Geology Reference
In-Depth Information
a
SiO
2
Olivine
Chromite
b
c
50
50
40
40
evolved
magma
F
hybrid
magma
30
30
E
hybrid
magma
D
H
G
20
20
C
Chromite
primitive
magma
A
10
primitive
magma
10
B
Chromite
B
Olivine
Olivine
A
0.4
0.8
1.2
1.6
0.4
0.8
1.2
1.6
Olivine Chromite
Contamination from wall-rock
Olivine
Chromite
Mixing of the evolved magma
with a primitive magma
Fig. 3.4 Irvine's mechanisms explaining how chromite can crystallize alone. (a) sketch of phase
diagram showing position of diagrams b and c, (b) Contamination model, (c) magma mixing
model
liquid crystallizes chromite alone. Most probably both processes - contamination
and magma mixing - operate together to produce the chromite deposits of the
Bushveld Complex. Obviously the process is not that simple; in particular, it is very
difficult to understand how the chromite crystals accumulated and how they were
extracted from the enormous volume of magma needed to yield metre-thick layers
of chromite.
Nonetheless, the formation of chromite deposits illustrates an important princi-
ple: the ore mineral, in this case chromite, is a normal constituent of many
ultramafic intrusions and it forms through normal magmatic processes. Under
ordinary circumstances it is present in low concentrations: normal magmatic
rocks are not ores. For a deposit to form, the normal geological process must be
perturbed so that the ore mineral accumulates in far higher concentrations. In the
case of the chromite deposits, contamination or magma mixing are among the
perturbing processes. As we will see in subsequent chapters, a large range of special
circumstances modifies other geological processes like sedimentation or the circu-
lation of hydrothermal fluids so as to create the unusual concentrations of minerals
that constitute an ore.
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