Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 43

Chapter 3
Magmatic Ore Deposits

3.1 Introduction

A magmatic ore deposit is an accumulation of magmatic minerals. Some of these
minerals are extremely rare and almost never encountered in common rocks, an
example being alloys of the platinum metals; other minerals, such as magnetite, are
common and can be seen in many thin sections. They form an ore deposit when they
accumulate in large amounts and at unusually high concentrations. The question is
how these concentrations come about.

3.2 Chromite Deposits of the Bushveld Complex

To illustrate the ore-forming process, we will take as our first example the deposits
of chromite in the Bushveld Complex of South Africa. Chromite, a Cr-Fe oxide, is
the only ore of the metal chromium. A brief description of the complex,
emphasizing its economic importance, is given in Box 4.1. Bushveld contains the
type examples of ore deposits in a large layered intrusion, but this is not the sole
type of chromite deposit; others are found in ophiolites, particularly in the Urals,
Turkey, Greece and India. In addition to their great economic value, the deposits of
the Bushveld were chosen because they illustrate several fundamental processes
governing ore formation in a magmatic setting.
Figure 3.1 , a photo of chromite veins from the Dwars River location in the
eastern side of the complex, clearly shows some important features of a chromite
deposit. The mineral occurs in layers that may reach a metre or more in thickness,
which alternate with layers composed of other magmatic minerals. The rock is
a cumulate, having formed by the transport of magmatic minerals to the floor of
the Bushveld magma chamber. The Bushveld Complex itself (Fig. 3.2 ) is a vast,
roughly funnel-shaped differentiated intrusion composed of the Lower Zone which
consists of alternating layers of cumulus mafic minerals such as olivine and