Open Pit Mine Planning and Design, Two Volume Set & CD-ROM Pack, Third Edition, 3rd Edition

CHAPTER 3

Orebody description

3.1 INTRODUCTION

Today, most potential orebodies are explored using diamond core drilling. The small diam-

eter core collected from each hole provides a continuous ‘line’ of geologic information.

Each of the recovered cores is studied in detail and the contained information recorded. The

process is called ‘logging’. Each ‘line’is subsequently subdivided into a series of segments

representing a particular rock type, structural feature, type of mineralization, grade, etc. By

drilling a pattern of such holes, a series of similarly segmented lines are located in space.

Using this information, together with a knowledge of the geologic setting and other factors,

the mining geologist proceeds to construct a 3-dimensional representation of the mineral-

ized body. The objective is to quantify, as best possible, the size, shape and distribution of

the observable geologic features. The distribution of ore grades are correlated to lithology,

alteration, structure, etc. The result is a mineral inventory or geological reserve.At this point

in the evaluation process, economics have not been introduced so that terms such as ‘ore’

or ‘ore reserve’ are not involved.

The development of a mineral inventory involves substantial judgement, assumptions

being made regarding sample and assay quality, and the interpretation and projection of

geologic features based upon very limited data. The geologic data base, properly gathered

and interpreted, should remain useful for many years. It forms the basis for current and

future feasibility studies, mine planning and financial analyses. The success or failure of a

project can thus be directly linked to the quality of its recorded data base, the drill logs and

the maps. This chapter covers some of the basic techniques involved in the development

and presentation of a mineral inventory.

3.2 MINE MAPS

The fundamental documents in all stages of mine planning and design are the maps.

Maps are essential for the purpose of:

– collecting,

– outlining, and

– correlating

a large portion of the data required for a surface mining feasibility study. These maps are

drawn to various scales. The ‘scale’is the ratio between the linear distances on the map and

186

Orebody description 187

Figure 3.1. Plan map drawn to scale

1:50.

the corresponding distances at the site. In the English system this scale relates ‘inches’ as

measured on the map to ‘feet’ in the field. This may be expressed as

Map distance (in) = K

E

×Actual distance (ft) (3.1)

where K

E

is the English map scale. A typical map scale might be

K

E

=

1

200

= 1:200

This means that an actual distance of 200 ft would be represented by a length of 1 inch on

the map. In the metric system, the map scale relates similar map and actual distance units:

Map distance (m) = K

M

×Actual distance (m)

or

Map distance (cm) = K

M

×Actual distance (cm)

(3.2)

where K

M

is the metric map scale. A scale of 1:1000 means that a length of 1 meter on the

map represents 1000 meters in the field. Similarly a length of 1 cm represents a distance of

1000 cm. A metric scale of 1:1250 is very close to the English scale of 1 in =100 ft.

One speaks of a map being of larger or smaller scale than another. Figure 3.1 shows a

particular area drawn to a scale of 1:50. In Figure 3.2 the region within the dashed lines of

Figure 3.1 has been drawn to a scale of 1:20. In this figure the building appears larger. Thus

the scale of the 1:20 map is larger than that of the 1:50 map. A map of scale 1:40 would be

of larger scale than one drawn to 1:200.

The general rule is ‘the greater the ratio (50 is greater than 20), the smaller is the scale.’

The selection of the most appropriate scale for any map depends upon:

1. The size of the area to be represented.

2. The intended uses for the map.

As more detail and accuracy is required, the scale should be increased.

Mine planning, for example, should be done at a scale that keeps the whole pit on one

sheet and yet permits sufficient detail to be shown. For medium to large size metal mines,

common planning scales are:

1in = 100 ft

1in = 200 ft