Evaluation of dimension stone in gneissic rocks Ð a case historyfrom southern Finland

H. Luodesa,*, O. Selonenb, K. PaÈaÈkkoÈnena

aGeological Survey of Finland, Regional Of®ce for Mid-Finland, P.O. Box 1237, FIN-70211 Kuopio, FinlandbFinska Stenindustri Ab, FIN-23200 VinkkilaÈ, Finland

Received 13 April 2000; accepted for publication 20 June 2000

Abstract

A dimension stone prospect in southern central Finland was assessed by a detailed mapping, geo-radar survey, and core

drilling. The prospect is a veined and bedded garnet±cordierite gneiss, consisting of a dark schistose medium-grained

palaeosome and a light coarse-grained granitic leucosome. Both components are found mainly as thin units, but the leucosome

can occur as individual veins several metres thick, which leads to disturbing variations in the appearance of the stone. The

soundness of the stone is de®ned by the amount of tight, but open cracks in the palaeosome and weakness zones in the

weathered leucosome. As a whole, the soundness is diminished down to 10 m below the outcrop surface. Furthermore, a

subhorizontal body of younger granite is identi®ed at approx. 8±20 m depth. Consequently, the prospect is not feasible for

production of dimension stone.

The core drilling was decisive in the ®nal evaluation of the prospect because the true density of the cracks and the subsurface

granite were identi®ed by the method. Only with the use of core drilling could the proper interpretation of the characteristics of

the prospect be made, demonstrating the importance of three-dimensional investigation of a dimension stone prospect. q 2000

Elsevier Science B.V. All rights reserved.

Keywords: Dimension stone; Building stone; Gneiss; Granite; Finland

1. Introduction

Dimension stone is a natural rock that satis®es

given qualitative requirements and is hence quarried

and processed into de®nite shapes and sizes. Rock

types used as dimension stone include, e.g. granite,

gneiss, gabbro, diabase, marble, limestone, sandstone,

soapstone, and slate. Dimension stone is mainly

utilized in the building, construction, monument and

tombstone industries. The de®nition of dimension

stone covers rough blocks and ®nished material, but

excludes crushed or powdered stone consumed as an

aggregate or reconstituted to form arti®cial stone (see,

e.g. Allison (1984) and Niini (1986)).

Dimension stone is used in areas where the aesthe-

tical properties of stone are crucial. A special feature

in the commerce of dimension stone is its dependence

on fashion. The popularity of stone types changes like

that of clothing, and the anticipation of coming

changes is often almost impossible. The appearance

of a stone is thus a very important criterion for good

dimension stone. Even if stone is a product of nature,

the quality requirements concerning the colour of the

Engineering Geology 52 (2000) 209±223

0013-7952/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.

PII: S0013-7952(00)00059-4

www.elsevier.nl/locate/enggeo

* Corresponding author.

E-mail addresses: hannu.luodes@gsf.® (H. Luodes),

olavi.selonen@®nskastone.® (O. Selonen),

kari.paakkonen@gsf.® (K. PaÈaÈkkoÈnen).

stone are very strict. The colour should be as uniform

as possible across the entire deposit. If the stone is

classi®ed as a one-coloured type, stripes, inclusions,

or veins of a differing colour cannot be accepted in a

stone of the ®rst class. However, if the stone is classi®ed

as a multicoloured type, an appropriate variation of the

colours is required, but also in this case the colour and

design of the stone must be homogeneous enough so

that the market can identify it as one and the same

product. The second crucial criterion for feasible

dimension stone is the soundness of the deposit. The

soundness is de®ned by the use of the stone and by the

demands of the processing industry. For example, a

suitable size of the production blocks for the modern

gang saws varies from approx. 2:40±2:90 £1:30±1:90±0:70±1:40 m; which means that the

spacing of the fracturing in the deposit must be at

least 2±3 m. The third important criterion for good

dimension stone deposit is the market demand for a

stone type. Even if the appearance and soundness are

at an acceptable level, the stone has no value without

demand from the market, which is again dependent

upon the ever-changing fashion. Other criteria for a

feasible dimension stone deposit include infrastruc-

tural and technical criteria. The location of the deposit

is essential for its long-term utilization. It should not

H. Luodes et al. / Engineering Geology 52 (2000) 209±223210

Fig. 1. Geological map of southern Finland. The location of the study area is indicated. Modi®ed after Simonen (1980).

be located near sensitive nature areas or objects, but

ought to be situated close to good transport facilities.

When stone is used by the construction and building

industry, it must satisfy strict physical and mechanical

requirements. These properties are measured in certi-

®ed laboratories by standardized methods. For the

quality demands of dimension stone, see Harben and

Prudy (1991), Jefferson (1993), Shadmon (1996),

Selonen (1998), and Selonen et al. (2000).

Geological investigations of dimension stone

deposits have two goals: identi®cation of ªnewº

deposits and development of ªoldº deposits/quarries

(see, e.g. Ribeiro et al., 1999; Taboada et al., 1999;

Perdahl, 2000; Selonen et al., 2000). A new deposit is

often localized through a stepwise regional explora-

tion survey where the identi®cation of a prospect with

potential for dimension stone is made by a desk study

and a ®eld mapping (Selonen et al., 2000). If the

prospect is interesting enough, detailed mapping,

geo-radar survey, core drilling, test production, test

processing, and commercial testing are available for

further assessment (Selonen et al., 2000). Some of

these methods, such as geo-radar survey or core

drilling, can be used, e.g. when developing an opera-

tional quarry (Selonen et al., 2000).

We have discussed earlier dimension stone evalua-

tion on a regional scale (Selonen et al., 2000) Ð in

this paper we shall describe a case history of a

prospect-scale assessment and discuss the applicabil-

ity of the different investigation methods.

2. Study area

The prospect is a garnet±cordierite gneiss located

in southern central Finland (Fig. 1) within an area

of highly deformed and metamorphosed rocks of

the Palaeoproterozoic age. The prospect was iden-

ti®ed by a regional exploration study and it was

primarily evaluated by a ®eld mapping during

which the general soundness and appearance of

the stone were de®ned. This preliminary assessment

indicated that the prospect had potential enough for a

detailed investigation as the appearance of the veined

gneiss was very interesting, and because there is a

market for such stone. Furthermore, on the exposed

outcrops the stone appeared to be relatively sound.

The location of the prospect with respect to roads,

nature objects, and houses is also suitable as it is

situated by a good forest road and because the nearest

house is at a distance of 500 m. The prospect,

which is a rounded hill with a relative elevation

of approx. 20 m, measures approx. 200 £ 250 m

(Fig. 2) and is well-exposed or covered only by

a thin layer of soil except for the eastern and

south-eastern parts.

On the basis of the ®eld mapping, a research plan

was designed, including detailed mapping, geo-radar

survey, core drilling, reserve assessment, test quarry-

ing, test processing, and quarry planning.

3. Methods of investigation

The site investigations commenced with topogra-

phical measurements by tachymeter and with the

planning of mapping traverses. The total length of

the 3±5 m wide traverses was approx. 820 m (Fig.

2). The exposures in the traverses were cleaned by

compressed air and pressurized water jets.

After cleaning and washing, the traverses were

measured and mapped in detail on a scale of 1:100.

During the detailed mapping, special attention was

paid to the composition, colour, and structure of the

stone, as well as the fracturing. A couple of block

samples were extracted, and a total of 60 specimens

were collected along the traverses by a hand-held

diamond drill. The specimens were split and polished

for examination of colour and mineral composition.

Important portions of the traverses were also photo-

graphed.

The subsurface fracturing along the traverses was

investigated by ground penetrating radar (geo-radar),

using antennae of 100 MHz and 400 MHz. The depth

penetration was 20±25 m, and the total lateral length

of the geo-radar pro®les was approx. 800 m. The geo-

radar pro®les were interpreted in order to study the

overall soundness of the prospect.

A total of 11 core drill holes were drilled (Fig. 2),

adding up to approx. 232 m of drill core (approx.

40 mm in diameter). The soundness and the variations

in the composition of the stone were special targets in

studying the drill cores.

Because of the poor production properties of the

prospect, the reserve assessment, test quarrying, test

processing, and quarry planning were later omitted.

H. Luodes et al. / Engineering Geology 52 (2000) 209±223 211

H. Luodes et al. / Engineering Geology 52 (2000) 209±223212

Fig

.2

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4. Evaluation of the prospect

4.1. Appearance of the stone

The migmatitic and veined garnet±cordierite gneiss

is composed of a dark gneissic palaeosome and a light

granitic leucosome (Fig. 3A). The main minerals in

the palaeosome are plagioclase, potassium feldspar,

quartz, cordierite, garnet, and mica, whereas the

leucosome consists of potassium feldspar, plagio-

clase, quartz, and garnet.

The schistose and bedded palaeosome consists of

pelitic and psammitic layers, the pelitic layers typi-

cally including granitic veins, while the psammitic

layers are more homogeneously gneissic in composi-

tion. The colour of the medium-grained pelitic

palaeosome is dark grey. The slightly folded

schistosity strikes N308E and dips approx. 608SE.

The bedding is de®ned by dark grey and ®ne-grained

psammitic layers (Fig. 3B) parallel to the schistosity.

The thickness of the beds is mostly less than 50 cm,

but occasionally can be up to 7 m thick. They occur

randomly in the prospect area (Fig. 4), and the spacing

is commonly tens of metres and rarely 30±50 cm.

The anatectic and granitic leucosome veins are

massive and medium or coarse-grained, striking

parallel or obliquely to the schistosity and bedding.

They occur as narrow veins, a few centimetres wide,

and as individual layers with a thickness of up to 6 m

(Fig. 3C). The contact between the thick leucosome

layers and the veined gneiss is often sharp. The

spacing of the randomly occurring individual

leucosome veins is mostly in the order of several

metres (Fig. 4). The colour of the leucosome is mainly

light grey or yellowish, but brownish and reddish

zones occur within the veins.

In the middle of the prospect area there are small

intrusions of a younger, red, and coarse-grained

granite (Fig. 4), clearly differing in appearance from

the leucosome. The granite is found in a couple of

H. Luodes et al. / Engineering Geology 52 (2000) 209±223 213

A

Fig. 3. (A) Garnet±cordierite gneiss typical for the prospect area. (B) Bedding in the garnet±cordierite gneiss. (C) Modes of occurrence for the

leucosome in the garnet±cordierite gneiss. (D) Non-penetrative cracks in the garnet±cordierite gneiss. The cracks are often con®ned to the

layers with psammitic material. The length of the compass is 12 cm.

exposures on the outcrop surface and as a 2±5 m thick

subhorizontal lens approx. 8±20 m beneath the

surface.

4.2. Soundness of the stone

The garnet±cordierite gneiss is typically fractured

by discontinuous (some tens of centimetres long),

non-penetrative, tight, almost closed cracks, which

strike perpendicularly (N608W) or diagonally

(N758E and N158W) to the schistosity (Fig. 3D).

The dip of the cracks is normally either subvertical

or subhorizontal. The fracture surfaces are brownish and

ªrustyº. The density of the cracks varies considerably

because of their tendency to occur in clusters, but in

general the fracturing is denser when the material

becomes more psammitic and less granitic.

The fractures in the leucosome are open and strike

N308E, parallel to the schistosity. The open fractures

typically thicken into zones where the stone is

brownish with a structure weakened by strong

weathering. In the separate leucosome veins, these

zones can be up to a couple of metres wide.

Commonly, the weathering reaches down to 10 m

below the surface, but some weathered zones are

found even at 20 m depth. The open fractures occa-

sionally occur in the palaeosome. Clustering of the

fractures is typical (Fig. 5).

The subsurface fracturing (except for drill holes 8

and 6, Fig. 2) is dense in the ®rst 5 m below the

outcrop surface throughout the prospect area, with a

spacing seldom exceeding 1 m. In the western parts of

the prospect (drill hole 7, Fig. 2), the portion of dense

fracturing reaches down to 11 m.

The site is bordered by late shear/fracture zones

approx. parallel to the schistosity in the western

(strike of N258W) and south-eastern (strike of

N158E) parts. The colour of the stone is often reddish

in these zones.

4.3. Feasibility of the prospect

In the international market for dimension stone the

garnet±cordierite gneiss of this study is classi®ed as a

H. Luodes et al. / Engineering Geology 52 (2000) 209±223214

B

Fig. 3. (continued)

multicoloured stone type, which is very sought after.

The product is de®ned by the appearance of the

contrasting narrow veins of dark grey gneissic

palaeosome and light grey/yellowish granitic

leucosome, forming a relatively regular design. The

appearance is further enhanced by a vivid weft of

clusters of red garnet and occasional blue cordierite.

Within the de®nition of the product, the leucosome

can occur as narrow separate veins cutting the main

schistosity.

The several metres thick leucosome layers will

pose a problem as they form an unacceptable variation

in the appearance of the product. In some cases there

could be blocks, which would almost entirely consist

of the light granitic material. The occurrence of the

leucosome as separate layers also indicates that, due

to the mineralogical differences, the sharp contact

between the leucosome and veined gneiss is a prob-

able zone of weakness along which the stone easily

breaks. The rust-brown colour of the weathered zones

in the leucosome is another defect in the appearance

of the stone, which diminishes the value of the

prospect.

In spite of the dif®culties described above, the

appearance of the stone is basically very interesting

and its defects do not destroy the production possibi-

lities of the prospect. The real problem is the lack of

soundness of the stone. As the garnet±cordierite

gneiss is composed of units with different grain size,

mineral composition, and thickness, the fracturing

varies accordingly and no regular joint system (e.g.

cubic) can be found. The total soundness of the stone

is determined by the amount of cracks and by the

weathering of leucosome. In general, the fracturing

is denser in the ®ne-grained layers than in the

coarse-grained layers and the cracks are especially

well-developed in the psammitic layers. Except for

the northern parts of the prospect area, the fracturing

is dense within 5 m below the outcrop surface, and in

the west the dense fracturing reaches even deeper. The

leucosome is often weathered down to 10 m, indicat-

ing that the physical and chemical weathering has

H. Luodes et al. / Engineering Geology 52 (2000) 209±223 215

C

Fig. 3. (continued)

advanced more easily in the coarse-grained granitic

material than in the less porous palaeosome. The

combined effect of the cracks and the weathering of

the leucosome is that the soundness of the stone is

diminished 10 m down from the outcrop surface,

meaning that the recovery rate of the extraction will

be very low during the ®rst two quarry layers. Below

this, the stone will be more sound, but there, the

several metres thick granitic intrusion will lower the

pro®tability of the extraction. Several thousands of

cubic metres (three production layers) would have to

be quarried with a very low output before stone of

reasonable quality is reached. In consequence, the

costs of founding a quarry would be too high to

make it pro®table.

The facts presented above show that the prospect is

not suitable for production of dimension stone.

5. Remarks on the individual evaluation steps

The evaluation described above is based upon a

combination of results from all the applied investiga-

tion methods, while in this section we discuss some of

the issues and problems connected to the individual

methods and evaluation steps during the assessment.

As the prospect was not thickly covered by vegeta-

tion and trees, it was possible to prepare the mapping

traverses almost to the planned length. Only in the

eastern and south-eastern parts, due to the thick soil

cover, did the traverses become shorter than desired.

The detailed mapping in this case was better suited for

assessment of the appearance of the stone than the

soundness of it. The palaesomes and leucosomes

had a relatively consistent and easily mappable strike

without considerable variations. Furthermore, the

main defects, the psammitic layers and the thick

leucosome veins, were easy to observe as their

appearance deviated notably from that of the veined

gneiss. By contrast, the evaluation of the soundness

was problematic due to the absence of a distinct joint

system. The attitude of the discontinuous and almost

closed cracks as well as the degree of weathering of

the leucosome were hard to interpret from the outcrop

H. Luodes et al. / Engineering Geology 52 (2000) 209±223216

D

Fig. 3. (continued)

H. Luodes et al. / Engineering Geology 52 (2000) 209±223 217

Fig

.4.

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H. Luodes et al. / Engineering Geology 52 (2000) 209±223218

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surface, leading to an uncertain assessment of the

amount of fracturing. While mapping the prospect,

only indications of the younger granite were observed,

and no knowledge of its shape could be obtained

because of the low relief of the study area.

The geo-radar survey in this study was done only

along the cleaned traverses. Except for the western

parts of the prospect, no distinct horizontal fracture

surfaces were identi®ed in the geo-radar pro®les,

agreeing with the observation of the absence of a

major joint system. However, features indicating

exfoliation connected to the weathering of the upper

layers were occasionally observed (Fig. 6). The

weathered zones of weakness and the clusters of the

open and tight fractures were saturated with water,

giving good radar re¯ections, whereas the individual

cracks seldom appeared in the pro®les. Features that

could represent tilted large-scale fractures were

observed occasionally (Fig. 6), but we were not able

to con®rm these features by the core drilling. No sign

of the younger granite body was obtained in the geo-

radar survey although it could have been possible

considering the penetration depth of the radar signal.

Apparently, the dielectric properties of the granite did

not differ enough from those of the garnet±cordierite

gneiss to give a clear re¯ection at their contact.

Furthermore, the contact was sharp and unweathered,

offering no surface for re¯ections.

The core drilling was focused on the study of the

soundness of the stone, especially on the aspects that

H. Luodes et al. / Engineering Geology 52 (2000) 209±223 219

Fig. 6. Thirty metres of interpreted geo-radar pro®le. Tilted large-scale fractures are enhanced with lines. Weathering of the outcrop surface can

be seen down to 3 m depth.

H. Luodes et al. / Engineering Geology 52 (2000) 209±223220

could not be exactly veri®ed by the detailed mapping

and the geo-radar survey: the density and depth

dimension of the small-scale cracks and the degree

and depth of the weathering of the leucosome. In

drill core, all the individual cracks (also the semi-

closed cracks) opened, thus de®ning the real density

of fractures (Fig. 7). Furthermore, the variation of the

intensity of the fracturing in the different layers with

different composition appeared well in the drill core.

The weathering of the leucosome was indicated by

loss of core, i.e. parts of the core where the stone

had totally disintegrated during drilling (Fig. 7).

Other important factors that could be con®rmed by

the core drilling included the absence of a distinct

joint system, the consistent migmatitic structure of

the stone, and the occurrence of the leucosome also

as separate veins with sharp contacts to the veined

gneiss. Only with the help of the core drilling could

the subsurface subhorizontal lens of younger granite

be identi®ed (Fig. 7), which was the decisive factor

in our evaluation of the prospect as being non-

economical.

6. Discussion

The investigation methods used in this case are

standard in geological ®eldwork. In dimension stone

evaluations their purpose is to provide the facts

needed for a decision to open a quarry or to abandon

the site. Hence, the assessment of a dimension stone

prospect is targeted to investigate its production

properties, i.e. the effects of the geological character-

istics on extracting, processing, and utilizing the

stone as building material. In most cases, reserves of

stone homogeneous and sound enough for at least 10

years' production should be secured. This emphasizes

the need for three-dimensional consideration of the

dimension stone prospects by utilizing the appropriate

methods.

The case presented in this paper is a good example

of the importance of the use of subsurface investiga-

tion methods, especially core drilling. While all the

previous evaluation steps were necessary to provide a

full understanding of the prospect and to reach the

conclusion that the prospect was not suitable for

dimension stone production, the results from the

core drilling were fundamental for the ®nal assess-

ment. After the detailed mapping, the prospect

appeared to be more sound than it was in reality. It

is typical that the true nature of the fracturing is very

dif®cult to judge exactly from the outcrop surface.

Commonly, a zone of intense fracturing is restricted

to the surface of the rock, and underneath is a

discontinuity, usually a distinct horizontal fracture

below which the fracturing diminishes substantially.

Therefore, the uppermost layer of the bedrock, i.e. the

®rst production layer, is often largely useless as

dimension stone. If there is any vertical exposure or

local relief, some models of the fracturing on the

outcrop surface can be gained by carefully examining

the precipices. If a deep enough vertical cross section

is not available, as in this case, the data gathered by

the detailed mapping should not be extrapolated

downwards below the exposed surface without further

controls by radar or drilling.

The appearance of the stone in this case could be

assessed quite well by the detailed mapping because

the outcrop surface was relatively fresh. However, it

must be pointed out that weathering, a process

enhanced in the humid/tropical regions, but active

also in the temperate/polar areas, is another important

feature that has a considerable effect on the reliability

of the detailed mapping. Depending upon the stone

type and the thickness of the soil cover, the weathering

can affect the surface of the rock from a few centimetres

down to a few metres depth. For example, the true

colour of a stone does not come out on the weathered

surface and small drilled samples or block samples

must be taken. Sometimes even shallow core drilling

(from 5 to 10 m deep) is needed to penetrate the

weathered zone. But, for example, due to the different

weathering rates of minerals, the heterogeneity in the

structure of the stone is intensi®ed and can well be

observed on the weathered surface.

The subsurface methods used in this study included

geo-radar survey and core drilling. The results of the

geo-radar survey indicated a fair general soundness,

giving the impression of a feasible prospect. But, as

the fracturing of the prospect was mainly in the form

H. Luodes et al. / Engineering Geology 52 (2000) 209±223 221

Fig. 7. Logged drill cores. Note the occurrence of the granite. For location of the drill holes see Fig. 2. Not to lateral scale.

of small and almost closed cracks, the ®nal soundness

could not be determined by the geo-radar survey; the

dry individual cracks did not show up well in the radar

pro®les. In general, the advantage with a geo-radar

survey is that it gives a large cross section of the

subsurface fracturing. In planning for dimension

stone production, the knowledge of the spacing and

dip of the major subsurface horizontal and sub-

horizontal fractures is essential for the estimation of

the production levels and the block sizes. The geo-

radar survey is nevertheless a more feasible method in

homogeneous rock types than in heterogeneous types.

In the latter case, the proper interpretation of the

results is more dif®cult as different internal structures

of the stone are often seen in the radar pro®les. If the

material is strongly deformed and folded, these

structures can be falsely interpreted as fractures, see

also HaÈnninen et al. (1991) and HaÈnninen (1992).

Compared to the geo-radar survey, the core drilling

gives very local information but the bene®t is that it

produces an actual sample of stone in which both the

fractures and the appearance can be studied. Selected

parts of the core are split and polished in order to

increase the reliability of the assessment of the

appearance. There are some geological features that

can only be observed in drill core. For example, as

demonstrated by this case, the small-scale cracks

often open only during drilling. Another example is

mineralogical weaknesses such as thin mica stripes,

barely visible to the eye, affecting the durability of

stone. A problem with core drilling is, however, that

a dimension stone prospect cannot be drilled as

densely as an ore body because the drill holes can

spoil otherwise suitable stone, emphasizing the need

for a careful planning of the drilling. In this case, the

core drilling proved to be the most ef®cient subsurface

method as it revealed the real soundness of the

prospect and the existence of the younger granite

below. By drilling we were able to construct the

®nal three-dimensional view of the prospect, best

exempli®ed by the discovery of the granite.

Although expensive, core drilling was the method

that showed that a quarry at this site would not

have been pro®table.

A prospect evaluation, like the one described in this

paper, can falsely be regarded as a series of operations

executed in a certain order to mechanically evaluate

the prospect. In all cases, however, a proper under-

standing and interpretation of geological phenomena

rising from the knowledge of the local geology

combined with the investigation operations give the

best result. We conclude that for a successful evalua-

tion study, the discontinuous and varied nature of

geological features in three dimensions must be

clearly understood.

Acknowledgements

This work was ®nancially supported by Finska

Stenindustri Ab and the Geological Survey of Finland,

which is gratefully acknowledged. Mr Pentti

Toivanen, Mr Erkki Niskanen, and Mr Hannu Repo

(Geological Survey of Finland) assisted in the ®eld

work, Mr Pekka MaÈaÈttaÈnen (Geological Survey of

Finland) was responsible for the topographical

measurements and Mr Jukka Leino (Geological

Survey of Finland) for the geo-radar survey. Mr Veli

Juhani HaÈnninen (Finska Stenindustri Ab) took the

block samples. Mr Ream C. Barclay (AÊ bo Akademi

University) corrected the English language. Dr I.W.

Farmer and an anonymous reviewer critically evaluated

the manuscript. Their contributions are highly appre-

ciated. Prof. Carl Ehlers (AÊ bo Akademi University,

Department of Geology and Mineralogy) is thanked

for his constructive comments on the manuscript and

for the use of the infrastructure of the department.

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