Marjoribanks R. Geological Methods in Mineral Exploration and Mining

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56 3 Mine Mapping

Floor

Pit

Batter

Berm

Pit FloorPit Floor

View along strike : True Dip View oblique to strike : Apparent Di

View oblique to strike : Apparent Dip

Fig. 3.5 Judging the direction of the strike and true dip of beds exposed on the wall of an open

cut. When the view is along strike, the trace of the bed on adjacent batters will line up

3.3 Mapping Underground Openings

Geological mapping in underground mines is normally done on to a plan or section

of the opening provided by the mine surveyor. The great detail that can be observed

requires mapping at very large scales, generally from 1:100 to 1:10.

Rock exposure is effectively 100%, but it is usually necessary to arrange to wash

down walls and backs (the roof) before detailed features can be seen.

Rock can generally be observed on the backs and walls of the opening, and also

on advancing faces. A fully detailed mapping job would therefore require that three

or four maps be produced, one for each surface. Such detailed mapping is often

necessary where particular complexity needs to be resolved in a critical part of an

ore body. In the example illustrated, narrow, gold bearing, quartz reefs are offset

by a series of faults resulting in a complex geometry which only detailed mapping

of all available exposed surfaces can resolve. The face, walls and backs maps are

displayed together on the same plan – informally called “exploded box” mapping

(Fig. 3.6). However, this sort of detail is often unnecessary and the mine geologist

will choose to map only one of the available surfaces. There is no hard r ule – the

choice of the reference frame on which to record observations depends on the nature

of the available surfaces, the attitude of rock structures, the time available and how

the map is to be used.

Flat-lying structures such as bedding, veins, etc., are best observed on vertical

surfaces and recorded on section. If such features are present and are considered

important, then mapping of the walls or face of the opening would be appropri-

ate, with observations being compiled onto standard mine cross sections and/or a

longitudinal section along the wall (Fig. 3.7). Conversely, steep-dipping structures

are best recorded on a plan view (Fig. 3.8). I n underground open stopes, virtually

the only exposure available may be the roof of the opening. In this case, a map of

the backs would be the obvious and indeed the only available choice. In cross-cuts,

3.3 Mapping Underground Openings 57

Fig. 3.6 In underground mines with complex geology it may be necessary to make a separate map

of all exposed surfaces – the roof (backs), both walls and advancing face. This is sometimes called

“exploded box” mapping. The separate maps made for each advance can then be compiled on to

level plans, longitudinal sections and standard mine cross sections

which are at a high angle to the dominant rock structure, there would be little point

in mapping the advancing face of an opening as this would lie parallel to the struc-

ture. A map of the wall and backs is indicated. However, in drives running along the

dominant strike, the advancing face will usually give the most effective cross-section

view of the structures (Fig. 3.7).

Another consideration in the choice of a mapping frame is the type of plans that

are required for mine planning purposes. For example, geological interpretation in

a mine is often based on underground or surface drilling, and it is necessary to map

geological data in such a fashion that they can be readily compiled, with a minimum

of projection or manipulation, on to the drill sections. An example of mapping which

is compiled on to a drill section in this way is shown in Fig. 3.7.

Clearly, mapping of shafts or other essentially vertical openings will have to be

done on to a section.

A good compromise, often adopted for mine mapping where steep-dipping struc-

tures predominate, is to project rock structures seen on walls and backs on to a single

level plan located at about waist height above the ﬂoor (Fig. 3.8). This is typically

the height of the level plan produced by the mine surveyor, and also the height of any

channel samples that might be collected for assay along the walls of a cross-cut. The

58 3 Mine Mapping

Figure 3.5 a

(a)

(b)

(c)

500 RL

450 RL

400 RL

1 Level

3 Level

2 Level

3 Level

10W

Heading

supergene

shale

porphyry

metabasalt

ultramafic

ENIGMA OPEN CUT

ENIGMA

DEEPS

9900 N

10000 N

ENIGMA SOUTH

10,250 N

Geologist mapping a

development heading

in ore. After each blast,

miners remove the ore

then bar down the roof

and walls to make safe.

The geologist hoses

down the rock,

measures the face

advance with a tape,

then maps face and

walls at 1:25 scale. A

portable, rechargeable

flood light illuminates

the scene.

Fig. 3.7 Shallow dipping structures are best mapped where they intersect the wall or advancing

face of the opening. In the example shown, mapping has been compiled along with surface drill

hole data on to a standard mine cross-section

3.3 Mapping Underground Openings 59

Composite geological plan of mid-

level of mine opening

Fig. 3.8 Steep dipping structure is best mapped in plan view. In this example, structures seen on

the walls and roof of the mine are projected down-dip and down-plunge onto a single composite

map at around the mid-point of the level. The resulting map will usually cover an area that is wider

than the opening. This type of mapping would not be appropriate for very complex structures, for

which “exploded box” mapping is more appropriate (Fig. 3.6)

geological map compiled in this way is thus a composite of observations made from

all available surfaces; the data is nowhere projected more than a few metres on to

the map plane, and is exactly located along both walls. For most geological mapping

purposes, but particularly for cross-cuts, such a map is sufﬁciently accurate.

A point to bear in mind is that by universal convention geological maps show

geology as if viewed from above. Care must be taken when projecting geology seen

on the backs (which are naturally viewed from below) on to such a plan, as the

different viewpoint causes a mirror image change in angular relationships. Folds

with an “S” proﬁle (for example) when viewed from below become “Z” proﬁle folds

on the geology map. Beds displaced by a fault in a left-lateral (or sinistral) sense

when viewed from below are shown with a right-lateral (dextral) displacement on

the map (Fig. 3.8). Mine geologists must learn to mentally “ﬂip over” the geology

they are looking at before plotting it on to their maps.

Whatever mapping choice is adopted, it is inevitable that many observed rela-

tionships will be seen which cannot be shown on the map. If considered signiﬁcant,

these can be recorded using detailed sketches, or separate maps and sections, drawn

to the side of the main map and keyed to it with a line or arrow.

Although compasses work well underground,

the nature of underground rock

exposure means that it is seldom possible to measure structures directly by laying a

compass on the structure. The best way to measure the strike of a planar structure is

But beware of adjacent iron objects that might affect the compass needle – there can be a lot of

these in the typical underground mine.

60 3 Mine Mapping

True dip of bed

Apparent dip of

bed on wall - 70°

75°

Fig. 3.9 The trace of a planar surface on the wall of a mine opening generally represents the

apparent dip, not the true dip of the surface. To obtain the true dip, it is necessary to locate the

surface on both walls of the opening so that a sighting can be made along strike

to take a bearing between the points where the structure is intersected by the walls

of the mine opening. The intersection of a planar structure with a vertical mine

surface is only a true dip if the structure is at right angles to the wall. In the general

case, intersections seen on mine walls are apparent dips. To obtain a true dip, it is

necessary to sight the compass along the strike bearing (Fig. 3.9).

3.4 Safety in Mines

Mines are one of the most dangerous work environments that any geologist can ﬁnd

themselves in. To name just a few hazards, there is the ever present danger of rock

falls, of being run over by heavy mine machinery or of falling into holes or over

steep rock faces. To maximise safety, the geologist should keep the following rules

at the forefront of his mind at all times:

• Make sure that a suitable responsible person (shift boss, mine foreman, immedi-

ate supervisor, etc...) knows where and when you are working in the mine. There

is usually some sort of tagging system for entering and leaving an underground

mine area. Use it religiously: this is vital for your safety.

• Familiarise yourself with all mine safety rules and procedures and follow them.

Reference 61

• A geologist needs to get close to a rock to see structure or details of minerali-

sation, but having one land from above on top of your head is much too close.

Do not approach any steep or overhanging rock face without ﬁrst checking it for

safety. If in doubt keep well clear.

• In open cut mines steep batters can collapse with little warning. Dangerous

ground is usually marked by small rock slips or by cracks opening on the berms

at the head of the face. Consult with mine engineers about potentially dangerous

ground. Faces that have been stable for many months or years are unlikely to

collapse just because you are underneath them – but beware all rock faces after

heavy rain or if there has been any recent digging or blasting or movement of

heavy equipment nearby.

• In underground mines, new mine advances generally need to be barred down and

perhaps secured with rock bolts or steel mesh before it is safe for anyone to enter.

Check with the mine foreman before entry to such areas.

• In open cut mines keep well clear of heavy earth moving machinery – the drivers

of these units have almost zero visibility for anything close to them or behind

them or indeed for anything as small as a human being – even if they are clothed

in ﬂuorescent orange, jumping up and down and waving their arms about. When

mapping active mining areas wait till the coast is clear before approaching the

face – not all areas of a mine are worked at the same time: shift changes or

meal breaks can often leave the mine area clear for periods long enough for the

geologist to get close to the active faces in order to carry out her job.

• Wear safety gear. This includes hard hats, safety glasses, steel-capped boots,

heavy duty coveralls and brightly coloured or reﬂective clothing.

Reference

Sales RH (1913) Ore deposits at Butte, Montana. Trans Am Inst Min Eng 46:1523–1626

Chapter 4

Trenching and Underground Development

4.1 Preamble

Pits and trenches, or to use the old Cornish mining term, costeans, can be a quick

and cheap way of obtaining lithological, structural and assay information in areas

of shallow cover. Underground development, although neither quick nor cheap, has

also been successfully employed in exploration programmes to gain direct access to

mineralised rocks for detailed mapping and collection of bulk samples for assay or

metallurgical studies.

4.2 Pitting and Trenching

Pitting is usually employed to test shallow, extensive, ﬂat-lying bodies of mineral-

ization. An ideal example of this would be a buried heavy mineral placer. The main

advantage of pitting over a pattern-drill programme on the same deposit is that pits

are capable of providing a very large volume sample. Large sample sizes are neces-

sary to overcome problems of variable grade distribution, which are a characteristic

feature of such deposits.

Trenches are usually employed to expose steep dipping bedrock buried below

shallow overburden, and are normally dug across the strike of the r ocks or mineral

zone being tested. Trenches are an excellent adjunct to RAB or RC drilling pro-

grammes, where the structural data from trench mapping are needed to complement

the lithological information obtained from the drill cuttings.

In some cases, it may be possible to completely strip shallow unconsolidated

overburden to expose large areas of bedrock. This is done by bulldozing and/or

by sluicing with high-pressure water hoses. The bedrock can then be mapped

and sampled in great detail. Since the process is environmentally destructive, and

rehabilitation would be expensive, extensive stripping would normally only be

attempted when a prospective mineralized zone had been deﬁned, and special sam-

pling/geological problems were present that needed this kind of 100% exposure for

their resolution.

R. Marjoribanks, Geological Methods in Mineral Exploration and Mining, 2nd ed.,

DOI 10.1007/978-3-540-74375-0_4,



Springer-Verlag Berlin Heidelberg 2010

64 4 Trenching and Underground Development

Pits and trenches can be dug by bulldozer, by excavator, by back-hoe or even by

hand. Excavators and back-hoes are generally much quicker, cheaper and environ-

mentally less damaging than bulldozers, and because of this are nowadays usually

the preferred options for costeaning. A large excavator can match a bulldozer in its

power to dig rock. Back-hoes are relatively light machines suitable for digging small

pits or narrow trenches. Back-hoe trenches are difﬁcult or impossible to enter and

back-hoes are really more of a geochemical sampling tool than a geological tool.

When digging a trench, an excavator that can dig a trench of at least 1 in width and

that is capable of penetrating a minimum of I m into recognizable bedrock should

be used. It is very hard to observe details of geology on the walls of trenches that

are smaller than this. Continuous trenching machines,

which can rapidly cut a nar-

row (around 20 cm) trench to 1–2 m depth in soft material, have also been used in

exploration (and grade-control sampling in the soft weathered material of some open

cuts) for providing a continuous geochemical sample. These trenches are generally

of little use for anything other than basic lithological mapping.

Hand-dug pits and trenches are a valid option in places where power excava-

tion equipment cannot be brought to a remote site, and abundant cheap labour is

available. In remote and rugged tropical areas, contour trails/trenches have been

successfully employed in order to locate the source of anomalous ﬂoat boulders

or stream sediment geochemical anomalies. These trenches are long continuous

notches, or incised tracks, cut along the contours of steep slopes. They can expose

narrow strips of weathered bedrock along their inside edge. The exposed bedrock

can be geologically mapped and channel sampled. The trails provide convenient

access to the property and they are readily surveyed using a tape and compass.

Contour trenches cause less environmental damage than trenches that run across

the slope since in high rainfall areas the latter tend to become drainage channels that

focus erosion.

4.3 Underground Development

Because of the expense and time involved, this option would only be considered in

mineral exploration if three requirements are met:

1. The prospect must have a very good chance of hosting a proﬁtable ore body.

2. The project is not suitable for testing by drilling. This can happen where the

highly variable nature of metal distribution requires a large sample to be taken

so that a true grade can be determined. It can also happen because the nature

of the property (remote, very rugged, lack of access) means that large drill rigs

cannot be brought to site or set up on level drill platforms.

3. There is access to underground mining expertise and equipment.

Usually adapted from machines designed to dig trenches for shallow pipes and drains.

4.4 Safety and Logistics in Trenching 65

In the special conditions obtaining in the Eastern Goldﬁelds of Western Australia,

gold exploration projects have in the past been explored by shaft sinking and devel-

opment on several underground levels. Contract miners are employed to do this in

a process known as “tributing”. The miners pay for their efforts by keeping any

gold they ﬁnd, less a small proportion paid as “tribute” to the property owners. The

owners of the property in their turn get underground access to map and sample the

mineralisation. Should a company-scale mine be deﬁned, the end of the contract

period sees the right to mine revert to the owners.

Where steep dipping veins or zones of mineralisation occur in regions of strong

relief, driving short horizontal adits at several levels from the surface can provide

relatively easy access the mineralisation on several levels.

4.4 Safety and Logistics in Trenching

When digging a trench, attention to the following points will make subsequent

mapping and sampling much safer and more convenient.

• Cut back both sides of the top of the trench for one bucket width and to a depth

of 50–100 cm as shown in Fig. 4.1. This prevents loose unconsolidated sur-

face material from falling into the trench (and on to the head of any geologist

below!).

• Stack all topsoil and any loose surface material from the trench on one side of

the opening; stack all bedrock material to the other side. This facilitates making

a quick assessment of the trench material from the bedrock spoil heaps and will

permit a bulk sample to be taken if required. When re-ﬁlling the trench (a normal

environmental requirement) the spoil should be replaced in reverse order so that

the topsoil is preserved on top.

• If the trench is deep (i.e. cannot be easily climbed into or out of) and more than

50 m long, provide an access ramp at its midpoint.

• Most trench wall collapses take place in the ﬁrst few hours after digging or

else after heavy rain. With deep trenches, it is therefore advisable to leave

them for at least 24 h before entering and not to enter them immediately after

rain.

• In any case, never enter a deep trench unless accompanied by another person

who should remain outside the trench and be ready to provide assistance if

necessary.

• Before entering any trench, but particularly an old one, walk it out along the

surface to check for incipient wall cave-ins. If in any doubt, do not enter! There

is generally plenty of information to be obtained from the spoil heaps along the

trench edge; the walls of old trenches are often covered in grunge anyway and

certainly not worth risking one’s life for.

66 4 Trenching and Underground Development

Fig. 4.1 The ideal proﬁle of an excavator trench. Put excavated topsoil on one side of trench, bed

rock spoil on the other. Stepping back the sides of the trench as shown will makes it much safer to

enter

4.5 Geological Mapping

Trenches should be geologically mapped. To make the map, the following procedure

has been found useful:

• Drive a peg into the ground at one end of the trench and run a tape measure from

there along the trench ﬂoor.

• Using the tape, mark and number the wall of the trench with spray paint every

two metres. If the ﬂoor of the trench is sloping, a clinometer will have to be used

to calculate slope corrections before marking the walls.

• Scales from 1:50 to 1:500 are appropriate for trench mapping.

• Shallow trenches are best mapped in plan view, with information seen on the

walls of the trench projected on to plan (see Fig. 4.2). The strike of individual

features which traverse the trench can be determined by their position on the two

walls.

• Where a good vertical proﬁle can be seen in the trench wall, make a vertical plan

of the wall, as well as a horizontal plan (see Fig. 4.3).