Page 56 - Introduction to Mineral Exploration

3: MINERAL DEPOSIT GEOLOGY AND MODELS 39

(Solomon 1976). They can be divided into three hollows in the base of the igneous body and
classes of deposit: (a) zinc–lead–copper, (b) generally forms sheets or irregular lenses con-
zinc–copper, and (c) copper. Typical tonnages formable with the overlying silicate rock. From
and copper grades are 0.5–60 Mt and 1–5%, the base upwards, massive sulfide gives way
but these are commonly polymetallic deposits through disseminated sulfides in a silicate
often carrying other base metals and significant gangue to lightly mineralized and then barren
precious metal values which make them plum rock (Fig. 3.9).
targets for exploration, e.g. Neves-Corvo (see
section 1.2.3, “Metal and mineral prices”). Metamorphic host rocks
The most important host rock is rhyolite and Apart from some deposits of metamorphic ori-
lead-bearing ores are only associated with this gin such as the irregular replacement deposits
rock type. The copper class is usually, but not already described and deposits generated in con-
invariably, associated with mafic volcanics. tact metamorphic aureoles – e.g. wollastonite,
Massive sulfide deposits commonly occur in andalusite, garnet, graphite – metamorphic
groups and in any one area they are found at rocks are important for the metamorphosed
one or a restricted number of horizons within equivalents of deposits that originated in sedi-
the succession (see section 15.2.5). These mentary and igneous rocks and which have
horizons may represent changes in com- been discussed above.
position of the volcanic rocks, a change from
volcanism to sedimentation, or simply a pause Residual deposits
in volcanism. There is a close association with These are deposits formed by the removal of
volcaniclastic rocks and many orebodies over- nonore material from protore (rock in which
lie the explosive products of rhyolite domes. an initial but uneconomic concentration of
These ore deposits are usually underlain by a minerals is present that may by further natural
stockwork that may itself be ore grade and processes be upgraded to form ore). For ex-
which appears to have been the feeder channel ample, the leaching of silica and alkalis from a
up which mineralizing fluids penetrated to nepheline–syenite may leave behind a surface
form the overlying massive sulfide deposit. All capping of hydrous aluminum oxides (bauxite).
these relationships are of great importance in Some residual bauxites occur at the present
the search for this orebody type. surface, others have been buried under younger
sediments to which they form conformable
Plutonic hosts. Many plutonic igneous intru- basal beds. The weathering of feldspathic rocks
sions possess rhythmic layering and this is (granites, arkoses) can produce important kao-
particularly well developed in some basic in- lin deposits which, in the Cornish granites of
trusions. Usually the layering takes the form of England, form funnel or trough-shaped bodies
alternating bands of mafic and felsic minerals, extending downwards from the surface for as
but sometimes minerals of economic interest much as 230 m.
such as chromite, magnetite, and ilmenite may Other examples of residual deposits include
form discrete mineable seams within such some laterites sufficiently high in iron to be
layered complexes. These seams are naturally worked and nickeliferous laterites formed by
stratiform and may extend over many kilo- the weathering of peridotites.
meters, as is the case with the chromite seams
in the Bushveld Complex of South Africa and
the Great Dyke of Zimbabwe. 3.2 WALL ROCK ALTERATION
Another form of orthomagmatic deposit is
the nickel–copper sulfide orebody formed by Many ore deposits, particularly the epigenetic
the sinking of an immiscible sulfide liquid ones, may have beside or around them a zone or
to the bottom of a magma chamber containing zones of wall rock alteration. This alteration of
ultrabasic or basic magma. These are known as the host rock is marked by color, textural,
liquation deposits and they may be formed in mineralogical or chemical changes or any com-
the bottom of lava flows as well as in plutonic bination of these. The areal extent of the altera-
intrusions. The sulfide usually accumulates in tion can vary considerably, sometimes being