Page 54 - Introduction to Mineral Exploration

3: MINERAL DEPOSIT GEOLOGY AND MODELS 37

stratigraphical column. Thus the veins, pipes, Other good examples of concordant deposits
and flats of the Southern Pennine orefield of in argillaceous rocks, or slightly metamor-
England can be designated as strata-bound, as phosed equivalents, are the lead–zinc deposits
they are virtually restricted to the Carbonifer- of Mount Isa, Queensland, many of the
ous limestone of that region. A number of Zambian Copperbelt deposits, and the copper
examples of concordant deposits which occur shales of the White Pine Mine, Michigan.
in different types of sedimentary rocks will be
considered. Arenaceous hosts. Not all the Zambian
Copperbelt deposits occur in shales and
Limestone hosts. Limestones are very com- metashales. Some bodies occur in altered
mon host rocks for base metal sulfide deposits. feldspathic sandstones such as Mufulira, which
In a dominantly carbonate sequence ore is of- consists of three extensive lenticular orebodies
ten developed in a small number of preferred stacked one above the other and where the
beds or at certain sedimentary interfaces. ore reserves in 1974 stood at 282 Mt assaying
These are often zones in which the permeabil- 3.47% Cu. The largest orebody has a strike
ity has been increased by dolomitization or length of 5.8 km and extends several
fracturing. When they form only a minor part kilometers down dip. Many other concordant
of the stratigraphical succession, limestones, sandstone-hosted orebodies occur around the
because of their solubility and reactivity, can world, such as those in desert sands (red bed
become favorable horizons for mineralisation. coppers), which are very important in China
For example the lead–zinc ores of Bingham, where they make up nearly 21% of the
Utah, occur in limestones which make up 10% stratiform copper reserves of that country
of a 2300 m succession mainly composed of (Chen 1988).
quartzites. Many mechanical accumulations of high
density minerals such as magnetite, ilmenite,
Argillaceous hosts. Shales, mudstones, rutile, and zircon occur in arenaceous hosts,
argillites, and slates are important host rocks usually taking the form of layers rich in heavy
for concordant orebodies which are often minerals in Pleistocene and Holocene sands.
remarkably continuous and extensive. In Ger- As the sands are usually unlithified, the de-
many, the Kupferschiefer of the Upper Permian posits are easily worked and no costly crushing
is a prime example. This is a copper-bearing of the ore is required. These orebodies belong
shale a meter or so thick which, at Mansfeld, to the group called placer deposits. Beach sand
occurred in orebodies which had plan dimen- placers supply much of the world’s titanium,
2
sions of 8, 16, 36 and 130 km . Mineralisation zirconium, thorium, cerium, and yttrium. They
occurs at exactly the same horizon in Poland, occur along present-day beaches or ancient
where it is being worked extensively, and beaches where longshore drift is well devel-
across the North Sea in north-eastern England, oped and frequent storms occur. Economic
where it is subeconomic. grades can be very low and sands running as
The world’s largest, single lead–zinc orebody little as 0.6% heavy minerals are worked along
occurs at Sullivan, British Columbia. The host Australia’s eastern coast.
rocks are late Precambrian argillites. Above
the main orebody (Fig. 3.6) there are a number Rudaceous hosts. Alluvial gravels and con-
of other mineralized horizons with concordant glomerates also form important recent and
mineralisation. This deposit appears to be ancient placer deposits. Alluvial gold deposits
syngenetic and the lead, zinc and other metal are often marked by “white runs” of vein quartz
sulfides form an integral part of the rocks in pebbles as in the White Channels of the Yukon,
which they occur. The orebody occurs in a the White Bars of California, and the White
single, generally conformable zone 60–90 m Leads of Australia. Such deposits form one of
thick and runs 6.6% Pb and 5.9% Zn. Other the few types of economic placer deposits
metals recovered are silver, tin, cadmium, anti- in fully lithified rocks, and indeed the majority
mony, bismuth, copper, and gold. This orebody of the world’s gold is won from Precambrian
originally contained at least 155 Mt of ore. deposits of this type in South Africa (see