Page 169 - Introduction to Mineral Exploration

152 J. MILSOM

specifically to complicate their lives. In con- cating their lives even further. Geochemists,
sidering geophysics, geologists must ask them- once beyond the reconnaissance stream sedi-
selves a number of questions. What information ment stage, also take samples along pegged
is needed? Is there a realistic expectation that lines, but like to be able to plot locations at re-
geophysical methods will provide it? What in- gular intervals on planimetric base maps. Plani-
formation will they not provide? Is this infor- metrically regular spacing (secant-chaining)
mation essential and can any incompleteness is also desirable for geophysical point data
be tolerated? Is geophysics affordable and (even (gravity, magnetic, and SP), but in resistivity,
if the answer is yes) would the money be better IP, and electromagnetic surveys it is the actual
spent on something else? Team geologists may distances between coils or electrodes that must
not be able to answer all these questions them- be constant and these may differ significantly
selves but they need to ask them, and to get from plan distances. Whether lines are secant-
answers. The necessary approaches can be or slope-chained will depend on how much,
illustrated by specific examples: of what, is being measured, but this apparently
1 An exploration area is some hundreds of trivial problem can be a major cause of friction
square kilometers in extent and is covered between team members.
by regional aeromagnetic maps. Are these The final requirements of all exploration
adequate or should a new, detailed survey be geologists, and particularly of those who in-
planned? (Seek geophysical advice, but not volve themselves in geophysics, are serendipity
from a contractor who may be hoping for and an awareness of the possible. Geophysical
work.) Is it reasonable to suppose that the surveys always provide some information,
targets will be magnetic or that there will be even though it may not seem directly relevant
critical structural or lithological information at first, and many mineral deposits have been
in the magnetic data? Will such data only be discovered by what may be considered luck but
obtained by high sensitivity or gradient surveys at least required geologists with their wits
or will a 1 nT magnetometer do? If aeromag- about them. In the 1960s, radiometric surveys
netic work is to be done, is there a case for were flown in the Solomon Islands primarily
adding other sensors (electromagnetic, radio- for detecting phosphates. A ground follow-up
metric), bearing in mind the inevitable in- team sent to Rennel Island expected to find
creases in cost? Is the area topographically phosphatic rock, of which there was very little,
suitable for fixed-wing survey or will a heli- but did recognize that the source of the
copter have to be used? How much help will anomaly was uraniferous bauxite, in mineable
have to be bought in for contract supervision quantities.
and interpretation, and can this be accom-
modated within the existing budget?
2 A lease of a few square kilometers is to be 7.18 DEALING WITH CONTRACTORS
explored for massive sulfides. SP surveys have
located some mineralized bodies in the region Exploration geologists may be involved with
but have missed others. Bearing in mind that geophysical contractors at many different
other, more expensive methods will also have stages, and will often need the support and
to be used if the area is to be fully evaluated, advice of an experienced geophysicist who is
is more SP worthwhile? Should it be kept in committed to them and not to the contractor.
reserve (the equipment is cheap) in case there The most important stage is when the contract
are field hands spare for a few days? Or, having is first being drafted. Geophysical consultants
gone to the trouble of gridding and pegging find it frustrating (but also sometimes very
lines, should not every possible method be used profitable) to be asked to sort out contracts,
everywhere? already written and agreed, which are either so
Geophysical work will involve surveying, loose that data of almost any quality must be
and may also require line clearing and cutting. accepted or which specify procedures that actu-
At this point the geologist responsible for geo- ally prevent useful data being acquired. Project
physics may well come into conflict with the geologists will share in the frustration but not
geologist responsible for geochemistry, compli- in the profit!