Black dimensional stones: geology, technical properties and deposit characterization of the dolerites from Uruguay

Major and minor elements were determined by X-ray fluorescence spectrometry at the GeoForschungsZentrum in Potsdam (Germany). A total of 24 samples from 14 dikes were already investigated by Bossi et al. (1993) and a total of 28 samples from 16 dikes were analyzed in this study. Another five dikes were identified, but they were unreachable for samples collection. In some dikes two samples were analyzed, one from Zone 2 (fine-grained) and the other from Zone 3 (medium-grained), in order to ascertain the geochemical differences that can be correlated to the petrography of these rocks.

The results of these analyses (Tables 2, 3) corroborate the observations made by Bossi and Campal (1991), who divided the dikes into two groups based on their geochemistry that correspond to the two petrographic groups. Group A is characterized by relatively high contents of TiO₂ (between 1.5 and 2.5%) and an andesitic composition, and Group B is defined by relatively low contents of TiO₂ (<1.2%) and an andesitic–basaltic composition. This is shown in the De La Roche diagram in Fig. 10. Group A also shows relatively higher values for the other major elements: FeO_t, MnO, Na₂O, K₂O, P₂O₅, but lower values of Al₂O₃, MgO and CaO (Fig. 11). They also exhibit different mg# values (MgO/MgO + FeO_t), where the values for Group A are around 0.17 and 0.25 and those for Group B are between 0.35 and 0.45.

Group B shows higher values of the following trace elements: Rb, Ni, Y, Zn and V and lower values of Cr with respect to Group A. The other trace elements, Nb, Zr, Sr, Ba and Ga, show a similar distribution between both groups.

In both groups, a different geochemical signature in the two dike zones considered can be recognized. Regarding the major elements, Zone 2 represents an enrichment of the following elements in Groups A and B: TiO₂, Na₂O, K₂O and P₂O₅, and a decrease of SiO₂, MgO and CaO. For some other major elements a different behavior can be observed. In Group A, an enrichment of FeO_t and MnO and a decrease in Al₂O₃ is observable, when comparing Zone 2 to Zone 3. In the Group B, the opposite behavior occurs.

Trace elements are distributed as follows: there is an enrichment of Rb, Nb, Zr and Ba in Zone 3 of both dike groups, whereas only Group A presents an enrichment of Sr and Ga in this zone. Cr, Ni and Zn behave differently between both groups, where enrichment occurs in Zone 2 of Group A and a decrease is observable in the same zone of Group B. Y and V also show a different behavior between both groups, with an enrichment of these elements in Zone 2 of Group B, and a decrease in Group A. Different geochemical signatures between both groups can be explained by various stages in the magmatic evolution within a tholeiitic series, with Group A being more evolved than Group B (Fig. 12).

Bossi and Campal (1991) described the petrography of the dike swarm based on the differences correlated with the two geochemical groups in the dikes. According to these authors, the main petrographical differences between both groups are mineralogical, where the high TiO₂ rocks (Group A) show lower plagioclase contents. The plagioclase is of andesine composition in Group A and in low TiO₂ rocks (Group B) plagioclase shows labradoritic composition.

The clinopyroxene present in both groups is augite, but in Group B orthopyroxene (broncite) can also occur. Bossi et al. (1993) determined the composition of the clinopyroxenes as augite to subcalcic augite and pigeonite with high Ca contents. In Group A, more opaques occur consisting of magnetite and ilmenite intergrowths, which are normally automorph crystals associated with pyroxene. Only small proportions of pyrite and chalcopyrite are present. In Group B, the most important opaque mineral is skeletal ilmenite, which is associated with quartz–feldspar intergrowths and sometimes with pyroxene. Amphibole is present as a product of uralitization of clinopyroxene and is more common in Group A.

The term “micropegmatite” is used to refer to quartz intergrowths, normally composed of quartz and alkali feldspar, but sometimes also composed of quartz and plagioclase. These intergrowths are much more abundant in Group A, where they are composed of quartz and oligoclase. When apatite occurs, it is always related to these intergrowths. According to Bossi and Campal (1991), the grain size between 0.1 and 0.2 mm of the plagioclase laths characterizes the “Granito Negro Absoluto (Fig. 13a). “Granito Negro Fino” (0.3 and 0.5 mm), “Granito Negro Oriental” (0.6 and 0.8 mm) (Fig. 13b) and “Granito Negro Grueso” (plagioclase laths > 1 mm) all show larger grain sizes.

Spoturno et al. (2004) have also studied these rocks, but did not differentiate them into two groups. They described the rocks as being composed of plagioclase with grain sizes around 0.3 mm, when measured parallel to the long axis (approximate composition: labradorite) and pyroxene, i.e., augite, pigeonite and occasionally orthopyroxene. Occasionally, microphenocrysts of plagioclase and augite have been observed with sizes up to 2 mm giving the rock a microporphiritic texture. Accessories include ilmenite with leucoxene rims and apatite. These authors also described the presence of amphibole of uralitic origin, a product of alteration of pyroxene.

The petrographical observations corroborate the previous research, with new information being added by this study. In Table 4 the main observations are summarized and the petrographic details are shown in Figs. 14, 15, 16, 17 and 18.

Using the Michel–Levy method, the plagioclase laths in Group A were determined to have an An₃₄–An₄₄ composition (andesine), whereas Group B shows an An₄₇–An₆₀ composition (labradorite). In both groups, plagioclase occurs as fine-grained euhedrally zoned laths (for grain sizes see Table 1; Figs. 7, 8). In Group A, the laths commonly contain inclusions of very fine-grained apatite crystals.

The clinopyroxene is generally augite, but also some pigeonite can be observed. The augite in Group A occurs as small crystals with grain sizes between 0.4 and 0.6 mm. They are euhedral to subhedral, very often exhibit twining and are surrounded by amphibole and to a lesser extent by biotite. In Group B, the augites are coarser grained, with sizes ranging between 0.4 and 2.4 mm. The crystals are euhedral to subhedral and generally show an aspect ratio up to four. Very often they show twining and are sometimes surrounded by amphibole and biotite.

Biotite is present as euhedral to subhedral crystals with sizes up to 0.9 mm (with an aspect ratio of 4.5) in Group A. In Group B, the grain size only reaches 0.2 mm with an aspect ratio of 2. In both groups it occurs as an accessory. The biotite present is always related to magnetite or augite.

All samples exhibit the presence of granophyric intergrowth as described by Shelley (1993), which Bossi and Campal (1991) and Spoturno et al. (2004) refer to as “micropegmatite” or interstitial quartz–feldspar intergrowths. In some cases, microcline can be recognized within the granophyric intergrowth, but in other thin sections it is difficult to determine the nature of the feldspar present. They are normally up to 0.8 mm in size in Group A, but up to 1.6 mm in some cases and smaller in Group B, up to 0.6 mm.

Opaques are also present as accessory minerals and are much more frequent in Group A, where magnetite crystals are euhedral to anhedral and the ilmenite crystals are skeletal. Both crystals always occur with augite and biotite as intergrowths, never surrounded by plagioclase. Their size is between 0.2 and 0.8 mm. In Group B, the opaques are skeletal, long and lath-shaped ilmenite up to 0.7 mm with an aspect ratio up to 14. The very fine-grained magnetite crystals are subhedral to anhedral with a size up to 0.3 mm. Both are often distributed around augite as well as around the plagioclase. The opaques are homogeneously distributed in the grain size zones (1, 2 and 3), and in Zones 1 and 2 their grain size is generally smaller than the plagioclase laths. In Group B, the opaques are sometimes surrounded by red spots of limonite or hematite.

Apatite is found as an accessory, as very small (up to 0.1 mm in length) euhedral crystals with basal hexagonal sections and needle-like longitudinal sections. They are always included in the granophyric intergrowth and can be very often found as inclusions in the plagioclase of Group A.

Hornblende occurs as a product of uralitization of clinopyroxenes, where they appear either with fibrous habit or as euhedral to subhedral crystals with sizes up to 0.4 mm. They are more frequent in Group A.

Calcite occurs only as an accessory in a dike from Group B in Zone 3 (sample U13). It appears as very fine-grained (up to 0.2 mm) anhedral to subhedral crystals normally associated with biotite.

The texture normally observed in both groups is subophitic, but there are some local variations. A porphyritic texture is observed in the chilled margins of two dikes from Group B and in one from Group A. In the dike from Group A the phenocrysts are plagioclase (sample U55) and in the dikes from Group B they are plagioclase and augite. In one sample (U13) they are altered to calcite and chlorite, respectively.

In both dolerite groups the normative mineral compositions show more or less comparable values (see Table 5). The M value (color index: total volume percent of mafic minerals) varies between 29 and 37% in Group A and between 31 and 34% in Group B.

For both groups the normative M values are comparable between Zones 2 and 3. For Group A, Zone 2 has a value of 31 and Zone 3 a value of 33%. In Group B, both zones have an M value of 33%. Only one sample of Group A shows an important difference: 37% in Zone 2 and 33% in Zone 3 (Quarry U55). Although, M values for zones 2 and 3 are comparable, the Absolute Black (Zone 2) appears much darker than the Moderate Black (Zone 3) due to the smaller grain size.

This color index, which is determined by the normative mineral composition, shows some differences compared to the modal mineral composition. In Group A, the modal mineral composition exhibits a M value of 49% in Zone 2 and 38% in Zone 3. Group B shows 49% in Zone 2 and 47% in Zone 3. All the samples have an M value higher than 30%, and therefore are classified as mesocratic.

A first classification based on the color of the rock and grain size of plagioclase was made by Bossi and Campal (1991) and continued by Bossi and Schipilov (2007). They determined that the Absolute Black is aphanitic with a mean grain size between 0.1 and 0.2 mm. Fine Black is a fine-grained variety with a grain size between 0.3 and 0.5 mm. Normal Black (known as Oriental) has a grain size between 0.6 and 0.8 mm and the Coarse Black is coarser than 1 mm.

Color measurements were performed at the Labor für Baudenkmalpflege in Naumburg (Germany) on 12 different black dimensional stones. In Fig. 19, all results are listed. The L* values ranged from 25.22 for the Absolute Black dolerite to 42.47 for the Kvemo quartz–phyllite. a* values are between −0.03 for the Impala gabbro/norite and 1.00 for the Galaxy Brazil. b* values range between −0.13 for Kvemo and 1.60 for Impala (Fig. 19).

Motoki and Zucco (2005) developed a classification that takes into account the B parameter (brightness) from the HSB color system (see Appendix). The category limits determined by these authors have been converted to the CIE Lab color system as follows: black (L < 27), dark gray (27 < L < 58), light gray (58 < L < 87) and white (L > 87). Utilizing this classification, the Absolute Black dolerite can be defined as black and the Moderate Black dolerite as dark gray.

The two groups investigated from the dike swarm intruded into an area of 20,000 km² and in all the lithologies of the Piedra Alta Terrane, which consists of granites, metagranites and metasediments (Fig. 20). Bossi et al. (1993) pointed out that Group A crops out more often in the northern part of the Piedra Alta Terrane and Group B more to the southern part.

The dikes and the quarrying were first mentioned in Bossi (1969). All the mining concessions and 15 locations where the dikes were mined were mentioned in an internal report of the DINAMIGE (National Geological Survey of Uruguay; Medina and Carrión 1986). In 1987, Medina and Carrión presented another report that described just the dikes affected by exploration and the mining permissions obtain from the Florida Department.

Bossi et al. (1989) presented the first photogeological map where the distribution of the dikes is shown. The map depicts thousands of parallel dikes intruding the “ancient orogenic cycle” or what it is now known as the Piedra Alta Terrane (PAT). The dikes follow a NE–E direction, but in the eastern margin of the terrane, near the Sarandí del Yí Shear Zone, they change direction to S–SE, due to the dextral shear activities during the Proterozoic. They were recognized using aerial photographs at 1:20,000 scale and corroborated with field observations (Bossi and Campal 1991). Since this map is based on photographic aerial surveys, it was impossible to verify the presence of all these dikes in the field, and therefore the number of dikes is probably smaller than originally proposed.

More recently, Spoturno et al. (2004) published a geological and mineral resources map for the San José Department at a scale of 1:100,000. This represents an area of almost 5,000 km², which corresponds to approximately the half of the area to where the Piedra Alta Terrane crops out. They recognized a total of 11 dikes; 6 of them have been mined in the past for the extraction of dolerite blocks for the dimensional stone market. Techera et al. (2004) studied in detail the different quarries present in the San José Department and their observations are in accordance with those of Bossi and Campal (1991) and Bossi and Ferrando (2001).

The field observations made in the current investigation support most of the conclusions of the previously mentioned authors. They are:

A summary of the dikes sampled are listed in Table 7.

The main controlling parameters affecting the deposits of dimensional stones are the volume of the deposit, the ensemble of lithological aspects (e.g. fabric, mineralogy, geochemistry, etc.) and the tectonic inventory.

The volume is important because enough material has to be present in order to furnish a steady product for regional or international markets, and thus such an occurrence can be considered as a suitable provider. The dolerite deposits are characterized by their tabular shape, being a two-dimensional deposit. The volume of the deposit is limited by the length of the dike and its width, whereas the depth is determined by how deep it is technologically possible to mine.

In Uruguayan deposits, the maximum depth that has been reached in a dolerite quarry is around 10 m. Medina and Carrión (1987) reported that in the quarries U12A and U12B drillings were performed, but any report on the results is lacking. This is also the case of drillings in the quarry U8, where the drill cores are available, but the drilling location and direction is unknown, so it is impossible to know the depth of drilling. The maximum mining depth in this kind of deposit has been attained in the dolerite quarries of Sweden, where the extraction of blocks can reach up to 50 m.

The second controlling parameter is the ensemble of lithological aspects that indicates if the rock is suitable as a dimensional stone, since these influence the décor, e.g. the color and fabric of the rock. For black dimensional stones, the intensity and homogeneity of the black color is very important, so that no other color spots are present. Exceptions to this assumption are the cases where such heterogeneities make an improvement in the quality of the décor, e.g. the Star Galaxy from India or Black Galaxy from Brazil.

Tectonic elements are the third controlling parameter. The main structural elements are joints, which are mainly distributed as orthogonal sets and are probably the result of the cooling and contraction of the rock.

In Fig. 22 the main joint sets are represented, which were observed in all the investigated dikes. Previous authors (e.g. Medina and Carrión 1986, 1987; Bossi and Campal, 1991) have also reported on these joint sets. Three main joint sets occur, two sub-vertical and one horizontal. The two sub-vertical sets are orthogonal: one parallel to the dike walls striking 070–085° and dipping toward the NW or SE (represented as Vt 1 in Fig. 22), and the other one perpendicular to the dike walls striking to 320–360° and dipping toward the NE or SW (represented as Vt 2 in Fig. 22). The sub-horizontal joint set shows dips up to 10° in all directions.

The distribution of the joints in several quarries has been measured in order to calculate the potential block size that can be extracted. The first two joint sets observed are represented in the plots of Fig. 23. A meaningful difference in the orientation of the joints cannot be observed between the dikes of Groups A (High TiO₂) and B (Low TiO₂). However, the dikes of Group A display a higher frequency of joints parallel to the dike wall, whereas the dikes of Group B show a higher frequency of joints perpendicular to the dike wall and of joints that dip to the SW. Since these joint sets are orthogonal to each other, regular-sized blocks can be quarried. If the spacing between the joints is large enough, the quarry is able to produce blocks on a profitable basis. The problem with this type of deposit is the existence of other joint sets that crosscut the orthogonal ones. Diagonal joints have also been previously described by Bossi and Campal (1991) and Medina and Carrión (1986). Although these joints are not very frequent, their significance for the quarrying operation is very important as they crosscut the regular blocks that could be extracted, increasing the proportion of waste material and minimizing the production of marketable blocks (Fig. 24). In statistical documentations they are often overseen (Fig. 23). Diagonal joints have been isolated in the plots of Fig. 25 in order to better visualize them.

The two diagonal sets are also orthogonal to the dike direction and both have dips between 11 and 75°. One diagonal set strikes N070-085° and dips toward the NW or SE (Dg 1 in Fig. 22), and the other one strikes N320-360 and dips to the NE or SW (Dg 2 in Fig. 22).

All the dikes can be subdivided into three zones by their grain size: Zone 1 (very fine-grained), Zone 2 (fine-grained) and Zone 3 (medium-grained). In quarries located in the dikes of Group A, in which their Zone 2 is unfractured, field observations indicate that the Absolute Black variety can be mined (e.g. Black Stone Quarry, see Fig. 9). The Moderate Black variety (Negro Oriental in Uruguay) is extracted from Zone 3. Since Zones 1 and 2 are not more than 11% of the dike, the ratio between Absolute Black and Moderate Black is at best 0.124.

Several decades ago the scarcity of joints in a dimensional stone deposit was a very negative aspect for the extraction, because they could not be used to facilitate the mining of the blocks (Bossi and Campal 1991). Today the diamond wire saw makes it possible to cut a block of dolerite without any previous discontinuity. However, the problem of a greater proportion of joints in a quarry persists, which is one of the controlling factors in the quarrying of dolerite deposits.

Before starting the extraction of any dimensional stone, the weathered material (including rounded boulders) and sedimentary deposits that covers the fresh rock must be removed. These materials are called overburden or simply waste rock. In addition to these materials, an important amount of country rock (no matter if fresh or weathered) must also be extracted for safety reasons (instabilities, etc.) while quarrying. Considering a mining depth of 50 m, a width of 30 m and a dike dip angle of 89°, the proportion of waste rock to be removed is about 6.5% of the total volume extracted and 26% when the dip is 70°. However, the amount of waste rock will significantly increase by lower depths of exploitation as realize today in Uruguay (maximum depth up to 10 m).

Another kind of waste rock produced comes from the sections of the quarry where a high frequency of fractures occurs. No production of profitable blocks can be performed in these sections (Fig. 26). When the extracted block, which differs from a rectangular cuboid, is squared into one, new waste rock is produced that is highly dependent on the joint systems. Internationally, the waste material produced in dimensional stone quarries is around 30–50% of the extracted volume, however, in Uruguayan dolerite quarries this value can reach up to 90% (Oyhantçabal et al. 2007a, b).

Density values are similar to gabbro-dioritic rocks, to which they can be compared. For the Absolute Black variety the apparent density value is 2.98 g/cm³ and for the Moderate Black the value is 3.02 g/cm³. In both varieties, the real density is quite similar, 2.99 g/cm³ for Absolute Black and 3.02 g/cm³ for Moderate Black. The density values of some comparable rocks vary between 2.8 (for Impala and Star Galaxy) and 3.3 g/cm³ (for Ebony Black) (see Table 7).

The dolerites have very small values of porosity under vacuum, around 0.03% for Absolute Black and 0.06% for Moderate Black. Their pore radii distribution shows that the most abundant pore radii class is 0.01 μm for Absolute Black and 0.02 μm for Moderate Black, whereas the average pore radii is 0.10 μm for Absolute Black and 0.06 for Moderate Black.

The vacuum and atmospheric water uptakes are very low, 0.01 for Absolute Black and 0.02% for Moderate Black due to the low porosity values. The values of the comparable rocks in Table 7 vary between 0.05 and 0.33%, for Preto Absoluto and Shanxi Black, respectively.

The thermal expansion differs between 6.53 × 10⁻⁶ K⁻¹ for Absolute Black and 6.90 × 10⁻⁶ K⁻¹ for Moderate Black (Table 8). These values can be compared to those determined by Hoffmann (2006) for a hornblendite (6.64 × 10⁻⁶ K⁻¹). There is no permanent change in length after the heating/cooling-cycles.

The dolerite Absolute Black shows uniaxial compressive strength (UCS) values that vary between 367.00 MPa in the z-direction and 399.56 MPa in the x-direction. Moderate Black shows lower values, with 266.22 MPa in the x-direction and 285.78 MPa in the y-direction (for coordinates references see Appendix). The UCS values for basic plutonic rocks range from around 90 to 400 MPa (Mosch 2008) (Fig. 27a, b). The Young′s modulus for Moderate Black was determined to be 17.40 GPa in the z-direction and 27.33 GPa in the x-direction, whereas for Absolute Black the values are 29.60 GPa in the y-direction and 32.90 GPa in the x-direction.

The tensile strength values determined for the Uruguayan dolerites are between 16.68 MPa in the x-direction and 19.49 MPa in the y-direction for the Absolute Black, and between 14.31 MPa in the z-direction and 16.72 MPa in the x-direction for the Moderate Black. According to Mosch (2008), the tensile strength for basic plutonic rocks is around 28 MPa.

The flexural strength values vary between 46.06 MPa in the z-direction and 52.75 MPa in the y-direction for the Absolute Black dolerite, and between 35.72 MPa in the x-direction and 30.81 MPa in the y-direction for the Moderate Black dolerite. The flexural strength values for basic plutonic rocks are around 5–30 MPa (Mosch 2008) (Fig. 27c, d). The breaking load at the dowel hole is 4.68 MPa for Absolute Black and 4.21 MPa for Moderate Black.

The abrasion strength varies between 2.16 cm³/50 cm² in the z-direction and 2.36 cm³/50 cm² in the y-direction for Absolute Black and 2.26 cm³/50 cm² in the x-direction and 2.53 cm³/50 cm² in the y-direction for Moderate Black. These are very low values considering the values given by Hoffmann (2006) for diorite, between 7.5 cm³/50 cm² and 15 cm³/50 cm², meaning that the Uruguayan dolerites show a high resistance to material loss due to abrasion.

Dolerites are valuable dimensional stones, mainly because of two distinct properties: their black color and the possibility of extracting them in larger blocks. The black color is directly related to the décor, and therefore to three properties of the rock: the mineral composition, the texture (which includes grain size, grain shape, and its interlocking flow structures and the observed reaction textures) and the structure (e.g. layering or inclusions). The latter two belong to the rock fabric.

The minerals that determine the black color are melanocratics, such as pyroxene, hornblende, opaques and biotite. Changes in the color can be produced when leucocratic minerals are present or minerals resulting from autohydrothermal reactions. Minerals, such as plagioclase, alkali feldspar and quartz–feldspar intergrowths create white spots in the rock. Green spots are developed when hornblende is present.

The value of black stones increases when the minerals are homogeneously distributed in the rock, exhibit no flow textures, visible intergrowths, nor reaction textures or any visible structure that alters the black color of the rock. If melanocratic minerals are homogeneously distributed, then the rock will have a homogenous color, which can be black or gray depending on their proportion and on the grain size of the rock. Rocks with a finer grain size and the same percentage of melanocratic minerals tend to be darker. Layering in a black rock affects it negatively, as well as the presence of veins of leucocratic minerals.

For all black dimensional stones three color qualities are defined: absolute black, moderate black and special black. The absolute black color in a dimensional stone can be developed in three different cases. First, when the rock is composed of 100% melanocratic minerals. Second, the rock is composed of melanocratic and leucocratic minerals and the latter contain inclusions of melanocratic minerals, and when the grain size is extremely fine.

The moderate black color can be generated when the melanocratic minerals make up 30–60% of the rock and the grain size is coarse (up to 2 mm) making both mineral colors distinguishable. Three special cases can arise which affects the color of moderate black stones. The first can occur in both varieties when there are minerals present that interrupt the normal color of the stone. Two examples of this are the presence of granophyric intergrowths that produce white spots and the presence of broncite in gabbro that produces golden spots (such as in the Star Galaxy variety of India). Autohydrothermal alteration creates the second special case where whole color changes are produced in the rock. This occurs by the process of uralitization that produces a slightly green coloration. The third situation is given when the rock is cut by veins of different color or layering is present that modifies the whole rock color.

The color index of the Uruguayan dolerites calculated using normative minerals varies between 31 and 33% in Group A and is 33% in Group B. When the color index is calculated using modal analysis, the values are higher. Zone 2 in both groups shows higher modal M values than Zone 3. Zone 3 of Group A shows lower M values (38%) with respect to Group B (47%), and in both groups Zone 2 can have the same M value (49%). The rock is therefore classified as mesocratic using both, the normative and the modal M value.

Color variations are related to changes in the grain size, which determines the quality of the main rock varieties. In Uruguayan dolerites, the color of the Absolute Black variety is until now only found in the dikes of Group A. The most important characteristic of Absolute Black is the very small grain size and its homogeneity. The porphyritic texture observed in Zone 2 of some deposits drastically reduces its quality.

The Moderate Black variety develops a dark gray color, in both groups. This is due to the medium grain size of the leucocratic and melanocratic minerals that are distinguishable without any optical aid.

Grain size variations are sometimes visible on the sides of the extracted and squared blocks. A clear layering of melanocratic and leucocratic minerals was not observed; however, in three quarries (two classified as Group A and one as Group B) the presence of leucocratic veins has been observed.

Autohydrothermal alteration can have a negative influence on the color in some parts of a deposit, especially in combination with a porphyritic texture as in Zone 2. When the autohydrothermal alteration is homogenously distributed throughout the entire deposit the quality of the Moderate Black variety is not negatively affected.

Granophyric intergrowths are visible as white spots within a black background, when they are larger than 1 mm. These intergrowths affect the quality of the Absolute Black more negatively than the Moderate Black variety, because the latter one already has coarse white spots (the plagioclase laths) that are homogeneously distributed.

From the color measurement results it can be concluded that Absolute Black dolerite is the darkest of the black dimensional stones considered in this study, followed by Shanxi Black, Yi and Dark Larvikite. The a* parameter (see Apendix) for the samples analyzed is generally positive (in one sample negative) and very low, indicating that there is a very small proportion of red color in these rocks. The b* value is also generally positive (in one case negative) and very low, indicating that the rocks studied also have a proportion of yellow in their color. The values a* and b* for the samples considered are so small that they have no influence on the color of the sample, which is determined only by the L* value.

Utilizing the classification scheme of Motoki and Zucco (2005), the rocks analyzed in this study are black and dark gray. Absolute Black dolerite is black, as well as Shanxi, Yi, Dark Larvikite and Brazil Black and Moderate Black dolerite is dark gray, as well as the rest of the rocks analyzed.

The application and use of dimensional stones are finally controlled by the technical and petrophysical properties. Under normal conditions in a building the technical requirements of a dimensional stone are moderate. Extreme applied loads are seldom. More often the dimensional stones have to show high flexural strength and high resistance to the climatic conditions or the chemical reactants to which they can be exposed to.

In comparison to other black rocks on the international market, the Uruguayan dolerites have similar or even better physical (technical) properties. Particularly the Absolute Black variety shows a maximal compressive strength of 400 MPa and a maximal flexural strength of around 50 MPa. With such high resistance values, these rocks can be used safely in many building parts that require a high compressive and flexural strength. Examples include building façades, free-standing constructions in a building, such as floating stairs, columns and architraves or counter-tops. The footbridge in Bad Homburg (Germany) is a very good example of the use of this type of high strength rock, where individual blocks of Nero Assoluto dolerite from Zimbabwe are used as load-transferring masts.

The low porosity, water uptake, thermal expansion and also the high abrasive resistance of the rock makes it an appropriate material for counter-tops and precision tables for technical/industrial purposes. These types of constructions usually require a high quality, pertaining to their physical, chemical and geometrical properties (e.g. length measures changes, susceptibility to chemical reactions, evenness of the surface, high strength, etc.).

The dimensions of the preferred blocks are determined by the industrial processes that follows the extraction. The blocks are required to be rectangular cuboids (or right rectangular prisms) without irregularities (such as veins or joints) and with the specified dimensions required for extractable raw blocks. These dimensions determine whether the block is of minimum, optimal, maximal or special size. The minimum block is determined by the smallest piece that can be commercialized, which is a gravestone. The optimal block must have the adequate dimensions to be cut in a diamond gang saw. The available transport condition possibilities determine the maximal weight of a block and therefore the dimensions of the maximal block. The special blocks are determined by the demand. For example, precision tables for technical/industrial purposes may require the blocks to have the dimensions of 3 m long, 1 m wide and half a meter thick.

Uruguayan dolerites have a density of 3 t/m³ and the maximum block weight cannot exceed 28 tons, because this is the maximum weight that can be transported in containers. The different block dimensions can be distinguished into: small ones (between 0.5 and 1 m³), medium blocks (between 1 and 3.3 m³), large blocks (between 3.3 and 6.6 m³) and very large blocks (larger than 6.6 m³). The maximum block must be smaller than 9 m³.

The Black Stone Quarry is a good example for illustrating block production from a dolerite deposit in Uruguay. Between the years 2005 and 2009, this quarry produced a total of 2,536 m³ as shown in the block size frequency diagram in Fig. 28. Most of the block sizes produced is of the small dimensions with 34% of the total production and the medium dimension making up 44%. Large blocks represent 17%, the maximum blocks and the very large blocks come to 2.8% of the total production. The proportion of Absolute Black in production is very low, being 1.8% for the production time span considered, 0.5% belongs to the small blocks, 0.8% to the medium and 0.5% to the large blocks.

Sections of three quarries were modeled using the program 3D-Block Expert developed at the University of Göttingen (see Siegesmund et al. 2007a, b; Mosch 2008 and Mosch et al. 2010, for details). The raw block size distribution for the models obtained is shown in Fig. 29.

For the Black Stone Quarry a section having the dimensions (8 × 8 × 6 m) has been modeled (Fig. 30a, c, d) and the block size distribution analyzed in order to determine its potential yield. The resulting prism is cut into slabs of 1 m each (Fig. 30b) and then a possible block distribution can be proposed (Fig. 31). From inclined joints it is evident that the block sizes do not match any minimum raw block volume, which corresponds to a higher amount of waste (Fig. 31).

The deposits are characterized by their two-dimensional shape and their limited width, where the extraction can either develop along the length or with depth. The length of one single dike can be up to 3,500 m, such as the dike of the Polonia-Pichinango area, but normally they only attain a length of 1,000 m. The depths of the dikes in the study area are unknown, but in other parts of the world economical mining in such deposits can reach 50 m. Normally, the dikes do not exceed a width of 40 m. The economically more interesting ones are those wider than 15 m, because initial observations show that the thinner dikes have a finer grain size and a greater proportion of Absolute Black. The dip of the quarry walls can present a problem for mining safety, especially when a certain depth is reached. The joint sets determine the efficiency of the quarry, because they control the block size and the amount of waste rock that will be produced.

When planning a mine based on the geometry of the deposit, the efficiency of the mining operation increases, and thus the production yield. Using the program 3D-Block Expert, the volume of blocks extracted in a sector of a quarry taking can be predicted, when taking into account the joint distribution and geometry.

The total quantity of dolerite dikes in the Piedra Alta Terrane is difficult to estimate. The available information can be used to estimate a maximum and minimum amount of extractable dolerites.

According to Bossi and Campal (1991), the number of dikes is around 300 and their average length and width is 460 and 20 m, respectively. The minimum value is based on observations published by Spoturno et al. (2004) for the San José Department, where they found a total of 11 dikes in an area of 2,500 km². If a similar dike distribution is assumed for the rest of the area where the dikes crop out, a total of 88 dikes may exist. The current investigation indicates that there are about 19 dolerite quarries, where four of them have been superficially mined and five of them are still in operation.

The geometry for this type of deposit is simplified to that of a cuboid or parallelepiped, where the width of the dike is “a”, the length is “b” and the depth of the dike is “c”. The volume can be calculated by V = abc. To calculate the reserves of dolerites in the Piedra Alta Terrane, an average dike is considered to have a width of 30 m, a length of 150 m and a maximum exploitation depth of 50 m. The total volume of this average deposit is 225,000 m³. Considering that 19 productive dikes at different stages of mining have already been identified in Uruguay, the volume of the reserve base is 4,275,000 m³. The efficiency of extraction for commercial blocks is not better than 10% (Oyhantçabal et al. 2007a, b) and that 20,310 m³ were already extracted (Bossi and Campal 1991; Morales Pérez 2004; DINAMIGE 2010), a volume of probable reserves of at least 407,500 m³ can be suggested. From this volume around 10% of the rock has the quality of Absolute Black.

Comparable dolerite quarries to those in Uruguay can be found in Sweden. At present there are seven quarries in Sweden where each of them produces an average of 8,000 m³ of stone per year (Swedish Geological Survey 2009). In comparison to Sweden, the production in Uruguay is very low. In the last several years the production ranged from 4,000 to 5,000 m³/year for the five active quarries in the country. This comparison indicates the production in Uruguay can be increased several times, provided that an adequate exploration strategy is followed.