Page 50 - Introduction to Mineral Exploration

3

Mineral Deposit

Geology and Models

Anthony M. Evans and Charles J. Moon

A detailed understanding of the geology of has to be cut back to maintain a safe overall
mineral deposits is required to explore effec- slope to the sides of the pit (see section 11.2.1).
tively for them. As this is beyond the scope Before discussing the nature of ore bodies we
of this volume, only some key features are must learn some of the terms used in describ-
discussed here as well as how a more advanced ing them.
appreciation of mineral deposit geology may If an orebody viewed in plan is longer in one
be used in exploration programs. For further direction than the other we can designate this
detailed consideration of mineral deposit geo- long dimension as its strike (Fig. 3.1). The
logy the reader is referred to the companion inclination of the orebody perpendicular to the
volume in this series by Robb (2004) and refer- strike will be its dip and the longest dimension
ences listed in section 3.4. of the orebody its axis. The plunge of the axis
is measured in the vertical plane ABC but its
pitch or rake can be measured in any other
3.1 NATURE AND MORPHOLOGY OF plane, the usual choice being the plane contain-
OREBODIES ing the strike, although if the orebody is fault
controlled then the pitch may be measured in
the fault plane. The meanings of other terms
3.1.1 Size and shape of ore deposits
are self-evident from the figure.
The size, shape, and nature of ore deposits It is possible to classify orebodies in the
affects the workable grade. Large, low grade same way as we divide up igneous intrusions
deposits which occur at the surface can be according to whether they are discordant or
worked by cheap open pit methods, whilst thin concordant with the lithological banding (often
tabular vein deposits will necessitate more bedding) in the enclosing rocks. Considering
expensive underground methods of extraction. discordant orebodies first, this large class can be
Open pitting, aided by the savings from bulk subdivided into those orebodies which have an
handling of large daily tonnages (say >30 kt), approximately regular shape and those which
has led to a trend towards the large scale mining are thoroughly irregular in their outlines.
of low grade orebodies. As far as shape is con-
cerned, orebodies of regular shape can generally
be mined more cheaply than those of irregular Discordant orebodies
shape, particularly when they include barren
zones. For an open pit mine the shape and Regularly shaped bodies
attitude of an orebody will also determine how
much waste has to be removed during mining. Tabular orebodies. These bodies are extensive
The waste will often include not only over- in two dimensions, but have a restricted devel-
burden (waste rock above the orebody) but also opment in their third dimension. In this class
waste rock around and in the orebody, which we have veins (sometimes called fissure-veins)