Dimensional stones in Uruguay: situation and perspectives

Coloured granites (pink and red) 

Pan de Azúcar pluton and associated syenite deposits:

The Pan de Azúcar Pluton, associated with the Sierra de las Animas Complex, is a circular intrusion with a diameter of approximately 5 km including syenites, quartz syenites, and granites. The shape of the intrusion and the association with volcanics of similar age suggest a ring-complex structure for both the pluton and the nearby intrusions. A recent Ar/Ar amphibole age indicates late Neoproterozoic age for this magmatism (Oyhantçabal et al. 2006).

The lithologies used as dimensional stones are syenites and granites. Syenites are present in the marginal zone of the Pan de Azúcar intrusion. They are grey to pink coloured and very coarse-grained. Perthitic orthoclase up to 4 cm and interstitial amphibole, pyroxene and biotite are the main minerals. Accessory minerals include apatite and zircon. Quartz syenites crop out in the central part of the pluton. They are medium-grained, colours ranging from grey to brownish-pink to red. The mineral composition is similar to that of the syenites, but a small amount of interstitial quartz is present.

As is typical for alkaline granitoids, the postorogenic setting is connected with fewer tectonic overprint and alkali feldspar exhibits a wide palette of colours in the different intrusions (grey, deep red, brownish red, etc). These facts together with the accessibility of the area determines that since the beginning of the 20th century this district constituted the main area of Uruguay producing a variety of colours and designs of dimensional stones. The most important commercial varieties are: Guazubirá (medium-grained and reddish pink), Artigas (coarse-grained and mixed grey and pink colour), Bodega (Medium grained and deep red), Sara Pink (fine grained and pink), etc. The average production for the period 1997 – 2005 was 1502 Ton with an exceptional maximum in 2000 of 3800 Ton (these data include the production of some small quarries of coloured granites from other parts of the country). The major market for these granitoids is regional (cities of Buenos Aires and Montevideo) and in the last decades also Japan. The heterogeneity of colour and inadequate market strategies were the main problems that led to the closure of most of the quarries. In spite of the present situation it is considered that the potential is good, being a prerequisite the evaluation of resources, cost/price ratios and market possibilities.

Black dolerite 

A significant black stone producing province is associated with a Paleoproterozoic dolerite dyke swarm intruded in the Piedra Alta Terrane of the Río de la Plata Craton (Fig. 1). The swarm extends for near 160 km trending ca. 060º, the length of the individual dykes reaches some kilometres and the width ranges between 10 and 50 m. The composition is tholeitic and two subprovinces with high and low Ti respectively have been recognised (Bossi & Campal, 1991; Bossi et al. 1993). The dolerites are composed of plagioclase, clinopyroxene an opaque minerals and the texture is subophitic. Biotite, apatite and interstitial intergrowths of quartz and alkali feldspar (micropegmatite) are the main accessory phases. Rims of uralitic amphibole and chlorite are scarce.

Two main varieties of dolerite are mined: one is deep black and fine grained (Negro absoluto) while the other is dark grey-black and medium grained (Negro oriental). The first is more appreciated in the market but blocks of commercial dimensions are not common. Colour inhomogeneities are seldom observed and are associated to pockets of white coloured quartz –alkali feldspar intergrowths. The main problem for the production of commercial blocks is the abundance of joints that determines low extraction yields (<10%). The production shows strong oscillations and ranges between 919 and 3797 Tons/year for the period 1997-2005.

This black stone has a great demand in the international market with prices over 1000 US$/m3. Good quality and the world demand for deep black stones creates a high potential for these rocks although research is needed for unsolved problems in exploration and mining.

Marble 

Marble occurrences in Uruguay are related with the Neoproterozoic low- to medium- grade metamorphic supracrustal successions of the Dom Feliciano Belt. In some cases the deposits are related with aureoles of contact metamorphism around granitic intrusions.

The main production provinces are:

The Polanco area: white and coarse-grained marbles related to the metamorphic aureole of the Polanco Granite. Aggregates of radial tremolite are common as contact metamorphism minerals in these marbles. Reserves are small because most of the deposits correspond to roof-pendants in the intrusion. Jointing is additionally important and therefore the blocks are small-sized. Coarse-grain, small size of the blocks and small volume of reserves make these marbles interesting only for internal markets. Most of the quarries are at the moment closed.

The Minas - Pan de Azúcar area: varicoloured marbles (white, grey, red, green and black) were exploited in small quarries using inadequate technologies and excess of explosives. Most of the quarries are presently closed. This province has a potential for exploration of valuable marble varieties due to the interesting palette of colours and designs.

Mining of marbles began in Uruguay in the first decades of the 20th century. The market was internal and regionally the expanding city of Buenos Aires (Argentina). The variability of colours together with the jointing associated to tectonics and to erroneous extraction techniques besides unfavourable cost/price ratios and poor marketing strategies led to a continuous decay in the production since 1990 (2900 Ton) and ruined most of the quarries (39 Ton in 2005). For the internal market the production must also compete with imported marbles from Argentina, Brazil and even Spain. Available Geotechnical parameters are presented in Table 2.

Phyllites 

These building stones are associated with the Neoproterozoic thrust and fold Schist Belt of the Dom Feliciano Belt. The mining province is located between the localities of Pan de Azúcar and Minas (Fig. 1). The quarries are located in the limbs of a folded calcsilicate sequence, where bedding and foliations are parallel.

The main lithotype is a fine banded calcareous phyllite. The mineralogy is sericite, chlorite, calcite and quartz and the texture is lepidoblastic to grano– lepidoblastic. The palette of colours is quite varied: light green, grey, dark grey, reddish and black. The mined stone is splitted in slabs 0.5-2 cm thick and manufactured in two ways: irregular slabs and dimensional floor tiles. Geotechnical parameters are presented in Table 2. The average production the period 1997-2005 is around 3991 Ton. A historical maximum of 13000 Ton is observed in 1993.

Potential resources are high being the variations in demand of the regional market the main limitation and the cause of several flourishing and decays cycles in the activity. Exploration for colours and qualities is a prerequisite for the development of the sector.

Sandstones 

Mildly lithified sandstones of the Tacuarembó formation (Triassic, Northern Basin, see location in Fig. 1) have been mined discontinuously. These quartzose medium-grained sandstones are light reddish to grey coloured and have parallel bedding. The sandstones were mined during the first decades of the 20th century only for the internal market. After several decades of inactivity a small production started a few years ago (774 Ton in 2004 and 1014 Ton in 2005). Geotechnical parameters are not available.