Page 57 - Introduction to Mineral Exploration

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

Surface

0 0.5 km

Norite
Footwall rocks

Interstitial sulfide in norite

Ragged disseminated sulfide
Gabbro-peridotite inclusion
sulfide and massive sulfide

FIG. 3.9 Generalized section through the Creighton ore zone, Sudbury, Ontario, looking west. (After Souch
et al. 1969.)

limited to a few centimeters on either side of synthesis of available data and should include
a vein, at other times forming a thick halo the most informative and reliable characteris-
around an orebody and then, since it widens tics of a deposit type, identified on a variety of
the drilling target, it may be of considerable scales and including definition of the average
exploration value. Hoeve (1984) estimated that and range of each characteristic (Adams 1985).
the drilling targets in the uranium field of the It is therefore subject to uncertainty and
Athabasca Basin in Saskatchewan are enlarged change; each new discovery of an example of a
by a factor of 10–20 times by the wall rock deposit type should be added to the data base.
alteration. The Atlas of Alteration by Thomp- In mineral deposit models there are two main
son and Thompson (1996) is a very well illus- types which are often combined; the empirical
trated place to start your study. model based on deposit descriptions and a
genetic model which explains deposits in term
of causative geological processes. The genetic
3.3 GEOLOGICAL MODELS OF MINERAL model is necessarily more subjective but can be
DEPOSITS more powerful, as it can predict deposits not
contained in the descriptive data base. Another
One of the aims of the planning stage (see type of model which is extremely useful for
Chapter 4) is to identify areas for reconnais- preliminary economic evaluations is a grade-
sance and to do this we must have some idea of tonnage model. This accumulates grade and
how the materials sought relate to geological tonnage data for known deposits and from this
factors including geophysics and geochemistry. it is possible to estimate the size and grade of an
This is best achieved by setting up a model or average or large deposit and the cash flow if one
models of the type of deposit sought. But what were found. Examples of this type of modeling
is a model? The term has been defined in vari- are given by Gorman (1994) for South American
ous ways but a useful one is that of “functional gold and copper deposits. Cost curves can also
idealization of a real world situation used to aid be calculated for differing deposit types. Exam-
in the analysis of a problem.” As such it is a ples for copper (Fig. 3.10) show the low cost of