Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 65

Jinchuan is the third largest Ni-Cu sulfide deposit (after Noril'sk and Sudbury).
Unlike the others that we have described in this chapter, which all formed at or near
the surface, the Jinchuan deposit is located at mid-crustal level in a series of
strongly metamorphosed and highly deformed gneisses and marbles. The host
intrusion is once again small (only about 6 km long and a few hundred metres
wide) and it is composed almost entirely of olivine-rich ultramafic rocks.
A conspicuous feature of the geological setting is the virtual absence of S-bearing
country rocks, which seems to rule out assimilation of external sulfur as the ore-
forming process. Tang ( 1993 ) and Lehmann et al. ( 2007 ) have suggested that the
first stage of ore formation involved the contamination of komatiitic magma in
a deeper staging chamber. Then a mush composed of olivine crystals and sulfide
droplets in a silicate liquid was injected into the present Jinchuan intrusion. Sulfide
segregation or transport may have been aided by interaction of the magma with
wall-rock marbles.
3.5 Other Magmatic Deposits
Table 3.3 lists several other types of deposits that are found in igneous rocks and are
thought to form mainly by magmatic processes. In fact it is not always straightfor-
ward to decide whether a deposit should be classed as magmatic or hydrothermal.
Porphyry deposits (described in the following chapter) are the most important
source of copper, and these deposits are indeed located within or adjacent to
granitic rocks. However, as will be shown in the following chapter, they form
through the precipitation of ore minerals from aqueous fluids and thus fit our
definition of a hydrothermal deposit. In the case of tin deposits in granites
(Table 3.3 ) similar ambiguity exists because many of these ore bodies are located
at the marginal zones of granitic plutons where late-magmatic fluids have interacted
with country rocks to produce a type of alteration, called greissen, which produces
rock enriched in tin as well as other metals such as Sb, Cu, Pb, Zn. This type of
deposit also lies at the limit between magmatic and hydrothermal. The most
important magmatic tin deposits are located in the Malaysia, Indonesia, China,
Australia, and Brazil. The South American countries Peru and Bolivia also produce
large amounts of tin from polymetallic (Ag-Pb-Zn-Sn) hydrothermal deposits.
Historically important deposits in Cornwall provided the metals that helped fuel
Britain's industrial revolution.
Box 3.4 The Tin Fiasco of the 1980s
For much of the last century Malaysia was the world's major tin producer. In
the 1980s it formed a cartel with other tin-producing countries to try to protect
tin prices. Substitutes had emerged for traditional tin applications, particularly
the use of protective plastic coatings inside what were once called “tin cans”.
This, together with increased recycling, had stifled demand for the metal.
(continued)