Page 51 - Introduction to Mineral Exploration

34 A.M. EVANS & C.J. MOON

Flat Thick impervious shale
Limestone
Strike of orebody
D B
Shale
Pitch or
A Dip rake Limestone
Shale
Plunge
E Sandstone

Hanging Footwall
wall
AB and CB lie in the same 20 m
Axis of orebody DB, AB and EB are in the
vertical plane.
same horizontal plane and
C EB is perpendicular to DB FIG. 3.2 Vein occupying a normal fault and
exhibiting pinch-and-swell structure, giving rise to
ribbon ore shoots. The development of a flat beneath
impervious cover is shown also.
Longitudinal section of an orebody Cross section
of the same
orebody The infilling of veins may consist of one
Surface Surface mineral but more usually it consists of an
intergrowth of ore and gangue minerals. The
boundaries of vein orebodies may be the vein
walls or they may be assay boundaries within
the veins.
Width or
Shaft Levels thickness Tubular orebodies. These bodies are relat-
Plunge length
Breadth ively short in two dimensions but extensive in
the third. When vertical or subvertical they
are called pipes or chimneys, when horizontal
Stope or level length or subhorizontal, “mantos.” The Spanish word
manto is inappropriate in this context for its
FIG. 3.1 Illustrations of terms used in the description literal translation is blanket; it is, however,
of orebodies. firmly entrenched in the English geological
literature. The word has been and is employed
by some workers for flat-lying tabular bodies,
and lodes (Fig. 3.2). These are essentially the but the perfectly acceptable word “flat” (Fig.
same and only the term vein is now normally 3.2) is available for these; therefore the reader
used. Veins are often inclined, and in such must look carefully at the context when he or
cases, as with faults, we can speak of the hang- she encounters the term “manto.” Mantos and
ing wall and the footwall. Veins frequently pipes may branch and anastomose and pipes
pinch and swell out as they are followed up or frequently act as feeders to mantos.
down a stratigraphical sequence (Fig. 3.2). This In eastern Australia, along a 2400 km belt
pinch-and-swell structure can create difficult- from Queensland to New South Wales, there
ies during both exploration and mining often are hundreds of pipes in and close to granite
because only the swells are workable. If these intrusions. Most have quartz fillings and some
are imagined in a section at right angles to that are mineralized with bismuth, molybdenum,
in Fig. 3.2, it can be seen that they form ribbon tungsten, and tin; an example is shown in
ore shoots. Veins are usually developed in frac- Fig. 3.3. Pipes may be of various types and
ture systems and therefore show regularities in origins (Mitcham 1974). Infillings of min-
their orientation throughout the orefield in eralized breccia are particularly common, a
which they occur. good example being the copper-bearing breccia