The scale chosen for mapping controls the type of data which can be recorded and hence the type of observations which are made in the field. The choice of appropriate scale depends on the purpose in making the map.
A small-scale map– say at 1:25,000 or smaller– shows broad regional patterns of rock distribution and major structures. From an exploration point of view this is the scale at which the prospectivity of a basin, fold belt, tectonic unit or other large geological subdivision might be determined.
It is a scale appropriate for developing ideas for new project generation. Explorationists do not often make maps at these small scales. There are two reasons for this: firstly, this is the type of mapping undertaken by Geological Surveys and can often be bought off the shelf; secondly, explorationists in most cases cannot obtain a sufficiently large tenement holding to make this kind of mapping worth while.
Maps with intermediate-range scales between 1:25,000 and 1:5,000 can be described as detailed regional maps. These are appropriate scales for the first pass mapping of large tenement holdings. They are also ideal scales to use when combining geological mapping with regional prospecting or regional geochemistry (such as stream sediment sampling).
At scales in this range, some of the larger features which might have had an effect on the localization of ore are capable of being shown, although the outline of an ore deposit itself could not generally be shown. The intermediate range of map scales is therefore suitable for the control and development of new prospect generation.
On maps at scales more detailed than 1:5,000, individual outcrops or outcrop areas and the surface expression of significant areas of mineralization can be shown. These scales are appropriate for showing the features that directly control and localize ore. Maps at these scales are often called outcrop maps and the need to make them generally arises after a prospect has been defined.
The purpose of such maps is to identify the size, shape and other characteristics of the potential ore body. The map is then used to help specify, control and evaluate all subsequent programmes of detailed prospect exploration including geophysics, geochemistry and drilling.
Author: Roger Marjoribanks
