Marjoribanks R. Geological Methods in Mineral Exploration and Mining

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7.6 Recognizing and Interpreting Structures in Core 109

Fig. 7.6 How to ﬁnd a

ﬁne-grained penetrative

mineral lineation in core. As

the core is slowly tilted at a

low angle to a strong direct

light, crystal faces of elongate

minerals that parallel the core

surface will often reﬂect a

ﬂash of light, thus revealing

their preferred orientation

(see also Laing, 1991,

Structural geology in

exploration and mining,

unpublished)

7.6.5 Folds

Fold axes are deﬁned by the line joining points of maximum curvature (an inﬂection

line) on the folded surface (Huddlestone, 1972). Where a fold is intersected by drill

core, the core surface will show two fold inﬂection points: one where the fold axis

enters the core, and one where it exits. As explained above, each planar limb of the

fold is also deﬁned by two inﬂection points that represent the ends of the long axis of

the intersection ellipse for the surface. With all these inﬂection points, the trace of a

folded bed on the surface of core can therefore have quite a complicated appearance

and identifying the position and orientation of the axes of folded surfaces in drill

core therefore requires some care.

The simplest case comprises those folds whose axes are oriented at right angles to

the core axis. Here the inﬂection lines for the two fold limbs (and for any axial-plane

cleavage present) will all coincide and lie at right angles to the fold axes (Fig. 7.7).

By looking at right angles to the core s urface, the true proﬁle shape of the fold can

be observed.

In the more general case, where fold axes are not normal to the core axis, the

inﬂection lines for the two fold limbs and the axial-plane cleavage do not coincide.

This produces a complex asymmetric fold shape on the core surface (Fig. 7.8). In

order to distinguish the fold axes from the inﬂection points for the two fold limbs, it

is necessary to carefully trace the folded bed right around the core. No view of the

core gives the true proﬁle of the fold, although, by looking along the fold axis, and

allowing for the effects of foreshortening, some idea of the true fold proﬁle shape

can still be obtained.

110 7 Diamond Drilling

CORE AXIS

FOLD AXIS

Point E for Limb 1

Point

E′

for Limb 2

Point

E′for cleavage

Point E for Limb 1

CORE AXIS

Limb 1

Limb 2

FOLD AXIS

Fig. 7.7 How folds look in core when the fold axes are normal to the core axis. In the illustration,

a bedded sequence deﬁnes a small fold pair. There is a strong penetrative axial plane cleavage that

bisects the fold limbs. Inﬂection lines for the two fold limbs and the axial plane cleavage coincide

and are symmetrical about the fold axes

FOLD AXIS

Point E′

for Limb 2

Point E′

for cleavage

Point E for Limb 1

CORE AXIS

Limb 2

Limb 1

FOLD AXIS

Fig. 7.8 How folds look in core when fold axes are not normal to the core axis. In this example,

the same fold pair illustrated in Fig. 7.7 is intersected by a drill hole that is oblique to the fold axes.

This results in a complex trace of the beds on the core surface. Inﬂection lines for the two fold

limbs and the axial plane cleavage do not coincide and are asymmetrical with the fold axes

7.6.6 The Scale Problem

In a small piece of drill core it is relatively easy to see small or close-spaced struc-

tures, but structures which are signiﬁcantly larger than the diameter of the core can

7.6 Recognizing and Interpreting Structures in Core 111

be hard to recognise. For example, a strong axial plane cleavage may be very obvi-

ous in core, but this may obscure a folded bed that lies at a high angle to the cleavage

(Fig. 7.9). In outcrop, the solution to this problem is to step back, let the busy fore-

ground detail go out of focus, and view a larger area of the outcrop. In drill core, this

is not so easy, but so long as the geologist is aware of the problem a solution can be

attempted. One method is to observe the structures present in the less deformed sec-

tions of the core, and examine the nature of the transition to the more deformed parts

of the core. Another way is to do what one would do at an outcrop, standing back

from the core and attempt to see any larger structure that may be present. Sometimes

the large structure is only apparent when a detailed log for the entire hole is plotted

on to the drill section, and comparisons made with adjacent holes. When a large fold

structure is suspected, detailed re-examination of the core will often yield the subtle

indirect evidence for its presence, which might have been missed in the ﬁrst-pass

logging.

Small structures often give a reliable indication of the style and attitude of associ-

ated larger structures. This relationship (mentioned above in relation to minor faults)

has long been known to geologists as “Pumpelly’s Rule” after Raphael Pumpelly

the USGS geologist who ﬁrst enunciated it (Pumpelly et al., 1894, p. 158). The rela-

tionship is now appreciated as a f undamental feature of many non linear systems,

and is referred to as “scale dependent self similarity”, or more simply as a fractal

relationship (from “fractal dimension” see: Blenkinsop, 1994; Mandelbrot, 1983).

Whatever it is called, such relationships can provide very valuable clues for resolv-

ing structure of large size from the evidence of small structures seen in drill core or

outcrop.

Fig. 7.9 Because of the small

size of drill core, identifying

large structures is difﬁcult.

Here a strong cleavage in a

fold hinge obliterates earlier

bedding surfaces in a small

piece of core. Only by

carefully observing

progressive changes over a

longer section of core can the

true structure be identiﬁed

He also had the mineral pumpellyite named after him.

112 7 Diamond Drilling

As a general rule, and this applies to any type of observation, the best way to

recognise subtle features in a rock is to be aware of the possibility that such a fea-

ture might be present, and then actively look for it. The famous nineteenth century

French scientist Louis Pasteur put the same idea well in 1854: “Where observation

is concerned, chance favours the prepared mind”. Good observation has nothing to

do with good eyesight.

7.6.7 Vergence

Vergence is the name given to the systematic changes in the symmetry of small

structures about the axis of a large fold.

Recognising and recording structural

asymmetry in outcrop or drill core can allow deduction of the approximate position

and geometry of the larger structure. Structures from which vergence information

can be gained are fold pairs (either S-shape or sinistral; Z-shape or dextral) or the

sense of angular relationship between bedding and cleavage (once again described

as dextral or sinistral). Note that, by convention, the terms dextral and sinistral refer

to an angular relationship as it appears looking down on the structure. When viewed

from below, the sense of asymmetry is reversed. For example, a sinistral (S shape)

fold pair becomes dextral (Z shape) when viewed from below. This means that

vergence relationships can only be recognised in oriented core.

Structures showing vergence relationships are readily seen in drill core and, if

the core is oriented, can be used to provide valuable data on the style, position and

geometry of larger scale folding. Even if the core is not oriented, some aspects of

changing vergence relationships can still indicate that a major fold axis has been

crossed. For example, cleavage at a high angle to bedding indicates a position on

a fold hinge: cleavage at a relatively low angle to bedding indicates a position on

the fold limb. However, if the core is oriented, the vergence observations can also

indicate whether the large fold that has been crossed is an antiform or a synform, as

well as providing data on the geometry of the fold itself (Fig. 7.10).

Once a vergence determination has been made, it can be recorded on the log or

drill section as a vector (an arrow) pointing up the hole or down the hole towards

the adjacent large antiform. Arrows pointing towards each other on the hole trace

will thus indicate the position of a map scale antiformal axis. Arrows pointing away

from each other will then indicate the position of a synformal axis.

For a discussion and explanation of the use of vergence in structural geology, the reader is

referred to any good structural geology textbook, such as Wilson (1961) or Hobbs et al. (1976).

An antiform is any upwards-closing folded surface. A synform is any downwards-closing folded

surface. The terms anticline and syncline can strictly only be used when describing sedimentary

sequences where the original stratigraphic top of the sequence is known. Thus in an anticline beds

become older towards the centre of the fold. In a syncline, beds become older away from the centre

of the fold.

7.7 Measuring and Recording Structures in Core 113

Fig. 7.10 Using the angular relationships between bedding and cleavage (vergence) in oriented

drill core to deﬁne large, prospect scale, antiformal or synformal folds. A small arrow plotted

beside the drill trace on section indicates the direction to the adjacent antiform. Opposed arrows

show that a fold axis has been crossed in the hole

7.7 Measuring and Recording Structures in Core

Down-hole orientation surveys record the deviation of a hole from its initial azimuth

and inclination (Sect. 7.2). However, the solid stick of drill core (sometimes, not so

solid) recovered from a hole is not fully oriented by a down-hole survey. Although

the azimuth and inclination of the core axis is known, it has another degree of free-

dom in that it can rotate about its long axis. This does not affect a lithological log

made of the core (the down-hole depth to any given point on the core can still be

measured), nor the ability to measure true thickness of beds (Fig. 7.14), but the

original attitude of structures cannot be directly determined. However, techniques

are available to fully orient the drill core.

In many cases rocks contain a penetrative planar fabric such as bedding or cleav-

age whose orientation is known from surface mapping. In these cases, if this surface

can be identiﬁed in the core, and the assumption can be made that its attitude is con-

stant, the surface can be used to orient the core. A cleavage is a better surface to use

for this than bedding because the attitude of a cleavage is generally more constant

than that of bedding (Annels and Hellewell, 1988). Once the core is oriented by this

means, other structures present can be directly measured. How to do this is fully set

out in Appendix B.

A common situation occurs when a hole is drilled at right angles to the strike

of a planar structure whose dip is either not known or else is suspected of changing

through the length of the hole, perhaps as the result of folding. This is a situation that

often arises when drilling surface geochemical or geophysical anomalies. All that

can be measured is the angle alpha – the angle that the surface makes with the long

axis of the core. Since in this case the strike is either known or can be conﬁdently

114 7 Diamond Drilling

Fig. 7.11 How to plot the attitude of surfaces in non-oriented core where the strike of the bed

is known, but the dip is not (i.e. assuming the hole azimuth is normal to strike). Of the two

possibilities plotted, one can often be discarded as unlikely

assumed, the surface can be plotted on the drill section in only two possible attitudes,

that is, symmetrically disposed at angle alpha about the hole trace (Fig. 7.11). In

many if not most cases, simple inspection of the two geometric possibilities for

the attitude of the measured surface will lead to one of them being discarded as

unlikely.

In the general case where the strike of a surface seen within the drill core cannot

be assumed, then the angle between that surface and the core axis (α) deﬁnes half

of the apical angle of the cone that would be produced if the surface were rotated

around the core axis. This is illustrated in Fig. 7.12, where the different possible

attitudes of the surface on rotating the core about its axis are shown. In this case,

no absolute measurement of the attitude of the surface is possible, but it is still

worthwhile to plot the boundaries of the cone of lines on to the drill section to

represent the limits to the range of possible solutions to the true orientation of the

surface.

Fig. 7.12 How to plot the attitude of a surface where neither its strike nor dip is known. On the

trace of the drill hole on section two lines with ticks indicate the range of possible attitudes deﬁned

by the alpha angle

7.7 Measuring and Recording Structures in Core 115

Fig. 7.13 How to plot the attitude of surfaces in non-oriented core where the surface is at 90

◦

the core axis (α = 90

◦

). This situation can only arise when drilling at right angles to strike. The

dip shown on the section is therefore a t rue dip. However, the core is still not fully oriented, as the

attitude of any surfaces not at 90

◦

to the core or any linear structure still cannot be measured

A special case exists where the surface seen in core is exactly normal to the core

axis. In this case, rotation about the axis gives no apparent change in attitude of the

bed. The surface can therefore be plotted directly on to the drill section as a short

line at right angles to the hole trace (Fig. 7.13). However, note that the core is still

not oriented, and other planar or linear structures that might occur within it (that are

not normal to the core axis) cannot be measured.

For the general case where no assumptions can be made about the attitude

of structures in the core, absolute measurement can only be made if the core is

mechanically oriented by means of a core-orienting device. The specialized pro-

cedures involved in handling and measuring oriented core are fully described in

Appendix B.

Even where core is not oriented, it is still possible to make a large number of

useful structural observations and measurements. For example:

• Qualitative observations on the style and nature of structures.

• The relative ages of structures and their relationships to lithology, veining and

alteration.

The only quantitative structural measurement possible in non oriented core from

a single drill hole is the alpha angle. Taking and recording alpha angles for planar

structures is quick and easy to do and should be a routine procedure in all core

logging. From the alpha angle is possible to use simple trigonometry calculate the

true thickness of any tabular bed or vein intersected by the core. How to do this is

illustrated in Fig. 7.14.

If a distinctive and unique marker surface is intersected in a minimum of three

drill holes, then the coordinates (northing, easting and height above datum) for each

intersection can be used to calculate the strike and dip of the surface. This situation

116 7 Diamond Drilling

Fig. 7.14 How to calculate

the true thickness of beds in

core. All that is needed is the

alpha angle. It is not

necessary for the core to be

oriented, or even to know the

hole azimuth and inclination

is frequently encountered in drilling programs and is referred to as the three-point

problem. The solution is set out fully in Appendix C.

Where there is no single marker bed that can be correlated between adjacent

drill holes it is sometimes still possible to determine the orientation of a set of

parallel surfaces (such as bedding planes, a cleavage, or a vein set) provided the

surfaces have been cored by a minimum of three non parallel drill holes (Mead,

1921; Bucher, 1943). The same technique has even been extended to a single hole,

provided the hole has sufﬁcient deviation along its length for the differently oriented

sectors of the same hole to be considered in the same way as three separate holes

(Laing, 1977). The solution involves an elegant stereo net procedure: how to do it is

described fully in Appendix C.

7.8 Core Logging Systems

The way data are to be recorded inﬂuences in a signiﬁcant way the amount and type

of data that are observed.

Using the most appropriate system for recording geolog-

ical observations of drill core is therefore very important. Although a huge number

of different logging forms are used in industry (almost every exploration group has

its own design), there are only three basic methods for recording observations on

drill core and cuttings. All individual logging systems correspond to one or other,

or a combination, of these basic methodologies. The three logging styles are char-

acterized here as prose logging, analytical spread-sheet logging and graphical scale

logging. They are described below.

7.8.1 Prose Logging

In prose logging, an interval is selected, identiﬁed by its down-hole depth limits,

and then described in words. Such a log will look like the example of Fig. 7.15.

This is a good illustration of the aphorism – “The Medium is the Message” – coined by Marshall

McCluhan (1964).

7.8 Core Logging Systems 117

NOITPIRCSEDHTPED

123.45 –

136.9

Sandstone with calcareous cement.Pale

khaki colour with a medium,somewhat

gritty grainsize.Bedding defined by

thin shale partings at 123,34m 126.58

& 132.12 also pebbly band at128.4m.

Bedding at generally 45° to LCA but

becomes a bit flatter to end section.

Numerous thin wispy qtz veins assoc.

with pyrite speckling along really

distinct margins(looks a lot like the

last hole).You can see a weakish

cleavage in places but mostly its not

there..

Fig. 7.15 An example of the prose style of recording geological observations of core or cuttings.

An interval is selected, deﬁned by precise down-hole depth intervals, and then described in words.

It is generally difﬁcult to extract precise, objective information from such logs and they are tedious

to read. The style is not recommended

Words are a powerful means of summarizing conclusions and prose is an invalu-

able way to present an argument, give an explanation or provide a discussion.

However, long passages of prose are a laborious and ineffective way of recording

the complex spatial relationships that can exist between the observable f eatures of

a rock. It is also unlikely that any two geologists would ever describe a rock using

quite the same words. This means that extracting precise and objective information

from a prose log, so as to construct a drill section or to understand the relationships

seen, is difﬁcult and time consuming. As a general rule, literary efforts should be

reserved for report writing and not be used as a means of routine core description.

It is therefore recommended that this style of logging be only used, if at all, in a

special “Comments” column to provide brief verbal commentaries to complement

one of the other two logging styles.

7.8.2 Graphical Scale Logging

Initial stage drilling needs to make use of a logging system that permits and

encourages detailed observation and presents the information in a manner that aids

interpretation. Pre-eminently, the best recording system for this is graphical scale

logging. In such logs, a down-hole strip map is made of the core at some cho-

sen scale, say 1:100. Structures are s imply drawn onto the map in the attitude that

they appear in the core. For example a 50 cm vein, which cuts the core at 45

◦

will be drawn (at 1:100 scale) on the core map as a 5 mm wide vein at 45

◦

to the

core trace. Different columns of the pictorial log (different specialised maps) can

be allocated for different features of the core: for example there could be separate

columns for lithology, alteration, veining and structure. An example of this type of

logging is shown in Fig. 7.16. All the columns are linked horizontally on the log by

118 7 Diamond Drilling

Fig. 7.16 An example of the use of graphical scale l ogging of core. Observations are recorded

graphically and numerically on the log sheet against a selected down-page scale. The use of colour

on such logs greatly enhances information content. Graphical logging is a powerful and ﬂexible

technique which supports detailed observation: its use is recommended for all ﬁrst stage explo-

ration drilling. A detailed description on this log form will be found in Appendix A (Refer Color

Plate 1 at end of book)