Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 138

5.5 Other Lateritic Deposits

The accumulation of gold in residual (eluvial) deposits has been described in
Sect. 5.2 . When the weathering takes place in hot humid climates, thick lateritic
profiles build up and when they develop on rocks with a significant gold content, the
weathering can transform protore (gold-bearing material in which the grade is too
low to mine) to ore. Good examples of this type of deposit are found in West Africa.
The same applies to platinum group elements which are concentrated in lateritic
soils over ultramafic rocks in the Carajas region of Brazil.
Another type of surficial deposit forms as “calcrete”, the name given to calcified,
calcite-rich soils that develop in arid environments. Important uranium deposits
(Langer Heinrich in Namibia and Yeelirie in Western Australia) have formed by the
precipitation of carbonate in surface waters that leads to the concentration of
carnotite,

the vivid yellow uranium mineral with the composition

K 2 (UO 2 ) 2 (VO 4 ) 2 ·3H 2 O.

5.6 Supergene Alteration

When sulfide ore bodies are exposed at the surface, the sulfide minerals become
oxidized and the ore metals leach downward to become concentrated in a layer near
the top of the water table (Fig. 5.10 ). These layers of “supergene enrichment”
contain two to five times more ore metals than the primary ore and they are
conveniently located close to the surface where they can be recovered at the start
of the mining operation. Developing a mine is a long and complicated procedure
that requires a huge investment, often of many millions of dollars. The time from
the start of the operation to the recovery and sale of the first metals may be
5-10 years, so the possibility of mining a layer of abnormally rich ore provides
welcome financial relief for the company and often makes the difference between a
viable and an uneconomic operation.
Figure 5.10 shows the profile through the supergene enrichment zone above a
copper sulfide ore body. The uppermost layer, of hydrated iron oxides, is called a
gossan. These layers are void of ore metals but they have distinctive textures that
signal the presence of buried sulfides and they are commonly used in mineral
exploration. Two enriched layers underlie the leached zone. The upper zone of
“oxidized” ores contains a wide variety of secondary minerals - carbonates,
silicates, sulfates, phosphates - that are commonly well crystallized, brightly
coloured, and highly prized by mineral collectors. It is underlain by a zone of
sulfide enrichment where the original iron-bearing sulfides, such as chalcopyrite
(CuFeS 2 ) are replaced by secondary sulfides that are iron-free or iron poor and have
high Cu contents. Examples include chacocite (Cu 2 S), covellite (CuS) and bornite
(Cu 5 FeS 4 ).