Marjoribanks R. Geological Methods in Mineral Exploration and Mining

Подождите немного. Документ загружается.

4.5 Geological Mapping 67

Fig. 4.2 An example of 1:500 scale geological mapping of a shallow excavator trench. The trench tests gold bearing quartz veins in steep dipping and deformed

Proterozoic metasediments

68 4 Trenching and Underground Development

Fig. 4.3 An example of 1:250 scale geological mapping of a relatively deep excavator trench. The trench, dug in deeply weathered Archaean metabasalt

overlain by Tertiary sediments, tests a surface gold soil anomaly. It unexpectedly encountered a thin coal seam and some old mine workings

4.6 Geochemical Sampling 69

4.6 Geochemical Sampling

Geochemical sampling of a trench involves taking a number of chip channels along

it. The procedure for collecting the sample will be described in some detail as it is

the same as for sampling any continuous rock exposure such as an outcrop, or the

walls of an open-cut or underground mine (see also Sect. 9.12).

Intervals for sampling should be marked out by the geologist on the exposed

rock and can be subsequently collected by a ﬁeld technician. Sample intervals

should be chosen to reﬂect natural geological boundaries that are considered to be

mineralization controls.

Where mineralized features are steep-dipping, the appropriate sample is a hor-

izontal channel along the trench wall (or ﬂoor, if that is where the best outcrop

is). Where mineralized features are ﬂat-lying, channel samples should be verti-

cal. Where there is no certainty as to the attitude of mineralized zones, a sample

consisting of both horizontal and vertical channels, composited over selected

horizontal intervals, should be used.

Soft materials can be sampled with a geology hammer or chisel, but there is a

danger that the harder bands (such as siliciﬁcation) might be undersampled, and

soft, easily collected material oversampled. In general, a good rock-chip channel

sample can only be collected with the help of a jackhammer or rock saw.

Hand-held, electric-powered, diamond saws allow a continuous channel to be cut

in the rock. The best technique is to cut out the sample using two angled cuts in the

form of a V. Alternatively, two parallel cuts can be made and the rock subsequently

broken free with a chisel. Diamond saws produce an excellent sample of even size,

but the procedure is slow and expensive, and would only normally be used for sam-

pling mineralized intervals. The use of a rock saw is more indicated for sampling in

underground mines than for trench sampling.

Small, electric jack-hammers are available which are powered by a portable

petrol generator; these are a relatively cheap and quick way of sampling most rocks

and are an ideal tool for trench sampling.

A recommended procedure which has been found useful for sampling a trench is

as follows:

• Cut a continuous chip channel sample along the trench. The maximum size of the

fragments should be around 50 mm. Any overlarge pieces will have to be broken

with a hand-held hammer and the over break discarded.

• Lay a canvas sheet along the bottom of the trench to collect the sample (Fig. 4.4).

• The sample volume is hard to control and will usually be too great for easy

bagging. It must therefore be split to a smaller representative portion (say 5–

10 kg) before it can be sent for assay. If the rock fragments are sufﬁciently

small, this can be done using a rifﬂe splitter. However, operating a splitter in the

restricted area of a trench ﬂoor can be difﬁcult, and cleaning the splitter between

each sample is tedious and time consuming.

• An effective and less labour-intensive way of collecting a split for assay is to

homogenize the sample by rolling it once or twice in the canvas tarpaulin on

70 4 Trenching and Underground Development

Fig. 4.4 Collecting a continuous chip-channel sample from a trench wall. The sampler is using a

small electric jackhammer powered by a portable generator. The broken rock falls onto a tarpaulin

laid along the trench ﬂoor. The same technique could be used for sampling any hard rock exposure

in the ﬁeld or in a mine

which it was collected, so as to form a long even pile of broken rock. The action is

somewhat similar to rolling a cigarette but unlike a cigarette requires at least two

people. A section of lengthways-cut 100–150 mm poly-pipe is then laid alongside

the rock pile and by rolling rock and pipe together in the tarpaulin once more, the

split pipe is ﬁlled with a portion of the broken rock pile. The contents of the half

poly-pipe section can then be easily slid into a sample bag (Fig. 4.5)

4.7 Examples of Successful Exploration Programmes 71

Fig. 4.5 Splitting a sample for assay from an even row of small rock fragments on the ground.

The rock fragments lie on a tarpaulin; a halved length of 100 mm poly pipe is laid along the

row and rocks and pipe are rolled together in the tarpaulin. Rock in the pipe is then slid into a

sample bag

4.7 Examples of Successful Exploration Programmes

Few histories of exploration programmes, even those that discover ore, are ever

adequately written up. This is a pity, because hindsight can show where the critical

decisions were made by the explorationist, both those that led to success and those

which added to the cost and inefﬁciency of the exploration process. The following

examples are all of successful exploration projects about which the author has some

personal knowledge.

1. In 1985, a geologist was contracted by Coolgardie Gold N.L. to make a detailed

geological outcrop map of the Coolgardie district in the Archaean age Yilgarn

craton of Western Australia. In a small valley with no outcrop, the geologist

realised that a series of low grassy mounds were the spoil from regularly-spaced

back-hoe pits that had been dug many years earlier in the course of an unsuccess-

ful search for placer gold. Each pit had been dug to the base of the alluvium (1–3

m), and along with river gravel had brought up samples of bedrock to surface.

This was a distinctive quartz gabbro that the geologist knew was a favoured host

rock for gold mineralisation elsewhere in the district. By sampling the bedrock

in the spoil heaps, an anomalous gold zone was quickly outlined. This was then

deﬁned by an extensive program of shallow excavator trenching. After evalua-

tion RC drilling, the prospect became the proﬁtable Greenﬁelds open cut gold

mine (Keele and Shelton, 1990). Prospect and mine were named in memory of

the pleasant valley, ﬁlled with spring wild ﬂowers at the time of discovery, which

disappeared below the open cut and waste dumps of the mine.

72 4 Trenching and Underground Development

2. In 1985, RGC Exploration discovered the Lucky Draw gold mineralisation,

near Burraga in the Australian state of New South Wales. The mineralisation

occurred within a stratabound iron-rich skarn in poorly-exposed metasediments

of Ordovician age. Initial diamond drilling with inclined holes showed that the

rocks had a dominant shallow-dipping metamorphic fabric. On the basis of this

observation, it was decided that all future holes should be vertical. Many drill

holes later, it became apparent that high grade gold intercepts could not be easily

correlated. A number of excavator trenches were then dug across the prospect

to expose the shallow bedrock and give a better idea of the structure. Trench

mapping showed that the ﬂat-lying fabric was a late cleavage which lay at a

high angle to sub-vertical bedding surfaces. Drilling was then resumed using

angle holes. A rational model for gold mineralisation soon emerged. Lucky Draw

subsequently became a successful open cut gold mine.

3. In 1995, Newcrest Mining was exploring for gold and copper in the Neogene

magmatic arc of Halmahera Island, Indonesia. Because of recent ash and collu-

vial cover, outcrop was limited. Along the bed of a small stream and ﬂanks of

a nearby ridge line, prospecting located abundant ﬂoat boulders of epithermal

quartz, some yielding high gold assays. A series of hand-dug trenches across

the slope above these occurrences exposed (sub) outcrop of low-sulphidation

epithermal quartz veining with a strike of over 300 m and widths to 30 m (Carlile

et al., 1998). Once the extent of the surface outcrop had been deﬁned by trench-

ing, an extensive diamond drilling program proved an open pittable resource

of 0.99 Mt at 27 g/t gold and 38 g/t Ag (Oldberg et al., 1999). This became

the highly proﬁtable Gosowong mine. Gosowong is now mined out, but through

persistent ongoing exploration in the surrounding district, Newcrest has located a

number of other similar high grade gold deposits, some of which (e.g. Kencana,

Toguraci) are currently being mined.

4. During 1999, in the remote, jungle covered eastern mountains of Burma, the

Canadian company Ivanhoe Limited, following up gold anomalies from regional

prospecting and geochemistry, located the Modi Taung prospect – a 4.5 km long

zone of steep-dipping mesothermal quartz veins that carry bonanza gold values

(Mitchell et al., 2004). The veins are hosted by cleaved mudstone of Mesozoic

age. Because of the difﬁculty of diamond drilling in such remote and rugged

terrain, and the extreme variability of gold distribution within the veins, it was

decided to explore the prospect by means of a series of adits driven from differ-

ent levels of the adjacent slopes. A total of 5 km of underground development

allowed detailed mapping of the veins and the collection of continuous channel

samples from the advancing faces and walls of drives in ore. Although the explo-

ration was successful in deﬁning a potentially proﬁtable company mine, Ivanhoe

was eventually forced withdraw for non technical reasons.

References

Carlile JC, Davey GR, Kadir I, Langmead RP, Rafferty WJ (1998) Discovery and exploration

of the Gosowong epithermal gold deposit, Halmahera Island, Indonesia. J Geochem Explor

60:207–227

References 73

Keele RA, Shelton MC (1990) Greenﬁelds gold deposit, Coolgardie. In: Hughes FE (ed) Geology

of the mineral deposits of Australia and Papua New Guinea. Australasian Institute of Mining

and Metallurgy, Melbourne, 463–466

Mitchell AHG, Ausa CA, Deiparine L, Hlaing T, Htay N, Khine A (2004) The Modi Taung-

Nankwe gold district, slate belt, central Myanmar: Mesothermal veins in a Mesozoic orogen. J

Asian Earth Sci 23:321–341

Oldberg DJ, Rayner J, Langmead RP, Coote JAR (1999) Geology of the Gosowong epithermal gold

deposit, Halmahera, Indonesia. In: Pacrim ’99 conference papers, Bali, Indonesia, 179–185.

Chapter 5

Drilling: A General Discussion the Importance

of Drilling

Drilling is one of the most important, and can be the most expensive, of all mineral

exploration procedures. In almost all cases, it is drilling that locates and deﬁnes

economic mineralization, and drilling provides the ultimate test for all the ideas,

theories and predictions that are generated in the preceding prospect generation and

target generation phases of the exploration process.

In any exploration group, the percentage of the budget that is put into drilling

worthwhile targeted

holes provides a measure of the efﬁciency of that group. To

use management school jargon, the amount of targeted drilling is the key perfor-

mance indicator (KPI) of an exploration group. Many well-managed and successful

exploration companies believe that, averaged over a period of time, at least 40%

of their exploration dollars should be spent on drilling targeted holes. The large

amount of space in this book devoted to drilling reﬂects the supreme importance of

this phase of exploration.

5.1 Types of Drilling

There are a large number of different drilling techniques. This book does not attempt

to describe them all, but focuses instead on the three basic types that are most com-

monly used in mineral exploration. In order of increasing cost, these are auger

drilling, rotary percussion drilling and diamond drilling (see Figs. 5.1, 5.2 and

Table 5.1). A detailed discussion of these drilling techniques will be found in Chaps.

6 and 7.

The term “targeted hole” is used here to refer to holes drilled on deﬁned prospects where there

is expectation of intersecting economic mineralization, as distinct from holes which are drilled

primarily to increase geological or geochemical background knowledge.

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

DOI 10.1007/978-3-540-74375-0_5,



Springer-Verlag Berlin Heidelberg 2010

76 5 Drilling: A General Discussion the Importance of Drilling

Hand Auger

Screw Auger

(post hole

digger)

Fig. 5.1 Simple drills for

collecting sub-surface

geochemical samples

Blade Bit Roller Bit

Chisel Bit

Types of bitReverse Circulaon

Drill

Drill Rods

Inner tube

for cuƫngs

& air return

Compressed Air

CirculaƟon

Standard Drill

Fig. 5.2 Cut-away sections through rotary percussion drills

5.2 Choosing the Right Technique

Selecting the right technique or combination of techniques is always a trade-off

between speed, cost, required sample quality, sample volume; logistics and environ-

mental considerations (see Table 5.1). Augering and RAB drilling provide relatively

low levels of geological knowledge but are quick and cheap and so are useful prin-

cipally as geochemical reconnaissance tools for collecting samples below areas of

shallow overburden.

5.2 Choosing the Right Technique 77

Table 5.1 Comparison of exploration drilling methods

Drill type Indications Advantages Disadvantages

Hand auger Geochemical sampling

in upper few meters

of unconsolidated

material

Hand portable and

operable.

Uncontaminated

sample. Cheap

Poor penetration

Power auger

(post-hole

digger)

Geochemical sampling

in upper few metres

of unconsolidated

material

Small lightweight

machine – vehicle

mounted or hand

operated. Quick,

cheap

Poor penetration (better

than hand auger).

Sample contamination

Rotary air blast

(RAB)

Geochemical sampling

to base of regolith

Ideal regolith

sampling tool

Large sample volume.

No site preparation

needed. Quick and

relatively cheap. Some

rock chip geological

data

Poor penetration of hard

rocks. Sample

contamination.

Limited depth. No

structural data

Air core Geochemical sampling

where good

characterisation of

bedrock required

Small rock core return.

Minimal

contamination.

Relatively quick and

cheap. Can penetrate

heavy clay/mud

Small sample size

Reverse

circulation

(RC)

Geochemical sampling

hard and soft rocks

to 200 m + Ore body

proving above water

table

Uncontaminated large

volume sample. Rock

chip geological data.

Relatively quick and

cheap cf. diamond

Large heavy rig may

need access

preparation. Limited

structural data. Poor

orientation control

Diamond Ore targeting and

proving to

1,000 m + High

quality sample.

Geological/structural

understanding

Maximises geological

information.

Uncontaminated,

undisturbed

high-recovery sample.

Accurate hole

positioning/control

Some site preparation

required. Water supply

required. Relatively

small sample size.

Slow. Expensive

Large rotary percussion rigs can quickly drill a large-diameter hole (100–

200 mm) with good sample volume and at reasonable cost. They are powerful

machines capable of penetrating much deeper and through harder rock than the RAB

rig. However, in normal rotary percussion drilling, the long sample return from drill

bit to the surface along the outside of the rods may produce contamination from the

walls of the hole. This problem can be especially acute when dealing with the low

and often erratic concentrations typical of gold mineralization. The sample recovery

system used in the RC drill rig (see Chap. 6) is designed to overcome the contam-

ination problem and for this reason RC rigs are nowadays speciﬁed in most rotary

percussion drilling programmes.

Diamond drilling provides the premier sample for both geology and geochem-

istry. A diamond impregnated cutting bit produces a solid cylinder of rock (Fig. 5.3).