Chemistry Reference
In-Depth Information
3.7 OREMINERALS
Metallic minerals exploited as ore are mostly sulfides, sulfosalts, oxides and,
to a lesser extent, native elements, tungstates, etc. Most of these minerals
are opaque: they cannot be studied with the petrographic microscope, but
require the reflected light microscope.
Some ore minerals (or associated with ores) transparent in thin section,
are presented in the CD.
3.7.1 Barite
BaSO
4
is a common mineral in the massive sulphide deposits, veins of low
temperature and stratiform deposits in carbonate (Mississippi Valley type)
or clastic environment (such deposits are sometimes called Red Beds). It
is very commonly associated with sphalerite, pyrite, galena and sometimes
fluorite. Barite is then a gangue for these ores or it may be exploited for
itself. The only significant substitution in this mineral is that of barium by
strontium.
3.7.2 Fluorite
Generally contains at least 99 wt% CaF
2
. The only notable substitutions are
the replacement of Ca by Sr or by Ce and Y.
Fluorite is a mineral of evolved igneous rocks, both subalkaline and
alkaline: granites, syenites, nepheline syenites and associated pegmatites. It
is probably not primary but rather appears at the tardi-magmatic (pneuma-
tolytic) and hydrothermal stages. It is a mineral of the greisens where it is
associated with topaz, lepidolite, cassiterite, etc.. It is rare in volcanic rocks,
where it occurs in geodes and fumarole deposits.
The major economic deposits of fluorite are hydrothermal veins of
low to medium temperature and stratiform of Mississippi Valley type as
well in carbonates as clastic environment. It is associated with sphalerite,
pyrite, galena, chalcopyrite and sometimes barite. In some districts, or even
some deposits, display a spatial zonation between barite and fluorite may
be observed.
3.7.3 Sphalerite
ZnS is the main ore of zinc. There are currently substitution of zinc by
iron; the maximum contain is about 26 wt% Fe, that is 45 mol% FeS. The
association sphalerite (Zn, Fe)S, pyrrhotite Fe
1-x
S, (pyrite FeS
2
) can be used
as a geothermometer and to determine f
S2
. Spharerite may contain some
cadmium, manganese, and trace of Ag, In, Ge, Ga, Tl, Se, Hg. These ele-
ments can greatly increase the value of the exploited sphalerite. Sphalerite
