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Exploration for dimensional stone — implications and examples from the Precambrian of southern Finland
Engineering Geology 56 (2000) 275–291 www.elsevier.nl/locate/enggeo
Exploration for dimensional stone — implications and examples from the Precambrian of southern Finland O. Selonen a, *, H. Luodes b, C. Ehlers c a Finska Stenindustri Ab, 23200 Vinkkila¨, Finland b Geological Survey of Finland, Regional Office for Mid-Finland, 70211 Kuopio, Finland. ˚ bo Akademi University, Department of Geology and Mineralogy, 20500 Turku, Finland cA Received 7 January 1999; accepted for publication 10 June 1999
Abstract Dimension stone is a natural rock that must fulfil high qualitative standards defined by both geology-based factors and non-geological factors. The stone itself (appearance/soundness) and the market demand are the two most important aspects in the quality assessment. The process of geological dimension stone exploration is a systematic and stepwise procedure, including individual steps of desk study, field mapping, detailed examination, geo-radar survey, and core drilling. The location of all economically feasible dimension stone deposits is strictly controlled by geological factors. Knowledge of these factors is fundamental in identifying new sites with potential for dimension stone. In this work the geological constraint on the occurrence of dimension stone in three areas, in the Precambrian of southern Finland, has been investigated. The deposits are localized to part of an area in which different geological features in combination have produced rock of good dimension stone quality. The study shows that, for example, vertical movements in the crust, magmatic evolution of an intrusion, and metamorphic grade define the location of rocks suitable for dimension stone. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Building stone; Dimension stone; Exploration; Finland; Granite; Precambrian
1. Introduction Stone is a durable and ecological building material which can be fully recycled. In particular granitic dimension stone, with its high-silica composition, effectively resists climatic stresses and the attacks of air impurities, so common in many of our industrial cities today. Furthermore, granite extraction (excluding blasting) and processing are purely mechanical processes with no chemicals or * Corresponding author. Fax: +3582-4332-600. E-mail addresses: [email protected] (O. Selonen), [email protected] (H. Luodes), [email protected] (C. Ehlers)
other polluting substances involved. Consequently, the use of granitic stone fits in well with the idea of sustainable development. The quality of the stone must thus meet very high standards, which means that marked variations in colour, structure, and technical properties cannot be tolerated. In the market for dimension stone there is a constant need for new products with enough reserves and quarrying potential to satisfy the demand of the market for a long time. Potential sites for good stone, meeting all the quality standards, are difficult to locate and require increasing investment from the industry. In order to cope with this situation, a better focused exploration for dimension stone is needed.
0013-7952/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 9 9 ) 0 0 09 1 - 5
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Systematic geological investigations and ‘knowhow’ in connection with exploration for dimensional stone have been sparse, and few scientifically based methods for exploration have been developed. By contrast, ore prospecting has become a sophisticated science involving geological, geochemical and geophysical methods which are today indispensable for the modern explorer. However, like ore deposits, the location of all industrial dimension stone deposits is governed by definable geological factors. By identifying such factors, we can locate new areas suitable for dimension stone in a systematical way. The first and third authors have studied geological controls on the location of dimension stone deposits in southern Finland [see Selonen (1998)]. The second author has carried out exploration surveys in central Finland and has published a brief study on the methodology of prospecting for dimension stone (Luodes, 1992). In this paper we review the criteria required for good dimension stone deposits and create guide lines for a geologybased exploration procedure. Finally, we discuss geological controls on the location of some selected deposits. We restrict the study to granites and our aim is to show that dimension stone can be regarded as geological entities that can be successfully investigated by geological methods.
2. Outline of the Precambrian in southern Finland The Precambrian bedrock of Finland is a part of the Fennoscandian Shield and comprises the Archaean craton (3100–2500 Ma) in the northern and eastern parts of the country and the Palaeoproterozoic Svecofennian orogenic belt (1930–1800 Ma) in the southern and western parts ( Figs. 1 and 2). In southern Finland, the Svecofennian rocks are cut by Palaeo/ Mesoproterozoic rapakivi granite intrusions (1650–1540 Ma) (Fig. 1). More than a half of the crust in southern Finland consists of plutonic rocks which by tradition have been divided into four groups in relation to the Svecofennian orogeny: 1. synorogenic; 2. late-orogenic; 3. post-orogenic; and
4. anorogenic intrusives ( Eskola, 1932; Nurmi and Haapala, 1986; Koistinen et al., 1996). For a recent description of the Svecofennian orogeny, see Nironen (1997). The I-type synorogenic intrusive rocks have U– Pb zircon ages of 1890–1870 Ma (Huhma, 1986; Suominen, 1991) but both older [1930–1900 Ma, Lahtinen (1994), Patchett and Kouvo (1986)] and younger ages [1860–1850 Ma, Suominen (1991)] have been reported. The synorogenic rocks form the large Central Finland Granitoid Complex (CFGC ) ( Fig. 1) and occur as separate plutons in the schist belts in southern Finland (Nurmi and Haapala, 1986; Koistinen et al., 1996). The CFGC consists mostly of granitoids, but intrusions of gabbros occur. The granitoids are typically foliated, coarse to medium-grained granites and granodiorites, with large potassium feldspar megacrysts (Front and Nurmi, 1987; Koistinen et al., 1996). The synorogenic intrusives found in the schist belts are either zoned batholiths and stocks consisting of successive pulses of magma with the mafic ones occurring at the margins of the plutons ( Front and Nurmi, 1987; Koistinen et al., 1996) or tonalitic–trondhjemitic layer-parallel intrusions ( Ehlers et al., 1993; Selonen and Ehlers, 1998). The S-type late-orogenic plutonic rocks yield U–Pb zircon ages of around 1840–1830 Ma ( Huhma, 1986; Suominen, 1991), and they form a distinct zone along the southern coast of Finland: the late Svecofennian granite–migmatite zone [the LSGM-zone, Ehlers et al. (1993)] ( Fig. 1). The late-orogenic rocks are K-rich potassium granitoids which occur as large subhorizontal, fairly homogeneous porphyritic granite sheets or heterogeneous in situ granites and migmatites ( Ehlers et al., 1993; Selonen et al., 1996). A few evengrained and less deformed granite stocks of the same age are also described ( Ehlers and Bergman, 1984). The post-orogenic granitoids (ca 1800 Ma) occur as small stocks and dykes in southern Finland ( Eklund et al., 1997). The anorogenic rapakivi granites occur in Finland as four large batholiths and as several smaller batholiths and stocks (Ra¨mo¨ and Haapala, 1995; Laitakari et al., 1996) ( Fig. 1), cutting sharply through the surrounding Svecofennian
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Fig. 1. The main geological features of the Precambrian in southern Finland. The 1.84–1.83 Ga-old potassium granites define the late Svecofennian granite–migmatite zone (LSGM ) in southern Finland. The 1.89–1.88 Ga-old granodiorites/tonalites in this map also include granites of the same age in the CFGC. O, The Orivesi granite batholith; RS, the Rantasalmi-Sulkava area; Ve, The Vehmaa rapakivi granite batholith; and Wi, the Wiborg rapakivi granite batholith. Modified from Simonen (1980), Ehlers et al. (1993) and Korsman et al. (1997).
bedrock. They yield U–Pb zircon ages varying from 1650 to 1540 Ma (Ra¨mo¨ and Haapala, 1995) and are composite intrusions with evidence of coeval felsic and mafic magmatism ( Eklund, 1993), forming plutons with a variety of granite types of different colours and appearances (Ra¨mo¨ and Haapala, 1995). Structurally they are thin (ca 10– 30 km) subhorizontal sheet-like intrusions with deep root zones (Lauren, 1970; Elo and Korja, 1993). A typical feature of the Svecofennian bedrock of southern Finland is an intense ductile deformation with several deformational phases ( Ehlers et al., 1993; Koistinen et al., 1996) and lowpressure/high-temperature metamorphism, locally reaching granulite-facies conditions ( Korja et al.,
1994). Shear zones are found throughout the Svecofennian domain (Fig. 1), often controlling the emplacement of the plutonic rocks (see, e.g. Ehlers et al., 1993).
3. Dimension stone in Finland Dimension stone is extracted in Finland from more than one hundred open-pit quarries (Fig. 2) (see, e.g. Pekkala, 1995), most of which are small or medium-sized and operative part of the year. There are 20–30 sites in which large quantities are quarried the year round. These sites are located in the Proterozoic granitic rocks, mainly in southern Finland. The production of Finnish dimension
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Fig. 2. Dimension stone deposits in Finland 1998. Lithology according to Simonen (1980).
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stone in 1997 was ca 450 000 tons of which ca 380 000 tons were exported with a value of ca FIM 385 million (P. Jauhiainen, personal communication 1998). Soapstone is today economically the most important rock in the Finnish stone business, but it is mainly used for utility articles and fireplaces, seldom as dimension stone. Marble is extracted in small quantities in northern Finland and slate is quarried from a small number of quarries in the schist belts in central and northern Finland. Anorogenic rapakivi granite is the most important raw material for dimension stone in Finland, being extracted from ca 20 sites in southwestern and southeastern Finland (Fig. 2). The porphyritic fine or medium-grained Balmoral Red is the bestknown Finnish dimension stone type worldwide. The deep red granite has been produced almost uninterruptedly in the Vehmaa rapakivi granite batholith ( Fig. 1) in southwestern Finland for almost a hundred years. In the Wiborg rapakivi granite batholith (Fig. 1) in southeastern Finland, the first dimension stone quarries were already established in the 1700s; today the batholith is quarried for red (Carmen Red, Eagle Red and Karelia Red ), brown (Baltic Brown and Monola Brown) and green (Baltic Green) rapakivi granite varieties. These rocks are coarse-porphyritic granites with a characteristic rapakivi texture of round ovoids, with or without a plagioclase mantle, in a medium-grained matrix. Anorthosites, which are intimately associated with the rapakivi intrusions, are extracted as spectrolite, mainly as gemstone, but also as dimensional stone. The synorogenic Svecofennian intrusives suitable as dimension stone are mostly granites and granodiorites, which are quarried from individual batholiths and stocks mainly in the CFGC, with stones such as the famous fine-grained Kuru Grey, and the coarse-porphyritic Viitasaari Red, Viitasaari Pink and Lappia Blue. Kuru Black and Blazing Black represent the mafic components (gabbro/diorite) of the synorogenic rocks and constitute raw material for the domestic tombstone industry. During this decade the selection of Finnish dimension stone has been strengthened with multicoloured stones of vivid ‘flamed’ appearances. The
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LSGM-zone in southern Finland is the location for the extraction of these migmatitic granites, granodiorites and gneisses, comprising Lieto Red, Aurora, and Amadeus.
4. Evaluation of a dimension stone deposit Dimension stone can be defined [modified from Allison (1984) and Niini (1986)] as a natural rock that fulfils certain qualitative criteria and is therefore extracted and processed to definite dimensions for use in the building, construction, monument and tombstone industries. The term includes raw stone, blocks and finished material, but excludes crushed or powdered stone used as an aggregate or reconstituted to form artificial stone. The suitability of a rock occurrence for dimension stone is controlled by several quality requirements. The factors defining the quality of a dimension stone deposit can be divided into two main categories: 1. geology-based factors; and 2. non-geological factors (Luodes, 1992). The two most important criteria in evaluating quality are, on the one hand, the stone itself (appearance/fracturing) and, on the other hand, the demand for the stone type. In the following, we review the criteria for a good dimension stone deposit [based on Luodes (1992)]. The factors are summarized in Table 1; in the text we highlight some of the most important ones. 4.1. Geology-based factors 4.1.1. Geological factors The essential criteria regarding the macroscopic geological factors are the appearance and fracturing of the stone. The appearance of a dimension stone should be aesthetically appropriate. It is defined by the colour and structure (e.g. porphyritic, even-grained or migmatitic) of the stone, and it is assessed by eye in the field and from polished samples. This aspect is very important because the marketability of a stone type is based on its appearance (see Section 4.2.1). Small variations in
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Table 1 Factors to be considered in evaluation of a dimension stone deposit. Modified from Luodes (1992) 1. Geology-based factors 1.1. Geological factors 1.1.1. Macroscopic factors —Appearance of the rock (colour, structure, inclusions, stripes, clusters) —Soundness of the deposit (fractures, faults, shears, veins) —Area, shape, and depth of the deposit 1.1.2. Microscopic factors —Mineral composition —Contacts between minerals —Form, distribution, deformation, and weathered state of minerals 1.2. Technical factorsa —Density —Porosity and water absorption capacity (vol.%) —Modulus of rupture —Compressive strength —Hardness/abrasion resistance —Flexural strength —Resistance to weather —Workability of stone 2. Non-geological factors 2.1. Economic factors —Market demand —Fashion and cultural value (colour/stone types) —Price —Product selection 2.2. Infrastructural factors —Legislation —Environmental aspects —Storage of the excess blocks —Logistics —Availability of labour a The technical requirements depend upon the final use of the stone.
colour and structure are tolerated, but uniformity in production-scale is necessary. Dimension stones are classified as one-coloured or multicoloured types. In one-coloured stones, the colour should be homogeneous across the entire deposit. Inclusions, stripes or clusters of minerals are regarded as flaws. By contrast, more variation in colour is asked for in the multicoloured stones, which are often gneisses or gneissose granites with a strong design. A dimension stone deposit should have a suitable pattern and spacing of fractures. The
character of the fractures (openness, fillings, microfractures, etc.) is also of importance when assessing the soundness of the stone. Blocks are often extracted by exploiting natural fracture directions in the bedrock; thus regular fracture patterns are to be favoured. The fracturing is primarily evaluated by measuring in the field, and later by georadar and by core drilling. A suitable spacing of the fractures in a deposit is defined by the intended usage for the extracted stone, and by the block size required by modern production machinery. The block size desired today is from 3 m3 upwards; smaller blocks are accepted in special cases. A dimension stone deposit should be large enough to allow extraction of homogeneous stone throughout the year (extraction during the winter season) and the perspective for future use should be >10 years. The size of the deposit is evaluated by topographical measurements and it must be assessed in relation to the intended quarrying of the stone (full-time or part-time extraction) and in relation to its value on the market. The microscopic-petrographical factors include the mineral composition of the stone, contacts between the minerals, their form, distribution, deformation, and weathering. The mineral composition largely determines the technical properties of the stone (hardness, abrasion resistance, resistance to weather, workability of stone etc.). High amounts of mica, amphibole or carbonate have a negative impact on the ability of a stone to accept polishing, while sulphides corrode with time and can cause discolouring. Contacts between deformed and metamorphosed minerals are often stronger than those between idiomorphic ones. The stone is strongest when the grains are interplaited (e.g. in diabase), whereas stripes of mica constitute zones of weakness along which the stone easily cracks. The importance of the study of microfracturing under the microscope cannot be emphasized enough. 4.1.2. Technical factors In assessing stone as a technical building material, its physical, mechanical, thermal etc. properties are measured in certified laboratories by standardized methods. These properties depend upon the mineralogy of the stone in a complicated
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way, as comprehensively reported by, for example, Alnæs (1995) and Winkler (1997). The technical requirements for a stone vary according to the final use of the material. The test standards can roughly be divided into three main categories covering: 1. construction systems for dimension stone; 2. standard specifications for different rock types; and 3. standard methods and practices for physical tests. In addition to this there are standards for terminology, statistical evaluation of test results etc. Standardization systems like ASTM (American Society for Testing and Materials) and DIN (Deutsche Industrie Norm) have several standard test methods for dimension stone. For example, ASTM defines the five most critical stone typespecific characteristics of dimension stone such as: water absorption/bulk specific gravity (ASTM C 97); modulus of rupture (ASTM C 99); compressive strength (ASTM C 170); abrasion resistance (ASTM C 241); and flexural strength (ASTM C 880) (see, e.g. Harben and Prudy, 1991). These standards also include absolute values that a stone should satisfy. However, in practice the usability of stone is often assessed project by project. At present, the European Committee for Standardization (CEN ) is preparing standards for: 1. natural stone terminology and denomination criteria; 2. test methods; and 3. natural stone products. The test methods include determination of density and porosity, compressive strength, resistance by recrystallization tests, flexural strength under concentrated load, water absorption by capillary action, breaking load at dowel hole etc. The CEN norms are still being worked out at the time of writing this article.
4.2. Non-geological factors 4.2.1. Economic factors It is not sufficient to localize a geologically or technically sound dimension stone deposit. The stone must also have a value on the market — if
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it cannot be sold, it will not be used regardless of its technical quality. Appearance, technical properties, availability and homogeneity of the material determine if it can be sold at a reasonable price. Considering the economic factors in the quality assessment, it must be remembered that the marketability of a stone is highly dependent upon prevailing fashion and personal taste, unlike that of products in the trade of industrial minerals, for example. It is often very difficult to predict future trends in popularity for stone materials. As a general rule, it can be said that strong and unusual colours always seem to be desirable. There are also national preferences concerning colour and style, often related to the culture and the religion of a country. For the grounds of selecting dimension stones, see, for example Hiltunen (1991). 4.2.2. Infrastructural factors Stone extraction is controlled by local legislation in different countries, and detailed extraction and reclamation plans are often required. The location of the deposit is thus an essential aspect. Environmentally sound deposits cannot be located close to sensitive nature areas or objects, and must be at an appropriate distance from the nearest houses. On the other hand, the deposit should have reasonably easy access to good transport facilities. Possibilities for storage and use of the excess blocks must be included in the final evaluation of the suitability of a location. It is a clear advantage for a deposit to be located so that the excess material can be economically used, for example, for construction purposes.
5. Exploration procedure The process of dimension stone exploration as defined in this paper is a systematic and stepwise ( Table 2) regional study, in which ‘new’ dimension stone deposits are identified in a large target area. The individual stages in this procedure can also be applied, for example, to site investigations or quarry development. The procedure of dimension stone exploration has earlier been discussed by,
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Table 2 Procedure for geology-based dimension stone explorationa
covered areas to determine boundaries of major rock formations and structures within them (e.g. diabase dykes). Larger tectonic features can be inferred from analyses of the topographic data. At this stage, environmental and infrastructural aspects must also be considered. The infrastructural factors are important when estimating the economic possibilities of a dimension stone prospect. Depending on the product to be quarried, a difficult location and high initial expense can prevent setting up a quarry. Environmentally sensitive regions and objects are noted and excluded from further studies. Furthermore, the status of regional planning in the area concerned should be taken into consideration. Geological aspects, controlling the location of the dimension stone deposits are compiled and analysed (see Sections 6 and 7). 5.2. Field mapping
a The process of applying for quarrying permits is not shown.
for example, Harben and Prudy (1991), Jefferson (1993) and Loorents (1997). 5.1. Desk study In the first stage of an exploration all the geological, mineralogical, geophysical, topographical and historical material available on the area of interest is gathered. The material includes (numerical ) maps and their explanatory texts, publications on specific topics related to the geology of the area, reviews and literature on previous stone extraction in the area, etc. The aim is to build a view of the geology and to point out specific areas where conditions favourable for finding dimension stone would be maximized, thus reducing the size and number of the target areas. An analysis of the collected material gives an overview of the rock types, mineralogy and physical properties, age relationships, metamorphism, and overall tectonics. For example, geophysical data are used in
Potential areas located by the analysis of the existing data are studied in the field. At this stage the mapping is done on a general level and on natural outcrops. At each site observations are made on the appearance and structure of the rock types. A few hand specimens and/or small block samples are taken for mineralogical studies and aesthetical evaluation. The general soundness of the rock and the least fractured portion of the prospect are estimated by observing the main tectonic features and types of fracturing. The size of the prospect is evaluated. The proper location in relation to the environmental and infrastructural aspects (natural objects, houses, roads, and waterways), should also be checked at this stage. By observing local stone constructions and by interviewing people in the area, valuable information on previous stone quarrying in the area can be gained. The most interesting prospects are then selected for a more detailed examination. 5.3. Detailed examination of a deposit prospect The selected prospects are examined in detail on washed excavations and traverses if the natural exposures are not sufficient. At this stage variations in the rock types, their colour, structure, inclusions,
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stripes, veins, dykes, and tectonic features are mapped on a scale of 1:100, for example. Density and pattern of fractures are measured. Sampling is more systematic than at the previous stage. Because of weathering, the true colour of the rock is often hard to determine and the sampling is normally done by a small diamond drill. Larger block samples should also be taken for a proper evaluation of the appearance of the rock. Large tectonic features, mapping traverses, topographical features, and possible geo-radar traverses are measured with a tachymeter and tied up to an appropriate coordinate system. Topographic information is further used to calculate the volume of the prospect. 5.4. Geo-radar survey The purpose of the survey with ground penetrating geo-radar is to study the subsurface fracturing of the prospect. The operating principle of the geo-radar is based on an electromagnetic wave sent into the ground. When the wave meets a layer or object of different electrical properties, some of its energy will be reflected back. The amplitude of the reflected part, the time elapsed between the pulse and the reflected wave, and the moment of transmission are recorded at a rate of ca 25–52 times per second. This produces a continuous cross-section of electrical properties in the ground (Hanninen, 1992). The waves are reflected from horizontal fractures and rock boundaries. The geo-radar survey gives quite accurate information down to 20 m below the outcrop surface. The horizontal and subhorizontal fractures appear clearly in the geo-radar profiles, while the vertical and subvertical fractures are more difficult to interpret (cf Hanninen et al., 1991). A good knowledge of the geological features of the prospect is an essential precondition for a successful interpretation of the geo-radar measurements. 5.5. Core drilling Core drilling gives invaluable information on the quality of the prospect by producing a drill core in which the variation in colour, mineralogy and fracturing can be observed in depth. To facili-
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tate this evaluation, selected parts of the core are split in half and polished. Correct placing of the inclined or vertical drill holes is very important and depends on the amount of geological information acquired during the preceding steps of the exploration process. The depth dimension of core drilling is up to 50 m. The normal diameter of the core is ca 45 mm. 5.6. Additional studies A prospect proven good by the geological investigation is ready for test quarrying and test production. Test quarrying is made on a small scale and normally a few hundred cubic metres are extracted. In this process the quarrying properties (extractability/drillability) of the stone are tested. The extracted material is then processed into final products to determine the production properties such as sawability, scorching ability, honability and polishing ability. The processed products are further used for commercial testing.
6. Geological controls on the location of dimension stone deposits In this Section we demonstrate some of the geological factors that constrain the occurrence of dimension stone deposits in the Precambrian of southern Finland. In all cases the deposits are concentrated in a small part of a larger area in which different geological factors coincide in a beneficial way. 6.1. The Orivesi granite batholith The Orivesi granite batholith (Selonen and Ehlers, 1996) is located at the southern margin of the CFGC ( Fig. 1) in an east–west trending belt of granite intrusions. The ca 1.87 Ga-old Orivesi batholith is composed of a red coarse-grained porphyritic granite, a blue-grey coarse-grained porphyritic granite and an aplite granite (Fig. 3). No mafic magmatic components are associated with the batholith. The red coarse-grained porphyritic granite with a magmatic foliation builds up the bulk of the batholith ( Fig. 3). The porphyritic
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Fig. 3. Geological map of the Orivesi granite batholith with surroundings. The blue-grey coarse-porphyritic granite occurs as scattered bodies inside the broken circle. Dimension stone deposits are marked by stars. Modified from Selonen and Ehlers (1996) and Selonen (1998).
texture is defined by K-feldspar phenocrysts, which usually are euhedral, but oval (and mantled ) grains also occur. The Orivesi batholith has intruded along regional shear zones in the southern parts of the CFGC (Fig. 1; Selonen and Ehlers 1996). The intrusion was diapiric, at least during the final emplacement, producing the magmatic structures in the batholith, and it is post-tectonic in relation to the ductile deformation in the host rocks (Selonen and Ehlers, 1996). Consequently, it is sparsely fractured and regionally well suited as a prospect for dimensional stone. The dimension stone deposits are confined to the red granite (Selonen, 1998) ( Fig. 3). All but one of the sites are situated in the western half of the batholith ( Fig. 3), indicating that the potential
for stone of good quality is higher in the western half of the batholith than in the eastern half. The blue-grey granite and the aplite granite lack value as dimension stone. The concentrated location of the dimension stone deposits in the Orivesi batholith can be explained by the geological history of the pluton (Selonen, 1998) (see Fig. 4). A NNW–SSE trending fault cuts the batholith into two halves. The western half has been uplifted relative to the eastern half, resulting in different structural characteristics in the halves at the present level of erosion. Representing a lower crustal level, the western half consists of sparsely fractured red granite with a well developed, steep, penetrative and concentric foliation. On the contrary, the upper parts of the batholith, in the eastern half, are characterized by
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Fig. 4. The different structures in the two halves of the Orivesi granite batholith can be explained by a west-side-up faulting, exposing a deeper crustal section on the western side. (A) Simplified geological map of the Orivesi granite batholith (cf Fig. 3). (B) Sketch map of the west-side-up movement in the batholith. Not to scale. Modified from Selonen (1998).
intrusions of late magmatic melts into the red granite: the blue-grey granite and the aplite dykes and sheets. The magmatic foliation in the red
granite is poorly developed and the late brittle fracturing on outcrops is dense in the east. The late melts have intruded as dykes and sheets and
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they are densely fractured and unsuitable as dimension stone. It is thus possible to explain why almost all the deposits of dimension stone are located in the western half of the batholith, and why the red granite there has better dimension stone properties compared to the other half. The west-side-up movement along the central fault zone exposes deeper levels of more homogeneous, sound and sparsely fractured red granite, restricting the locations of good dimension stone to the western half of the Orivesi batholith. 6.2. The Rantasalmi-Sulkava area The Rantasalmi-Sulkava area is located in the eastern part of the LSGM-zone (Fig. 1) and consists of highly metamorphosed and migmatized rocks ( Fig. 5). The main rock types are metapelites and potassium granite. Granodiorite/tonalite and gabbro are also present in smaller occurrences. The most common types of metapelites are mica schists and mica gneisses. They are strongly migmatized by coarse potassium granite, forming veined gneisses. The amount of potassium granite increases towards the southeast. In the northwestern parts of the area mica schists have preserved primary sedimentary structures, such as bedding ( Korsman, 1977) and graded bedding (Gaa´l and Rauhamaki, 1971), while in the southeastern parts the structures are only occasionally visible. The metamorphism in the area is progressive, and the grade increases from the northwest to the southeast, reaching granulite facies conditions in the centre of the area (Fig. 5). Metamorphic zoning in the metapelites has been described by Korsman et al. (1988) as follows: sillimanite+K-feldspar; cordierite+K-feldspar; garnet+cordierite+sillimanite+biotite; and garnet+cordierite+sillimanite (Fig. 5). Pyroxene is present in the granodiorites– tonalites in the area of granulite facies. The U–Pb zircon/monazite age of 1840–1830 Ma in the metapelites reflects the peak metamorphism of the area ( Korsman et al., 1984). Dimension stone deposits are found in two types of rock: garnet–cordierite gneiss; and potassium granite. The deposits are located in the area of the highest metamorphic grade. The garnet– cordierite gneisses are highly metamorphosed met-
apelites with a paleosome consisting mainly of garnet, cordierite and plagioclase. The neosome is rich in K-feldspar and quartz. The proportions of paleosome and neosome vary and the amount of neosome can be as high as half the total volume. The medium and coarse-grained potassium-rich granites are intensely red and include ‘ghostlike’ restites of mica gneiss and clusters of garnet/cordierite. The restricted location of the dimension deposits in the Rantasalmi-Sulkava area is geologically controlled by the metamorphism, confining the deposits to the area of granulite facies. The almost complete melting and homogenization of the parent material in that high-grade area are the prerequisites for forming a rock sound enough for industrial extraction of dimension stone. 6.3. The Vehmaa rapakivi granite batholith The Vehmaa rapakivi granite batholith is situated in southwestern Finland ( Fig. 1), covering an area of 700 km2 (Lindberg and Bergman, 1993). The Vehmaa batholith has a concentric structure consisting of five major rapakivi granite varieties: a pyterlitic rapakivi granite; a coarse-grained porphyritic rapakivi granite; a medium-grained porphyritic rapakivi granite; an even-grained rapakivi granite; and a porphyry aplite (Lindberg and Bergman, 1993) (Fig. 6). Lindberg and Bergman (1993) report two U–Pb zircon/monazite ages for the batholith: 1573±8 Ma for the even-grained rapakivi granite and 1582±4 Ma for the pyterlitic rapakivi. The anorogenic Vehmaa rapakivi granite batholith has intruded after the Svecofennian orogeny and sharply transects the Svecofennian bedrock. There are no traces of orogenic movements in the Finnish bedrock after the emplacement of the rapakivi granites. Therefore, the undeformed batholith is sparsely fractured and is a location with a high potential for dimension stone. Structural criteria indicate that the Vehmaa batholith is a subhorizontal laccolith-like intrusion (Selonen, 1998), resembling the other rapakivi intrusions in Finland (Lauren, 1970; Elo and Korja, 1993). Geometrically this means that the present level of erosion is subparallel to the lacco-
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Fig. 5. Geological and metamorphic map of the Rantasalmi-Sulkava area. Dimension stone deposits are marked by stars. Sill, Sillimanite; Ksp, K-feldspar; Gar, garnet; Cord, cordierite; Bio, biotite. Modified from Korsman et al. (1988) and (1997).
lithic intrusion, exposing a wide area of rapakivi granite with a high potential for dimension stone. Portions of granite with sparse fracturing are abundant in all the intrusive phases in the batholith, except for the densely fractured porphyry aplite. Although all the sparsely jointed magmatic components have technical potential as dimension stone (Selonen, 1998), the deposits in the Vehmaa batholith are strongly concentrated in only two types of rapakivi granite: the medium-grained porphyritic rapakivi granite in the middle of the
batholith and the even-grained rapakivi granite in the eastern part of the batholith (Fig. 6). This is because only those granites have a colour and a general textural outlook that are commercially viable. The location of the economically useful dimension stone deposits in the Vehmaa batholith is thus strictly controlled by the magmatic evolution of the batholith. The brightest coloured stone qualities belong to distinct intrusion phases, sometimes separated from less useful varieties only by gently dipping intrusional contacts that are barely
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Fig. 6. Geological map of the Vehmaa rapakivi granite batholith. The dimension stone deposits are marked by stars. Modified from Lindberg and Bergman (1993).
visible. For future stone material prospecting in this large batholith, the proper identification of the intrusion phases is of great importance.
7. Discussion and conclusions There is not much literature on methods of prospecting for dimension stone, or on the geological controls of dimension stone deposits. However, the localization of deposits is no trivial matter and the factors governing their location are strictly
geological. By applying normal methods of structural geology and petrology, and by carefully mapping both active and abandoned deposits, we can draw conclusions about the geological factors that in combination will produce a stone of suitable quality. The ultimate goal is, of course, to create systematic prospecting methods for dimension stone and to identify the geological conditions required for successful prospecting. The examples cited above show some of the primary geological factors of importance for the location of dimension stone deposits: vertical
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movements along major fractures; history of intrusion; different intrusion phases; and metamorphic grade. Other important geological features are the type and direction of deformational structures such as foliations and lineations. These are the results of common geological processes, easily addressed by ordinary geological investigation methods. The consequence of this is very important for the exploration for dimension stone: if we know the factors controlling the occurrences, we are able to predict geological environments critical for finding new deposits. The best effect can be obtained if this knowledge can be applied at the very beginning of the exploration process, that is, during the desk study, when the main purpose is to divide the target area into smaller units with different potential for dimension stone. The prospecting can thus be systematically directed to those subareas with the highest potential for dimension stone. It is essential in the desk study to gather data on the target area from many different sources, since sites with a high potential for dimension stone are located in areas where several geological factors coincide. By a careful and thorough analysis of the collected data, the potential areas (down to outcrop scale) can often be identified at that early stage. Consequently, the desk study should never be neglected or underestimated, but regarded as an integral part of the exploration process. The desk study is followed by field mapping, during which the prospects are identified, and the decision, whether a more detailed examination is worthwhile at the sites, is taken. The appearance and soundness of the rock are the most important criteria in these evaluations. Soil cover and weathered surfaces are difficulties which can lead to misjudgements even if some flaws (e.g. inclusions and variations in mineral composition) can be better recognized on a weathered surface. The success of the field mapping is very much dependent upon the experience of the mapping geologist, since the recognition of good appearance and soundness of a rock is mostly done by eye. The use of geo-radar surveys and core drilling may follow but at this stage the visual assessment is crucial. It is also important to have experience and knowledge of what can be achieved by different investigation methods.
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The quality and usability of the promising prospects are assessed during the detailed examination. At this stage the evaluation is ‘easier’ as the area is narrowed down and since the exposure is improved by stripping and washing of carefully selected mapping traverses. In the detailed examination the types and density of fracturing and the number and type of different flaws that can spoil large volumes of otherwise sound stone should be studied with special care. The supporting methods like geo-radar surveys and core drilling can be regarded as parts of the detailed examination ( Table 2), as the study of the outcrop surface alone normally does not give enough information for an adequate evaluation of the prospect. But they can be effectively applied only after careful geological mapping of the prospect. Because of the high cost, core drilling is often carried out as a final stage of the exploration procedure only in the most promising prospects. However, the use of the costly methods is well justified, because of the high level of initial investment for quarrying. The detailed examination also gives valuable information concerning the future quarrying and production planning. Our conclusions are: the occurrences of dimension stone are determined by local and regional geological characteristics, and can be studied by geological methods. Exploration for dimension stone can be done systematically with a geological approach, but a good knowledge of the quality demands is the prerequisite for a successful exploration study. In a properly executed survey, knowledge of the critical geological factors and knowledge of the required quality criteria are combined and related to the geological features of the target area. A systematic and stepwise dimension stone exploration based upon geological investigations, as well as a thorough geological quantity/quality inventory of a deposit prospect, will considerably enhance the profitability of stone extraction.
Acknowledgements This work was financially supported by the K.H. Renlund Foundation, Finska Stenindustri
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˚ bo Ab, Geological Survey of Finland and A Akademi University, Department of Geology and Mineralogy, which is gratefully acknowledged. Ms ˚ bo Akademi University) drew Merja Kuusisto (A the line drawings. Mr Pekka Jauhiainen ( The Finnish Natural Stone Association) kindly delivered statistics on the production and usage of Finnish dimension stone. Mr Ream C. Barclay ˚ bo Akademi University) corrected the English. (A Dr I.W. Farmer and Professor Heikki Niini critically reviewed the manuscript. Finally, Mr VeliJuhani Ha¨nninen ( Finska Stenindustri Ab) is thanked for intensive and enlightening discussions on relations between geological knowledge and dimension stone exploitation during the final stages of the study.
References Allison, P., 1984. Dimension stone — a rock steady market. Industrial Minerals. 202, 19–35. Alnæs, L.-I., 1995. Kvalitet og bestandighet av naturstein. Pa˚virkningsfaktorer og prøvemetoder. (Quality and durability of natural stone). Academic dissertation. The Norwegian Institute of Technology, University of Trondheim, (in Norwegian with an English summary). Ehlers, C., Bergman, L., 1984. Structure and mechanism of intrusion of two postorogenic granite massifs, southwestern Finland. In: Kro¨ner, A, Greiling, R. ( Eds.), Precambrian Tectonics Illustrated. E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany, pp. 173–190. Ehlers, C., Lindroos, A., Selonen, O., 1993. The late Svecofennian granite–migmatite zone of southern Finland — a belt of transpressive deformation and granite emplacement. Precambrian Res. 64, 295–309. Eklund, O., 1993. Coeval contrasting magmatism and magma ˚ land, mixing in Proterozoic post- and anorogenic granites, A ˚ bo Akademi UniverSW Finland. Academic dissertation. A sity. Department of Geology and Mineralogy. Turku, Finland. Eklund, O., Rutanen, H., Konopelko, D., Fro¨jdo¨, S., Shebanov, A., 1997. 1.8 Ga postorogenic lamprophyre-granitoid magmatism in the Fennoscandian shield Extended abstract, in: Publications of ISGAM II for 2nd International Symposium on Granites and Associated Mineralizations, Salvador, Bahia Brazil, 190–192. Elo, S., Korja, A., 1993. Geophysical interpretation of the crustal and upper mantle structure in the Wiborg rapakivi granitoid area, southeastern Finland. Precambrian Res. 64, 273–288. Eskola, P., 1932. On the origin of granitic magmas. Mitteilungen Mineralogie und Petrologie 42, 455–481.
Front, K., Nurmi, P.A., 1987. Characteristics and geological setting of synkinematic Svecokarelian granitoids in southern Finland. Precambrian Res. 35, 207–224. Gaa´l, G., Rauhama¨ki, E., 1971. Petrological and structural analysis of the Haukivesi area between Varkaus and Savonlinna, Finland. Bull. Geol. Soc. Finland., 265–337. Ha¨nninen, P., 1992. Application of ground penetrating radar and radio wave moisture probe techniques to peatland investigations.. Geol. Surv. Finland, Bull. 361. Ha¨nninen, P., Pa¨a¨kko¨nen, K., Tervo, T., 1991. Geo-radar in dimension stone explorations. Geol. Surv. Finland, Special Paper 12, 117–118. Harben, P., Prudy, J., 1991. Dimension stone evaluation. From cradle to gravestone. Industrial Minerals. 281, 47–61. Hiltunen, J., 1991. Bases for selecting natural stones from a quarrier’s point of view. In: Exterior Wall Systems: Glass and Concrete Technology, Design, and Construction. ASTM STP 1034 American Society for Testing and Materials, Philadelphia, 121–127. Huhma, H., 1986. Sm–Nd, U–Pb and Pb–Pb-isotopic evidence for origin of the early Proterozoic Svecokarelian crust in Finland. Geol. Surv. Finland, Bull. 337. Jefferson, D.P., 1993. Building stone: the geological dimension. Q. J. Eng. Geol. 26, 305–319. Koistinen, T., Klein, V., Koppelmaa, H., Korsman, K., Lahtinen, R., Nironen, M., Puura, V., Saltykova, T., Tikhomirov, S., Yanovskiy, A., 1996. Paleoproterozoic Svecofennian orogenic belt in the surroundings of the Gulf of Finland. Geol. Surv. Finland, Special Paper 21, 21–57. Korja, T., Luosto, U., Korsman, K., Pajunen, M., 1994. Geophysical and metamorphic features of Palaeoproterozoic Svecofennian orogeny and Palaeoproterozoic overprinting on Archaean crust. Geol. Surv. Finland, Guide 37, 11–20. Korsman, K., 1977. Progressive metamorphism of the metapelites in the Rantasalmi–Sulkava area, southeastern Finland. Geol. Surv. Finland, Bull. 290. Korsman, K., Ho¨ltta¨, P., Hautala, T., Wasenius, P., 1984. Metamorphism as an indicator of evolution and structure of the crust in Eastern Finland. Geol. Surv. Finland, Bull. 328. Korsman, K., Niemela¨, R., Wasenius, P., 1988. Multistage evolution of the Proterozoic crust in the Savo schist belt, eastern Finland. Geol. Surv. Finland, Bull. 343, 89–96. Korsman, K., Koistinen, T., Kohonen, J., Wennerstro¨m, M.Ekdahl, E., Honkamo, M., Idman, H., Pekkala, Y. ( Eds.), Bedrock map of Finland 1:1 000 000 1997. Geol. Surv. Finland, Finland. Lahtinen, R., 1994. Crustal evolution of the Svecofennian and Karelian domains during 2.1–1.79 Ga, with special emphasis on the geochemistry and origin of 1.93–1.91 Ga gneissic tonalites and associated supracrustal rocks in the Rautalampi area, central Finland. Geol. Surv. Finland, Bull. 378. Laitakari, I., Ra¨mo¨, T., Suominen, V., Niin, M., Stepanov, K., Amantov, A., 1996. Subjotnian: Rapakivi granites and related rocks in the surroundings of the Gulf of Finland. Geol. Surv. Finland, Special Paper 21, 59–97. Laure´n, L., 1970. An interpretation of negative gravity anomalies associated with the rapakivi granites and the jotnian
O. Selonen et al. / Engineering Geology 56 (2000) 275–291 sandstone in southern Finland. Geol. Fo¨ren. Stockholm Fo¨rh 92, 21–34. Lindberg, B., Bergman, L., 1993. Pre-Quaternary rocks of the Vehmaa map-sheet area. Geological map of Finland 1:100 000. Explanation to the maps of Pre-Quaternary rocks. Sheet 1042 Vehmaa. Geol. Surv. Finland, Espoo. (in Finnish with an English summary). Loorents, K.-J., 1997. Petrology, brittle structures and prospecting methods in the ‘‘Offerdalskiffer’’ from the central part of the Caledonian allochthon in the county of Ja¨mtland, Sweden. Earth Sciences Centre. Go¨teborg University. Thesis for licentiate degree. A21. Luodes, H., 1992. Rakennuskiviesiintymien arviointi (Evaluation of dimension stone deposits). In: Parkkinen, J. ( Ed.), Malmiarvioseminaari 26.11.1992 esitelma¨kooste Sarja B 53. Vuorimiesyhdistys r.y. Geologijaosto, Otaniemi, 122–130. (in Finnish). Niini, H., 1986. Classification and development of bedrock resources in Finland. Bull. Geol. Soc. Finland 58, 335–350. Nironen, M., 1997. The Svecofennian Orogen: a tectonic model. Precambrian Res. 86, 21–44. Nurmi, P.A., Haapala, I., 1986. The Proterozoic granitoids of Finland: granite types, metallogeny and relation to crustal evolution. Bull. Geol. Soc. Finland 58, 203–233. Patchett, J., Kouvo, O., 1986. Origin of continental crust of 1.9–1.7 Ga age: Nd isotopes and U–Pb zircon ages in the
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Svecokarelian terrain of South Finland. Contrib. Mineral. Petrol. 92, 1–12. Pekkala, Y., 1995. Finland’s mineral industry. Emphasis on paper pigments and stone. Industrial Minerals., 33–39. Ra¨mo¨, O.T., Haapala, I., 1995. One hundred years of Rapakivi Granite. Miner. Petrol. 52, 129–185. Selonen, O., 1998. Exploration for dimension stone — geologi˚ bo Akademi Univercal aspects. Academic dissertation. A sity. Department of Geology and Mineralogy. Turku, Finland. Selonen, O., Ehlers, C., 1996. The Orivesi granite batholith, southern central Finland — characteristics and emplacement. Bull. Geol. Soc. Finland 68 Part 1, 11–24. Selonen, O., Ehlers, C., 1998. Structural observations on the Uusikaupunki trondhjemite sheet. SW Finland GFF 120 Part 4, 379–382. Selonen, O., Ehlers, C., Lindroos, A., 1996. Structural features and emplacement of the late Svecofennian Pernio¨ granite sheet in southern Finland. Bull. Geol. Soc. Finland 68, Part 2, 5–17. Simonen, A., 1980. The Precambrian in Finland. Geol. Surv. Finland, Bull. 304. Suominen, V., 1991. The chronostratigraphy of southwestern Finland with special reference to Postjotnian and Subjotnian diabases. Geol. Surv. Finland, Bull. 356. Winkler, E.M., 1997. Stone in Architecture, Properties, Durability. 3rd ed., Springer, Heidelberg.
Exploration for dimensional stone — implications and examples from the Precambrian of southern Finland O. Selonen a, *, H. Luodes b, C. Ehlers c a Finska Stenindustri Ab, 23200 Vinkkila¨, Finland b Geological Survey of Finland, Regional Office for Mid-Finland, 70211 Kuopio, Finland. ˚ bo Akademi University, Department of Geology and Mineralogy, 20500 Turku, Finland cA Received 7 January 1999; accepted for publication 10 June 1999
Abstract Dimension stone is a natural rock that must fulfil high qualitative standards defined by both geology-based factors and non-geological factors. The stone itself (appearance/soundness) and the market demand are the two most important aspects in the quality assessment. The process of geological dimension stone exploration is a systematic and stepwise procedure, including individual steps of desk study, field mapping, detailed examination, geo-radar survey, and core drilling. The location of all economically feasible dimension stone deposits is strictly controlled by geological factors. Knowledge of these factors is fundamental in identifying new sites with potential for dimension stone. In this work the geological constraint on the occurrence of dimension stone in three areas, in the Precambrian of southern Finland, has been investigated. The deposits are localized to part of an area in which different geological features in combination have produced rock of good dimension stone quality. The study shows that, for example, vertical movements in the crust, magmatic evolution of an intrusion, and metamorphic grade define the location of rocks suitable for dimension stone. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Building stone; Dimension stone; Exploration; Finland; Granite; Precambrian
1. Introduction Stone is a durable and ecological building material which can be fully recycled. In particular granitic dimension stone, with its high-silica composition, effectively resists climatic stresses and the attacks of air impurities, so common in many of our industrial cities today. Furthermore, granite extraction (excluding blasting) and processing are purely mechanical processes with no chemicals or * Corresponding author. Fax: +3582-4332-600. E-mail addresses: [email protected] (O. Selonen), [email protected] (H. Luodes), [email protected] (C. Ehlers)
other polluting substances involved. Consequently, the use of granitic stone fits in well with the idea of sustainable development. The quality of the stone must thus meet very high standards, which means that marked variations in colour, structure, and technical properties cannot be tolerated. In the market for dimension stone there is a constant need for new products with enough reserves and quarrying potential to satisfy the demand of the market for a long time. Potential sites for good stone, meeting all the quality standards, are difficult to locate and require increasing investment from the industry. In order to cope with this situation, a better focused exploration for dimension stone is needed.
0013-7952/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 9 9 ) 0 0 09 1 - 5
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Systematic geological investigations and ‘knowhow’ in connection with exploration for dimensional stone have been sparse, and few scientifically based methods for exploration have been developed. By contrast, ore prospecting has become a sophisticated science involving geological, geochemical and geophysical methods which are today indispensable for the modern explorer. However, like ore deposits, the location of all industrial dimension stone deposits is governed by definable geological factors. By identifying such factors, we can locate new areas suitable for dimension stone in a systematical way. The first and third authors have studied geological controls on the location of dimension stone deposits in southern Finland [see Selonen (1998)]. The second author has carried out exploration surveys in central Finland and has published a brief study on the methodology of prospecting for dimension stone (Luodes, 1992). In this paper we review the criteria required for good dimension stone deposits and create guide lines for a geologybased exploration procedure. Finally, we discuss geological controls on the location of some selected deposits. We restrict the study to granites and our aim is to show that dimension stone can be regarded as geological entities that can be successfully investigated by geological methods.
2. Outline of the Precambrian in southern Finland The Precambrian bedrock of Finland is a part of the Fennoscandian Shield and comprises the Archaean craton (3100–2500 Ma) in the northern and eastern parts of the country and the Palaeoproterozoic Svecofennian orogenic belt (1930–1800 Ma) in the southern and western parts ( Figs. 1 and 2). In southern Finland, the Svecofennian rocks are cut by Palaeo/ Mesoproterozoic rapakivi granite intrusions (1650–1540 Ma) (Fig. 1). More than a half of the crust in southern Finland consists of plutonic rocks which by tradition have been divided into four groups in relation to the Svecofennian orogeny: 1. synorogenic; 2. late-orogenic; 3. post-orogenic; and
4. anorogenic intrusives ( Eskola, 1932; Nurmi and Haapala, 1986; Koistinen et al., 1996). For a recent description of the Svecofennian orogeny, see Nironen (1997). The I-type synorogenic intrusive rocks have U– Pb zircon ages of 1890–1870 Ma (Huhma, 1986; Suominen, 1991) but both older [1930–1900 Ma, Lahtinen (1994), Patchett and Kouvo (1986)] and younger ages [1860–1850 Ma, Suominen (1991)] have been reported. The synorogenic rocks form the large Central Finland Granitoid Complex (CFGC ) ( Fig. 1) and occur as separate plutons in the schist belts in southern Finland (Nurmi and Haapala, 1986; Koistinen et al., 1996). The CFGC consists mostly of granitoids, but intrusions of gabbros occur. The granitoids are typically foliated, coarse to medium-grained granites and granodiorites, with large potassium feldspar megacrysts (Front and Nurmi, 1987; Koistinen et al., 1996). The synorogenic intrusives found in the schist belts are either zoned batholiths and stocks consisting of successive pulses of magma with the mafic ones occurring at the margins of the plutons ( Front and Nurmi, 1987; Koistinen et al., 1996) or tonalitic–trondhjemitic layer-parallel intrusions ( Ehlers et al., 1993; Selonen and Ehlers, 1998). The S-type late-orogenic plutonic rocks yield U–Pb zircon ages of around 1840–1830 Ma ( Huhma, 1986; Suominen, 1991), and they form a distinct zone along the southern coast of Finland: the late Svecofennian granite–migmatite zone [the LSGM-zone, Ehlers et al. (1993)] ( Fig. 1). The late-orogenic rocks are K-rich potassium granitoids which occur as large subhorizontal, fairly homogeneous porphyritic granite sheets or heterogeneous in situ granites and migmatites ( Ehlers et al., 1993; Selonen et al., 1996). A few evengrained and less deformed granite stocks of the same age are also described ( Ehlers and Bergman, 1984). The post-orogenic granitoids (ca 1800 Ma) occur as small stocks and dykes in southern Finland ( Eklund et al., 1997). The anorogenic rapakivi granites occur in Finland as four large batholiths and as several smaller batholiths and stocks (Ra¨mo¨ and Haapala, 1995; Laitakari et al., 1996) ( Fig. 1), cutting sharply through the surrounding Svecofennian
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Fig. 1. The main geological features of the Precambrian in southern Finland. The 1.84–1.83 Ga-old potassium granites define the late Svecofennian granite–migmatite zone (LSGM ) in southern Finland. The 1.89–1.88 Ga-old granodiorites/tonalites in this map also include granites of the same age in the CFGC. O, The Orivesi granite batholith; RS, the Rantasalmi-Sulkava area; Ve, The Vehmaa rapakivi granite batholith; and Wi, the Wiborg rapakivi granite batholith. Modified from Simonen (1980), Ehlers et al. (1993) and Korsman et al. (1997).
bedrock. They yield U–Pb zircon ages varying from 1650 to 1540 Ma (Ra¨mo¨ and Haapala, 1995) and are composite intrusions with evidence of coeval felsic and mafic magmatism ( Eklund, 1993), forming plutons with a variety of granite types of different colours and appearances (Ra¨mo¨ and Haapala, 1995). Structurally they are thin (ca 10– 30 km) subhorizontal sheet-like intrusions with deep root zones (Lauren, 1970; Elo and Korja, 1993). A typical feature of the Svecofennian bedrock of southern Finland is an intense ductile deformation with several deformational phases ( Ehlers et al., 1993; Koistinen et al., 1996) and lowpressure/high-temperature metamorphism, locally reaching granulite-facies conditions ( Korja et al.,
1994). Shear zones are found throughout the Svecofennian domain (Fig. 1), often controlling the emplacement of the plutonic rocks (see, e.g. Ehlers et al., 1993).
3. Dimension stone in Finland Dimension stone is extracted in Finland from more than one hundred open-pit quarries (Fig. 2) (see, e.g. Pekkala, 1995), most of which are small or medium-sized and operative part of the year. There are 20–30 sites in which large quantities are quarried the year round. These sites are located in the Proterozoic granitic rocks, mainly in southern Finland. The production of Finnish dimension
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Fig. 2. Dimension stone deposits in Finland 1998. Lithology according to Simonen (1980).
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stone in 1997 was ca 450 000 tons of which ca 380 000 tons were exported with a value of ca FIM 385 million (P. Jauhiainen, personal communication 1998). Soapstone is today economically the most important rock in the Finnish stone business, but it is mainly used for utility articles and fireplaces, seldom as dimension stone. Marble is extracted in small quantities in northern Finland and slate is quarried from a small number of quarries in the schist belts in central and northern Finland. Anorogenic rapakivi granite is the most important raw material for dimension stone in Finland, being extracted from ca 20 sites in southwestern and southeastern Finland (Fig. 2). The porphyritic fine or medium-grained Balmoral Red is the bestknown Finnish dimension stone type worldwide. The deep red granite has been produced almost uninterruptedly in the Vehmaa rapakivi granite batholith ( Fig. 1) in southwestern Finland for almost a hundred years. In the Wiborg rapakivi granite batholith (Fig. 1) in southeastern Finland, the first dimension stone quarries were already established in the 1700s; today the batholith is quarried for red (Carmen Red, Eagle Red and Karelia Red ), brown (Baltic Brown and Monola Brown) and green (Baltic Green) rapakivi granite varieties. These rocks are coarse-porphyritic granites with a characteristic rapakivi texture of round ovoids, with or without a plagioclase mantle, in a medium-grained matrix. Anorthosites, which are intimately associated with the rapakivi intrusions, are extracted as spectrolite, mainly as gemstone, but also as dimensional stone. The synorogenic Svecofennian intrusives suitable as dimension stone are mostly granites and granodiorites, which are quarried from individual batholiths and stocks mainly in the CFGC, with stones such as the famous fine-grained Kuru Grey, and the coarse-porphyritic Viitasaari Red, Viitasaari Pink and Lappia Blue. Kuru Black and Blazing Black represent the mafic components (gabbro/diorite) of the synorogenic rocks and constitute raw material for the domestic tombstone industry. During this decade the selection of Finnish dimension stone has been strengthened with multicoloured stones of vivid ‘flamed’ appearances. The
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LSGM-zone in southern Finland is the location for the extraction of these migmatitic granites, granodiorites and gneisses, comprising Lieto Red, Aurora, and Amadeus.
4. Evaluation of a dimension stone deposit Dimension stone can be defined [modified from Allison (1984) and Niini (1986)] as a natural rock that fulfils certain qualitative criteria and is therefore extracted and processed to definite dimensions for use in the building, construction, monument and tombstone industries. The term includes raw stone, blocks and finished material, but excludes crushed or powdered stone used as an aggregate or reconstituted to form artificial stone. The suitability of a rock occurrence for dimension stone is controlled by several quality requirements. The factors defining the quality of a dimension stone deposit can be divided into two main categories: 1. geology-based factors; and 2. non-geological factors (Luodes, 1992). The two most important criteria in evaluating quality are, on the one hand, the stone itself (appearance/fracturing) and, on the other hand, the demand for the stone type. In the following, we review the criteria for a good dimension stone deposit [based on Luodes (1992)]. The factors are summarized in Table 1; in the text we highlight some of the most important ones. 4.1. Geology-based factors 4.1.1. Geological factors The essential criteria regarding the macroscopic geological factors are the appearance and fracturing of the stone. The appearance of a dimension stone should be aesthetically appropriate. It is defined by the colour and structure (e.g. porphyritic, even-grained or migmatitic) of the stone, and it is assessed by eye in the field and from polished samples. This aspect is very important because the marketability of a stone type is based on its appearance (see Section 4.2.1). Small variations in
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Table 1 Factors to be considered in evaluation of a dimension stone deposit. Modified from Luodes (1992) 1. Geology-based factors 1.1. Geological factors 1.1.1. Macroscopic factors —Appearance of the rock (colour, structure, inclusions, stripes, clusters) —Soundness of the deposit (fractures, faults, shears, veins) —Area, shape, and depth of the deposit 1.1.2. Microscopic factors —Mineral composition —Contacts between minerals —Form, distribution, deformation, and weathered state of minerals 1.2. Technical factorsa —Density —Porosity and water absorption capacity (vol.%) —Modulus of rupture —Compressive strength —Hardness/abrasion resistance —Flexural strength —Resistance to weather —Workability of stone 2. Non-geological factors 2.1. Economic factors —Market demand —Fashion and cultural value (colour/stone types) —Price —Product selection 2.2. Infrastructural factors —Legislation —Environmental aspects —Storage of the excess blocks —Logistics —Availability of labour a The technical requirements depend upon the final use of the stone.
colour and structure are tolerated, but uniformity in production-scale is necessary. Dimension stones are classified as one-coloured or multicoloured types. In one-coloured stones, the colour should be homogeneous across the entire deposit. Inclusions, stripes or clusters of minerals are regarded as flaws. By contrast, more variation in colour is asked for in the multicoloured stones, which are often gneisses or gneissose granites with a strong design. A dimension stone deposit should have a suitable pattern and spacing of fractures. The
character of the fractures (openness, fillings, microfractures, etc.) is also of importance when assessing the soundness of the stone. Blocks are often extracted by exploiting natural fracture directions in the bedrock; thus regular fracture patterns are to be favoured. The fracturing is primarily evaluated by measuring in the field, and later by georadar and by core drilling. A suitable spacing of the fractures in a deposit is defined by the intended usage for the extracted stone, and by the block size required by modern production machinery. The block size desired today is from 3 m3 upwards; smaller blocks are accepted in special cases. A dimension stone deposit should be large enough to allow extraction of homogeneous stone throughout the year (extraction during the winter season) and the perspective for future use should be >10 years. The size of the deposit is evaluated by topographical measurements and it must be assessed in relation to the intended quarrying of the stone (full-time or part-time extraction) and in relation to its value on the market. The microscopic-petrographical factors include the mineral composition of the stone, contacts between the minerals, their form, distribution, deformation, and weathering. The mineral composition largely determines the technical properties of the stone (hardness, abrasion resistance, resistance to weather, workability of stone etc.). High amounts of mica, amphibole or carbonate have a negative impact on the ability of a stone to accept polishing, while sulphides corrode with time and can cause discolouring. Contacts between deformed and metamorphosed minerals are often stronger than those between idiomorphic ones. The stone is strongest when the grains are interplaited (e.g. in diabase), whereas stripes of mica constitute zones of weakness along which the stone easily cracks. The importance of the study of microfracturing under the microscope cannot be emphasized enough. 4.1.2. Technical factors In assessing stone as a technical building material, its physical, mechanical, thermal etc. properties are measured in certified laboratories by standardized methods. These properties depend upon the mineralogy of the stone in a complicated
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way, as comprehensively reported by, for example, Alnæs (1995) and Winkler (1997). The technical requirements for a stone vary according to the final use of the material. The test standards can roughly be divided into three main categories covering: 1. construction systems for dimension stone; 2. standard specifications for different rock types; and 3. standard methods and practices for physical tests. In addition to this there are standards for terminology, statistical evaluation of test results etc. Standardization systems like ASTM (American Society for Testing and Materials) and DIN (Deutsche Industrie Norm) have several standard test methods for dimension stone. For example, ASTM defines the five most critical stone typespecific characteristics of dimension stone such as: water absorption/bulk specific gravity (ASTM C 97); modulus of rupture (ASTM C 99); compressive strength (ASTM C 170); abrasion resistance (ASTM C 241); and flexural strength (ASTM C 880) (see, e.g. Harben and Prudy, 1991). These standards also include absolute values that a stone should satisfy. However, in practice the usability of stone is often assessed project by project. At present, the European Committee for Standardization (CEN ) is preparing standards for: 1. natural stone terminology and denomination criteria; 2. test methods; and 3. natural stone products. The test methods include determination of density and porosity, compressive strength, resistance by recrystallization tests, flexural strength under concentrated load, water absorption by capillary action, breaking load at dowel hole etc. The CEN norms are still being worked out at the time of writing this article.
4.2. Non-geological factors 4.2.1. Economic factors It is not sufficient to localize a geologically or technically sound dimension stone deposit. The stone must also have a value on the market — if
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it cannot be sold, it will not be used regardless of its technical quality. Appearance, technical properties, availability and homogeneity of the material determine if it can be sold at a reasonable price. Considering the economic factors in the quality assessment, it must be remembered that the marketability of a stone is highly dependent upon prevailing fashion and personal taste, unlike that of products in the trade of industrial minerals, for example. It is often very difficult to predict future trends in popularity for stone materials. As a general rule, it can be said that strong and unusual colours always seem to be desirable. There are also national preferences concerning colour and style, often related to the culture and the religion of a country. For the grounds of selecting dimension stones, see, for example Hiltunen (1991). 4.2.2. Infrastructural factors Stone extraction is controlled by local legislation in different countries, and detailed extraction and reclamation plans are often required. The location of the deposit is thus an essential aspect. Environmentally sound deposits cannot be located close to sensitive nature areas or objects, and must be at an appropriate distance from the nearest houses. On the other hand, the deposit should have reasonably easy access to good transport facilities. Possibilities for storage and use of the excess blocks must be included in the final evaluation of the suitability of a location. It is a clear advantage for a deposit to be located so that the excess material can be economically used, for example, for construction purposes.
5. Exploration procedure The process of dimension stone exploration as defined in this paper is a systematic and stepwise ( Table 2) regional study, in which ‘new’ dimension stone deposits are identified in a large target area. The individual stages in this procedure can also be applied, for example, to site investigations or quarry development. The procedure of dimension stone exploration has earlier been discussed by,
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Table 2 Procedure for geology-based dimension stone explorationa
covered areas to determine boundaries of major rock formations and structures within them (e.g. diabase dykes). Larger tectonic features can be inferred from analyses of the topographic data. At this stage, environmental and infrastructural aspects must also be considered. The infrastructural factors are important when estimating the economic possibilities of a dimension stone prospect. Depending on the product to be quarried, a difficult location and high initial expense can prevent setting up a quarry. Environmentally sensitive regions and objects are noted and excluded from further studies. Furthermore, the status of regional planning in the area concerned should be taken into consideration. Geological aspects, controlling the location of the dimension stone deposits are compiled and analysed (see Sections 6 and 7). 5.2. Field mapping
a The process of applying for quarrying permits is not shown.
for example, Harben and Prudy (1991), Jefferson (1993) and Loorents (1997). 5.1. Desk study In the first stage of an exploration all the geological, mineralogical, geophysical, topographical and historical material available on the area of interest is gathered. The material includes (numerical ) maps and their explanatory texts, publications on specific topics related to the geology of the area, reviews and literature on previous stone extraction in the area, etc. The aim is to build a view of the geology and to point out specific areas where conditions favourable for finding dimension stone would be maximized, thus reducing the size and number of the target areas. An analysis of the collected material gives an overview of the rock types, mineralogy and physical properties, age relationships, metamorphism, and overall tectonics. For example, geophysical data are used in
Potential areas located by the analysis of the existing data are studied in the field. At this stage the mapping is done on a general level and on natural outcrops. At each site observations are made on the appearance and structure of the rock types. A few hand specimens and/or small block samples are taken for mineralogical studies and aesthetical evaluation. The general soundness of the rock and the least fractured portion of the prospect are estimated by observing the main tectonic features and types of fracturing. The size of the prospect is evaluated. The proper location in relation to the environmental and infrastructural aspects (natural objects, houses, roads, and waterways), should also be checked at this stage. By observing local stone constructions and by interviewing people in the area, valuable information on previous stone quarrying in the area can be gained. The most interesting prospects are then selected for a more detailed examination. 5.3. Detailed examination of a deposit prospect The selected prospects are examined in detail on washed excavations and traverses if the natural exposures are not sufficient. At this stage variations in the rock types, their colour, structure, inclusions,
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stripes, veins, dykes, and tectonic features are mapped on a scale of 1:100, for example. Density and pattern of fractures are measured. Sampling is more systematic than at the previous stage. Because of weathering, the true colour of the rock is often hard to determine and the sampling is normally done by a small diamond drill. Larger block samples should also be taken for a proper evaluation of the appearance of the rock. Large tectonic features, mapping traverses, topographical features, and possible geo-radar traverses are measured with a tachymeter and tied up to an appropriate coordinate system. Topographic information is further used to calculate the volume of the prospect. 5.4. Geo-radar survey The purpose of the survey with ground penetrating geo-radar is to study the subsurface fracturing of the prospect. The operating principle of the geo-radar is based on an electromagnetic wave sent into the ground. When the wave meets a layer or object of different electrical properties, some of its energy will be reflected back. The amplitude of the reflected part, the time elapsed between the pulse and the reflected wave, and the moment of transmission are recorded at a rate of ca 25–52 times per second. This produces a continuous cross-section of electrical properties in the ground (Hanninen, 1992). The waves are reflected from horizontal fractures and rock boundaries. The geo-radar survey gives quite accurate information down to 20 m below the outcrop surface. The horizontal and subhorizontal fractures appear clearly in the geo-radar profiles, while the vertical and subvertical fractures are more difficult to interpret (cf Hanninen et al., 1991). A good knowledge of the geological features of the prospect is an essential precondition for a successful interpretation of the geo-radar measurements. 5.5. Core drilling Core drilling gives invaluable information on the quality of the prospect by producing a drill core in which the variation in colour, mineralogy and fracturing can be observed in depth. To facili-
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tate this evaluation, selected parts of the core are split in half and polished. Correct placing of the inclined or vertical drill holes is very important and depends on the amount of geological information acquired during the preceding steps of the exploration process. The depth dimension of core drilling is up to 50 m. The normal diameter of the core is ca 45 mm. 5.6. Additional studies A prospect proven good by the geological investigation is ready for test quarrying and test production. Test quarrying is made on a small scale and normally a few hundred cubic metres are extracted. In this process the quarrying properties (extractability/drillability) of the stone are tested. The extracted material is then processed into final products to determine the production properties such as sawability, scorching ability, honability and polishing ability. The processed products are further used for commercial testing.
6. Geological controls on the location of dimension stone deposits In this Section we demonstrate some of the geological factors that constrain the occurrence of dimension stone deposits in the Precambrian of southern Finland. In all cases the deposits are concentrated in a small part of a larger area in which different geological factors coincide in a beneficial way. 6.1. The Orivesi granite batholith The Orivesi granite batholith (Selonen and Ehlers, 1996) is located at the southern margin of the CFGC ( Fig. 1) in an east–west trending belt of granite intrusions. The ca 1.87 Ga-old Orivesi batholith is composed of a red coarse-grained porphyritic granite, a blue-grey coarse-grained porphyritic granite and an aplite granite (Fig. 3). No mafic magmatic components are associated with the batholith. The red coarse-grained porphyritic granite with a magmatic foliation builds up the bulk of the batholith ( Fig. 3). The porphyritic
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Fig. 3. Geological map of the Orivesi granite batholith with surroundings. The blue-grey coarse-porphyritic granite occurs as scattered bodies inside the broken circle. Dimension stone deposits are marked by stars. Modified from Selonen and Ehlers (1996) and Selonen (1998).
texture is defined by K-feldspar phenocrysts, which usually are euhedral, but oval (and mantled ) grains also occur. The Orivesi batholith has intruded along regional shear zones in the southern parts of the CFGC (Fig. 1; Selonen and Ehlers 1996). The intrusion was diapiric, at least during the final emplacement, producing the magmatic structures in the batholith, and it is post-tectonic in relation to the ductile deformation in the host rocks (Selonen and Ehlers, 1996). Consequently, it is sparsely fractured and regionally well suited as a prospect for dimensional stone. The dimension stone deposits are confined to the red granite (Selonen, 1998) ( Fig. 3). All but one of the sites are situated in the western half of the batholith ( Fig. 3), indicating that the potential
for stone of good quality is higher in the western half of the batholith than in the eastern half. The blue-grey granite and the aplite granite lack value as dimension stone. The concentrated location of the dimension stone deposits in the Orivesi batholith can be explained by the geological history of the pluton (Selonen, 1998) (see Fig. 4). A NNW–SSE trending fault cuts the batholith into two halves. The western half has been uplifted relative to the eastern half, resulting in different structural characteristics in the halves at the present level of erosion. Representing a lower crustal level, the western half consists of sparsely fractured red granite with a well developed, steep, penetrative and concentric foliation. On the contrary, the upper parts of the batholith, in the eastern half, are characterized by
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Fig. 4. The different structures in the two halves of the Orivesi granite batholith can be explained by a west-side-up faulting, exposing a deeper crustal section on the western side. (A) Simplified geological map of the Orivesi granite batholith (cf Fig. 3). (B) Sketch map of the west-side-up movement in the batholith. Not to scale. Modified from Selonen (1998).
intrusions of late magmatic melts into the red granite: the blue-grey granite and the aplite dykes and sheets. The magmatic foliation in the red
granite is poorly developed and the late brittle fracturing on outcrops is dense in the east. The late melts have intruded as dykes and sheets and
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they are densely fractured and unsuitable as dimension stone. It is thus possible to explain why almost all the deposits of dimension stone are located in the western half of the batholith, and why the red granite there has better dimension stone properties compared to the other half. The west-side-up movement along the central fault zone exposes deeper levels of more homogeneous, sound and sparsely fractured red granite, restricting the locations of good dimension stone to the western half of the Orivesi batholith. 6.2. The Rantasalmi-Sulkava area The Rantasalmi-Sulkava area is located in the eastern part of the LSGM-zone (Fig. 1) and consists of highly metamorphosed and migmatized rocks ( Fig. 5). The main rock types are metapelites and potassium granite. Granodiorite/tonalite and gabbro are also present in smaller occurrences. The most common types of metapelites are mica schists and mica gneisses. They are strongly migmatized by coarse potassium granite, forming veined gneisses. The amount of potassium granite increases towards the southeast. In the northwestern parts of the area mica schists have preserved primary sedimentary structures, such as bedding ( Korsman, 1977) and graded bedding (Gaa´l and Rauhamaki, 1971), while in the southeastern parts the structures are only occasionally visible. The metamorphism in the area is progressive, and the grade increases from the northwest to the southeast, reaching granulite facies conditions in the centre of the area (Fig. 5). Metamorphic zoning in the metapelites has been described by Korsman et al. (1988) as follows: sillimanite+K-feldspar; cordierite+K-feldspar; garnet+cordierite+sillimanite+biotite; and garnet+cordierite+sillimanite (Fig. 5). Pyroxene is present in the granodiorites– tonalites in the area of granulite facies. The U–Pb zircon/monazite age of 1840–1830 Ma in the metapelites reflects the peak metamorphism of the area ( Korsman et al., 1984). Dimension stone deposits are found in two types of rock: garnet–cordierite gneiss; and potassium granite. The deposits are located in the area of the highest metamorphic grade. The garnet– cordierite gneisses are highly metamorphosed met-
apelites with a paleosome consisting mainly of garnet, cordierite and plagioclase. The neosome is rich in K-feldspar and quartz. The proportions of paleosome and neosome vary and the amount of neosome can be as high as half the total volume. The medium and coarse-grained potassium-rich granites are intensely red and include ‘ghostlike’ restites of mica gneiss and clusters of garnet/cordierite. The restricted location of the dimension deposits in the Rantasalmi-Sulkava area is geologically controlled by the metamorphism, confining the deposits to the area of granulite facies. The almost complete melting and homogenization of the parent material in that high-grade area are the prerequisites for forming a rock sound enough for industrial extraction of dimension stone. 6.3. The Vehmaa rapakivi granite batholith The Vehmaa rapakivi granite batholith is situated in southwestern Finland ( Fig. 1), covering an area of 700 km2 (Lindberg and Bergman, 1993). The Vehmaa batholith has a concentric structure consisting of five major rapakivi granite varieties: a pyterlitic rapakivi granite; a coarse-grained porphyritic rapakivi granite; a medium-grained porphyritic rapakivi granite; an even-grained rapakivi granite; and a porphyry aplite (Lindberg and Bergman, 1993) (Fig. 6). Lindberg and Bergman (1993) report two U–Pb zircon/monazite ages for the batholith: 1573±8 Ma for the even-grained rapakivi granite and 1582±4 Ma for the pyterlitic rapakivi. The anorogenic Vehmaa rapakivi granite batholith has intruded after the Svecofennian orogeny and sharply transects the Svecofennian bedrock. There are no traces of orogenic movements in the Finnish bedrock after the emplacement of the rapakivi granites. Therefore, the undeformed batholith is sparsely fractured and is a location with a high potential for dimension stone. Structural criteria indicate that the Vehmaa batholith is a subhorizontal laccolith-like intrusion (Selonen, 1998), resembling the other rapakivi intrusions in Finland (Lauren, 1970; Elo and Korja, 1993). Geometrically this means that the present level of erosion is subparallel to the lacco-
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Fig. 5. Geological and metamorphic map of the Rantasalmi-Sulkava area. Dimension stone deposits are marked by stars. Sill, Sillimanite; Ksp, K-feldspar; Gar, garnet; Cord, cordierite; Bio, biotite. Modified from Korsman et al. (1988) and (1997).
lithic intrusion, exposing a wide area of rapakivi granite with a high potential for dimension stone. Portions of granite with sparse fracturing are abundant in all the intrusive phases in the batholith, except for the densely fractured porphyry aplite. Although all the sparsely jointed magmatic components have technical potential as dimension stone (Selonen, 1998), the deposits in the Vehmaa batholith are strongly concentrated in only two types of rapakivi granite: the medium-grained porphyritic rapakivi granite in the middle of the
batholith and the even-grained rapakivi granite in the eastern part of the batholith (Fig. 6). This is because only those granites have a colour and a general textural outlook that are commercially viable. The location of the economically useful dimension stone deposits in the Vehmaa batholith is thus strictly controlled by the magmatic evolution of the batholith. The brightest coloured stone qualities belong to distinct intrusion phases, sometimes separated from less useful varieties only by gently dipping intrusional contacts that are barely
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Fig. 6. Geological map of the Vehmaa rapakivi granite batholith. The dimension stone deposits are marked by stars. Modified from Lindberg and Bergman (1993).
visible. For future stone material prospecting in this large batholith, the proper identification of the intrusion phases is of great importance.
7. Discussion and conclusions There is not much literature on methods of prospecting for dimension stone, or on the geological controls of dimension stone deposits. However, the localization of deposits is no trivial matter and the factors governing their location are strictly
geological. By applying normal methods of structural geology and petrology, and by carefully mapping both active and abandoned deposits, we can draw conclusions about the geological factors that in combination will produce a stone of suitable quality. The ultimate goal is, of course, to create systematic prospecting methods for dimension stone and to identify the geological conditions required for successful prospecting. The examples cited above show some of the primary geological factors of importance for the location of dimension stone deposits: vertical
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movements along major fractures; history of intrusion; different intrusion phases; and metamorphic grade. Other important geological features are the type and direction of deformational structures such as foliations and lineations. These are the results of common geological processes, easily addressed by ordinary geological investigation methods. The consequence of this is very important for the exploration for dimension stone: if we know the factors controlling the occurrences, we are able to predict geological environments critical for finding new deposits. The best effect can be obtained if this knowledge can be applied at the very beginning of the exploration process, that is, during the desk study, when the main purpose is to divide the target area into smaller units with different potential for dimension stone. The prospecting can thus be systematically directed to those subareas with the highest potential for dimension stone. It is essential in the desk study to gather data on the target area from many different sources, since sites with a high potential for dimension stone are located in areas where several geological factors coincide. By a careful and thorough analysis of the collected data, the potential areas (down to outcrop scale) can often be identified at that early stage. Consequently, the desk study should never be neglected or underestimated, but regarded as an integral part of the exploration process. The desk study is followed by field mapping, during which the prospects are identified, and the decision, whether a more detailed examination is worthwhile at the sites, is taken. The appearance and soundness of the rock are the most important criteria in these evaluations. Soil cover and weathered surfaces are difficulties which can lead to misjudgements even if some flaws (e.g. inclusions and variations in mineral composition) can be better recognized on a weathered surface. The success of the field mapping is very much dependent upon the experience of the mapping geologist, since the recognition of good appearance and soundness of a rock is mostly done by eye. The use of geo-radar surveys and core drilling may follow but at this stage the visual assessment is crucial. It is also important to have experience and knowledge of what can be achieved by different investigation methods.
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The quality and usability of the promising prospects are assessed during the detailed examination. At this stage the evaluation is ‘easier’ as the area is narrowed down and since the exposure is improved by stripping and washing of carefully selected mapping traverses. In the detailed examination the types and density of fracturing and the number and type of different flaws that can spoil large volumes of otherwise sound stone should be studied with special care. The supporting methods like geo-radar surveys and core drilling can be regarded as parts of the detailed examination ( Table 2), as the study of the outcrop surface alone normally does not give enough information for an adequate evaluation of the prospect. But they can be effectively applied only after careful geological mapping of the prospect. Because of the high cost, core drilling is often carried out as a final stage of the exploration procedure only in the most promising prospects. However, the use of the costly methods is well justified, because of the high level of initial investment for quarrying. The detailed examination also gives valuable information concerning the future quarrying and production planning. Our conclusions are: the occurrences of dimension stone are determined by local and regional geological characteristics, and can be studied by geological methods. Exploration for dimension stone can be done systematically with a geological approach, but a good knowledge of the quality demands is the prerequisite for a successful exploration study. In a properly executed survey, knowledge of the critical geological factors and knowledge of the required quality criteria are combined and related to the geological features of the target area. A systematic and stepwise dimension stone exploration based upon geological investigations, as well as a thorough geological quantity/quality inventory of a deposit prospect, will considerably enhance the profitability of stone extraction.
Acknowledgements This work was financially supported by the K.H. Renlund Foundation, Finska Stenindustri
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˚ bo Ab, Geological Survey of Finland and A Akademi University, Department of Geology and Mineralogy, which is gratefully acknowledged. Ms ˚ bo Akademi University) drew Merja Kuusisto (A the line drawings. Mr Pekka Jauhiainen ( The Finnish Natural Stone Association) kindly delivered statistics on the production and usage of Finnish dimension stone. Mr Ream C. Barclay ˚ bo Akademi University) corrected the English. (A Dr I.W. Farmer and Professor Heikki Niini critically reviewed the manuscript. Finally, Mr VeliJuhani Ha¨nninen ( Finska Stenindustri Ab) is thanked for intensive and enlightening discussions on relations between geological knowledge and dimension stone exploitation during the final stages of the study.
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