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Quite simply, if geologists can classify mineral deposits based on rock types, geologic settings and
formation conditions, then they can start to make useful predictions of where other similar deposits
might be. For example:
• porphyry copper deposits are associated with the circum-Pacific ring of fire;
• lateritic bauxite deposits in stable tectonic areas exposed to tropical weathering;
• Epithermal gold deposits are associated with younger volcanic systems.

Ways To Classify Mineral Deposits

Most mineral deposits are formed by more than one process, so attaching a single label to them is
difficult. Hence, there is a tendency to talk about Stillwater Complex type or Carlin-type gold deposits,
which rather defeats the purpose of finding a general from the specific.
There are two main approaches to mineral deposit classification. The descriptive models are a more
objective label, which describes the rocks and tectonic setting of the deposit. These have now evolved to
what is known as a genetic model, which includes theories on deposit formation as well as the physical
properties of the mineral deposit.
More sophisticated are occurrence probability models, which predict the probability of a deposit type
occurring within a particular location, based on known rock types and structural geology. While any
discussion of classification will no doubt result in an over-simplification of what is a very complex
subject, there are some general and useful concepts that can be shared.

Classification of Host Rocks
The Earth’s crust primarily composed of lighter minerals formed of lighter elements. As we travel
deeper within the earth heavier elements and minerals dominate. One method of classifying rocks is by
this association, which considers both the composition and conditions of formation. The table below
outlines these main rock types: Ultramafic, mafic and felsic rocks.

Rock Type Formation Composition Example Economic
Associations
Ultramafic Deepest, Highest Very low silica, high Kimberlite, PGM, Nickel,
Temperature magnesium and iron Peridotite Diamonds
Mafic Deep, High Low silica, elevated Gabbro, Basalt Nickel, Silver, Zinc
Temperature magnesium and iron

Felsic Shallow, Lower High silica Granite, Tin, REE’s, Tungsten,
Temperature Rhyolite Molybdenum

Although rocks may form at a variety of temperatures and depths within the Earth, they may also be
exposed at the surface by erosion, tectonic processes or volcanic eruption. This brings us to our next
classification.

A rock face showing (highly weathered) felsic volcanic rocks. The layers show periods of erosion
between eruptions. The uppermost layer contains angular mafic boulders which indicates a

particularly violent volcanic event.

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