accurate grade estim-
subsequent trench may be necessary. Burrow- ates as well as for undertaking pilot processing
ing animals may also be of help. Moles or rab- plant test work to determine likely recoveries.
bits, and termites in tropical areas, often bring Some operators in remote areas, particu-
small fragments to the surface. larly in central Canada, strip relatively large
In glaciated areas boulders may be moved areas of the overburden to enable systematic
up to tens of kilometers and distinct boulder mapping of bedrock. However, this would
trains can be mapped. Such trains have been now (probably) be regarded as environmentally
followed back to deposits, notably in central unfriendly.