Page 90 - Introduction to Mineral Exploration

5: FROM PROSPECT TO PREFEASIBILITY 73

exploration targets because of rising commod- Before a sampling program can be put into
ity prices, cheaper mining and processing costs, operation, a map of the old workings will be
the development of new technology which required, if none is available from archives. The
may improve recovery, or the development of exploration geologist is often the first person
a new geological model which could lead to at the site and it is up to him or her to produce
undiscovered mineralisation. The presence of a a plan and section of the old workings. This
mining district indicates mineral potential, would be done using a tape and compass survey
which must reduce the exploration risk. How- (Ritchie et al. 1977, Reedman 1979, Peters
ever, there will be a premium to pay, as the 1987, Majoribanks 1997) (Table 5.1, Figs 5.1 &
property will probably already be under option 5.2). Once the layout of the old workings
to, or owned by, a rival company. is known, the mapping and complementary
The type of examination warranted by an sampling program can begin. The survey pegs
old mine will depend on its antiquity, size, and established during the surveying will be used
known history. In Europe and west Asia old to locate the sample points and guide map-
mines may be over 2000 years in age and be the ping. With the tape held between the pegs, the
result of Roman or earlier activity. In this case sample points are marked on the drive or cross-
there are few, if any, records and the target cut walls and the distance from one peg to
commodity can only be guessed at. In such the sample point recorded in the field note
cases small areas of disturbed ground, indicat- book along with the sample number. The same
ing the presence of old prospecting pits or number is written on a sample ticket and
trenches, can best be found from aerial photo- included with the sample in the sample bag.
graphy. Field checking and grab sampling will The samples are normally collected at regular
confirm the presence and indicate the possible intervals from channels cut normal to the dip
type of mineralisation. of the mineralized rock (Fig. 5.3). The sample
Nineteenth or twentieth century mines are interval varies depending upon the type of min-
likely to be larger and have more extensive eralisation. A vein gold deposit may well be
records. Available records should be obtained sampled at 1 m intervals along every drive,
but should be treated with some caution, as while a copper deposit may only be sampled
many reports are unreliable and plans likely every 5 or 10 m.
to be incomplete. An aim of this type of inves- If old records are available and reliable, then
tigation is to check any records carefully by their data should be evaluated in conjunction
using systematic underground sampling above with new sample data acquired during the
the water level, as old mines are frequently remapping and resampling exercises. Geolog-
flooded. Evaluation of extensive underground ical controls on mineralisation should be estab-
workings requires considerable planning and lished using isopach and structure contour
will be more expensive than surface explora- maps as discussed in section 5.2.2. It may be
tion because equipment and labor for develop- necessary to apply a cut-off value below which
ment and securing old underground workings the mineralisation is not considered mineable.
are costly. A key consideration is safety and In Table 5.2 two cut-off parameters (Lane 1988)
access to the old workings must be made safe are used, in one a direct cut-off and in the
before any sampling program is established. It second a weighted average value is used.
may be necessary to undertake trenching and In the first case the upper sample cut-off is
pitting in areas adjacent to the old workings, taken where the individual grade falls below
and eventually drilling may be used to examine 1.5%. Some assays within this sample are also
the deeper parts of the inaccessible mineralisa- below 1.5%, but they are surrounded by higher
tion. Guidelines on safe working practices in values, which, when averaged out locally, have
old workings can be found in Peters (1987) and a mean greater than 1.5%. The samples be-
Berkman (2001), and in the UK in publications tween the two lines are then averaged using the
of the National Association of Mining History sample thickness as the weighting function,
Organizations (NAMHO 1985) and the Institu- giving an average of 2.26% Zn over 2.10 m.
tion of Geologists (now the Geological Society In the second case, the samples are averaged
of London) (IG 1989). from the base upwards using thickness as the