Page 36 - Introduction to Mineral Exploration

2: MINERALOGY OF ECONOMIC DEPOSITS 19
2

The Mineralogy of

Economic Deposits

Anthony M. Evans

2.1 INTRODUCTION pegmatites and in the mill the ore is processed
to produce chemical grade spodumene and
Ore minerals are the minerals of economic ceramic spodumene concentrates, mica and
interest for which the explorationist is search- feldspar concentrates, and a quartz–feldspar
ing. They can be metallic or nonmetallic. Min- mix marketed as sandspar. The amphibolite
eralogy is used to understand the relationships host rock is crushed, sized, and sold as road
between the ore mineral and the uneconomic aggregate. Of course such comprehensive ex-
host rock for their eventual separation. ploitation of all the material mined is not
Economic mineral deposits consist of every possible in isolated locations, but too often
gradation from bulk materials or aggregates, in the potential of waste material is overlooked.
which most of the rock or mineral is of com- A comprehensive mineralogical examination
mercial value, to deposits of precious metals of a mineral deposit and its waste rocks may
(gold, silver, PGM) from which only a few ppm mean that additional valuable materials in
(or ppb in the case of diamond deposits) are the deposit are identified and the presence of
separated and sold. The valuable mineral in one deleterious substances detected. This may add
deposit may be a gangue mineral in another, value if the project is ever brought to the pro-
e.g. quartz is valuable in silica sands, but is duction stage and will help to avoid embarrass-
a gangue mineral in auriferous quartz veins. ing undervaluation.
Thus the presentation of lists of ore and gangue Ore minerals may be native metals (ele-
minerals without any provisos, as given in ments), of which gold and silver are examples,
some textbooks, can be very misleading to the or compounds of metals with sulfur, arsenic,
beginner. This may lead to an erroneous ap- tellurium, etc., such as lead sulfide – the min-
proach to the examination of mineral deposits, eral galena (PbS) – or they may be carbonates,
i.e. what is recovered and what is discarded? silicates, borates, phosphates. There are few
An alternative question is, how can we process common minerals that do not have an eco-
everything we are going to mine and market nomic value in some mineralogical context or
the products at a profit? There are few mineral other. Some of the more important ore min-
operations where everything is mined gain- erals are listed in Table 2.1 and those which
fully. Fortunes can be made out of the waste are often classified as gangue minerals in Table
left by previous mining and smelting opera- 2.2. Ore minerals may be classed as primary
tions, but not usually by the company that (hypogene) or secondary (supergene). Hypogene
dumped it! A good example of a mine where minerals were deposited during the original
everything is mined is at the King’s Mountain period of rock formation or mineralisation.
Operation in the Tin–Spodumene Belt of North Supergene minerals were formed during a later
Carolina. It is in the world’s most import- period of mineralisation, usually associated
ant lithium-producing area (Kunasz 1982). with weathering and other near-surface proc-
The spodumene occurs in micaceous granite– esses, leading to precipitation of the secondary