Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 68

demand for high-technology metals such as Li and the rare earth elements, which
are used in batteries or find multiple applications in the electronics industry, they
are increasing sought after by mineral exploration companies and governmental
agencies (see Chap. 6).
A notable exception to the small size of such deposits is the enormous Palabora
deposit in South Africa, which is the country's largest copper deposit and a source
of numerous other commodities, including Zr, Ti. U, apatite (fertilizer), vermiculite
(a clay mineral used as an insulator or a growing medium in agriculture -those fluffy
shiny grains you buy from your local garden store) and magnetite. The Palabora
deposit is hosted by a carbonatite pipe within an alkaline pyroxenitic intrusion and
the open-cast mine that was excavated in the carbonatite is said to be the largest
man-man hole in Africa, if not in the world (see Fig. 3.18 ).
3.5.1 Diamond
Diamond in kimberlite is perhaps the best-known type of magmatic deposit.
Kimberlite is a special type of ultramafic magma, one that is charged with volatile
components such as water and CO 2 . The magma is rich in potassium and incom-
patible trace elements and is probably produced by low-degree partial melting of a
volatile-rich, geochemically enriched source deep in the mantle. Controversy exists
as to whether this source is located in the basal continental lithosphere or at greater
depths in the asthenosphere (Fig. 3.19a ). Lamproite, a rock type similar to kimber-
lite, can also contain commercial diamond deposits. Kimberlites and lamproites are
emplaced during highly explosive volcanic eruptions and form small, circular,
funnel-shaped craters called maars (Fig. 3.19b ). Most kimberlites are restricted to
continents, and, according to Clifford's Rule (Clifford 1966 ), diamond deposits
occur preferentially near or at the margins of stable Archean cratons. There are,
however, some important exceptions, most notably the giant Argyle mine in
Western Australia, which is located in a Proterozoic setting. This deposit is the
greatest producer of diamonds, in terms of quantity but not value because the
quality of the diamonds generally is poor. Unlike most other deposits, the Argyle
mine is hosted by lamproite, not kimberlite.
Several decades ago almost all diamond mines were located in southern Africa
but many large and important deposits have recently been found in Russia,
Australia, and Canada. The small African country Botswana is now the world's
second largest diamond producer (after Russia, measured by the values of the
gems), followed by Canada. This is a remarkable turnaround because diamond
deposits were quite unknown in Canada in 1990. Since then the discovery and
development of two major deposits, Ekati and Diavik, both of which produce a
large proportion of high-quality gems, has led to the country becoming a major
force on the global diamond market.