Page 42 - Introduction to Mineral Exploration

2: MINERALOGY OF ECONOMIC DEPOSITS 25

TABLE 2.2 List of common gangue minerals.

Name Composition Primary Supergene
Quartz SiO 2 X
Chert SiO 2 X
Limonite Fe 2 O 3 .nH 2 O X
Calcite CaCO 3 X X
Dolomite CaMg(CO 3 ) 2 X X
Ankerite Ca(Mg,Fe)(CO 3 ) 2 X X
Baryte BaSO 4 X
Gypsum CaSO 4 .2H 2 O X X
Feldspar All types X
Fluorite CaF 2 X
Garnet Andradite most common X
Chlorite Several varieties X
Clay minerals Various X X
Pyrite FeS 2 X
Marcasite FeS 2 X
Pyrrhotite Fe 1–x S X
Arsenopyrite FeAsS X

minerals from descending solutions. When Aluminum is of course abundant in many
secondary mineralisation is superposed on silicate rocks, but it must be usually in the form
primary mineralisation the grade increases of hydrated aluminum oxides, the rock called
and this is termed supergene enrichment. bauxite, for economic recovery. The mineral-
ogy of the ore mineral will also place limits on
the maximum possible grade of the concen-
2.2 MINERALOGICAL INVESTIGATIONS trate. For example, in a mineral deposit con-
taining native copper it is theoretically possible
Before looking at some of the many methods to produce a concentrate containing 100% Cu
that may be used, the economic importance of but, if the ore mineral chalcopyrite (CuFeS 2 ) is
these investigations will be emphasized by dis- the principal source of copper, then the best
cussing briefly the importance of mineralogical concentrate would only contain 34.5% Cu.
form and undesirable constituents.
Undesirable substances
Mineralogical form
Deleterious elements may be associated with
The properties of a mineral govern the ease both ore and gangue minerals. For example,
with which existing technology can extract tennantite (Cu 12 As 4 S 13 ) in copper ores can
and refine certain metals and this may affect introduce unwanted arsenic and sometimes
the cut-off grade (see section 10.4.2). Thus mercury into copper concentrates. These, like
nickel is far more readily recovered from phosphorus in iron concentrates and arsenic
sulfide than from silicate minerals and sulfide in nickel concentrates, will lead to custom
minerals can be extracted down to about 0.5%, smelters imposing financial penalties or refus-
whereas silicate minerals must assay about ing the shipment. The ways in which gangue
1.5% to be economic. minerals may lower the value of an ore are
Tin may occur in a variety of silicate minerals very varied. For example, an acid leach is
such as stanniferous andradite (Ca 3 Fe 2 Si 3 O 12 ) normally employed to extract uranium from
and axinite ((Ca,Fe,Mn)Al 2 BSi 4 O 15 OH), from the crushed ore, but if the carbonate, calcite
which it is not recoverable, as well as in its (CaCO 3 ), is present, there will be excessive acid
main ore mineral form, cassiterite (SnO 2 ). consumption and the less effective alkali leach