Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 44

Fig. 3.1 Chromite veins at
Dwars River, Bushveld
Complex. The black bands,
whose average thickness is
about 10-20 cm, are chromite
cumulates; the white bands
are plagioclase cumulates

9000 m

diorite
and
gabbro

8000 m

Bushveld Red Granite
Bushveld Complex

Magnetite
Reef

$

East Lobe

Main Magnetite Layer
Merensky Reef
Main Chromite Layer

norite
and
gabbro

6000 m

Various country rocks
and later intrusives

5000 m

fault
West Lobe

norite
and
anorthosite

4000 m

Merensky
Reef

$

Chromite
layers

norite
and
pyroxenite

$

$ 2000 m

pyroxenite
and
péridotite

1000 m

0 m

Fig. 3.2 The best examples of magmatic ore deposits: magnetite, chromite and platinum group
elements in the Bushveld Complex, South Africa

pyroxene, the mafic Main Zone of pyroxene and plagioclase cumulates, and the
more evolved Upper Zone of diorite or gabbro. Intervening between Lower and
Main Zone is the Critical Zone, which contains the chromite and PGE deposits.
More detailed descriptions of the complex and its ores are found in Cawthorn et al.
( 1996 , 2005 ).
Chromite, the ore mineral, is present throughout the lower ultramafic part of the
intrusion, but normally its concentration is less than 1-2%. At these levels the rock
is not ore. Only at specific levels in the Complex, in olivine

orthopyroxene
cumulates in the upper part of the Critical Zone (Fig. 3.2 ) is chromite present in
sufficient quantities and at sufficient concentrations, to constitute an ore deposit.
The chromite cumulate layers consist of close to 100% of the ore mineral, and
typical ore grades are around 25-35% Cr. The concentration of the same element in