Page 88 - Introduction to Mineral Exploration

5: FROM PROSPECT TO PREFEASIBILITY 71

then it is probable that married staff will work drilling program surveying to a few millimeters
more effectively if their families are moved to will be required for accurate borehole location.
this town. In general, the town should be less The usual practice when starting work on
than 1 hour’s commuting time so that a visit a prospect is to define a local grid for the pro-
to a drilling rig is not a chore. In the remotest spect using GPS to relate the local grid to the
areas staff will be housed in field camps and national or international grid systems (see sec-
will commute by air to their home base. If pos- tion 9.1.7). Some convenient point such as a
sible exploration staff should not be expected wind pump or large rock is normally taken as
to stay for long periods in field camps as this is the origin. The orientation of the grid will be
bad for morale and efficiency declines. parallel to the regional strike if steeply dipping
Budgeting for the project is more detailed mineralisation is suspected (from geophysics)
than at the reconnaissance stage and will take but otherwise should be N–S or E–W.
account of the more expensive aspects of ex-
ploration, notably drilling. A typical budget
sheet is shown in Table 4.2. Usually the major Surveying methods
expenditure will be on labor and on the various Geologists generally use simple and cheap
surveys. If the area is remote then transport techniques in contrast to those used by profes-
costs, particularly helicopter charter, can be- sional surveyors. Surveying has recently been
come significant. Labor costs are normally cal- transformed by the advent of GPS, based on
culated on the basis of man months allocated a network of satellites installed by the US
and should include an overhead component to Department of Defense. These enable a fix of
cover office rental, secretarial and drafting sup- approximately 5 m accuracy to be made any-
port. Commonly overheads equal salary costs. where on earth, with an inexpensive (currently
For geophysical, and sometimes geochemical approximately $US200) receiver, where three
surveys, contractors are often hired as their satellites can be viewed. The major problems
costs can be accurately estimated and compa- are in forested areas.
nies are not then faced with the possibility of One of the simplest and most widely used
having to generate work for staff. Estimates are techniques, before the advent of GPS, is the
normally made on the basis of cost per line- tape and compass survey. This type of survey
kilometer or sample. In remote areas careful starts from a fixed point with directions meas-
consideration should be given to contractor ured with a prismatic or Brunton compass
availability. For example in northern Canada and distances measured with a tape or chain.
the field seasons are often very short and a Closed traverses, i.e. traverses returning to the
number of companies will be competing for initial point, are often used to minimize the
contractors. For other types of survey and for errors in this method. Errors of distance and
drilling, estimates can be obtained from reput- orientation may be distributed through the
able operators. traverse (section 5.1.4). Grids are often laid out
using this technique with baselines measured
along a compass bearing. Distances are best
5.1.2 Topographical surveys
measured with a chain which is less vulnerable
The accurate location of exploration surveys to wear and more accurate than a tape. Longer
relative to each other is of crucial importance baselines in flat country can be measured using
and requires that the explorationist knows the a bicycle wheel with a cyclometer attached.
basics of topographical surveying. The effort Lightweight hip-mounted chains are com-
put into the survey varies with the success monly used in remote areas. Straight grid lines
and importance of the project. At an early stage are usually best laid out on flat ground by back
in a remote area a rough survey with the ac- and forward sighting along lines of pegs or
curacy of a few meters will be adequate. This sticks. Sturdy wood or metal pegs should be
can be achieved using aerial photographs and used, the grid locations marked with metallic
handheld Global Positioning Satellite (GPS) re- tags and flagged with colored tape. The tape
ceivers (Ritchie et al. 1977, Sabins 1987) where should be animal proof; goats have a particular
only a few survey points are needed. For a major fondness for colored tape.