Cartography of Industrial Qualities of Ornamental Rocks in the optimisation of granite exploitations

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The solid rock of O Porriño is located in the SO part of the province of Pontevedra (Galicia) and with a subcircular morphology it extends some 25 km in the N-S direction with width varying from 15 km in the north to 10 km to the south.

The variety Rosa Porriño is extracted in the concession of the exploitation Benedicta (nº 1544) located in the northwest part of the rock occupying a surface of 6.8 square km. The exploitations (39 quarries) are concentrated in an area of about 2 square km in the eastern part of the solid mass.

Of the Rosa Porriño layer, granite is extracted in blocks, stone for construction, and arid mainly for cement. The ornamental rock is destined both for the local market as well as exports, being commercialised in a large number of countries (emblematic works realised with this granite are, among others, the Tokyo City Hall and the headquarters of the European Parliament in Brussels).

The company, Pocasa-Porriñesa de Canteiras S.A., is the owner of the concession for exploitation of "Benedicta" located in the municipality of O Porriño and in which several operators are grouped together. With the aim of realising a project for the renovation of the concession of the exploitation, the above named company ordered a geological-mineral based study for calculating the reserves and planning the use of mining resources in the current area of exploitation, as well as a general cartography of the entire concession.

The extent of the cartography of industrial qualities

There is no doubt that once identified the deposit of commercial interest as is the case of some natural stones, it is convenient to design its exploitation in such a way that the output in economic terms be the highest possible within the range of sustainability, both from the socio-economical as well as environmental perspective.

To deal with the task of designing and planning the exploitation of a deposit with the aim of optimising and to be profitable in the use of the mineral resource, it is necessary to have geological-mineral information on a scale adequate to the dimensions of the deposit, its useful substance, and its potential destination. This information ought to be captured in a paper or digital support in a way that it can be used with different criteria, and allows not only the calculation of reserves in a useful way, but also the election of more adequate areas to establish both the temporary as well as fixed installations, as also for the design of the pathways and the gaps in exploitations, the direction of the advance, etc.

The working method to be used and the level of detail one aims at arriving, has much to do with the previously available information and the observation conditions. In each specific case, one has to make the best adaptation possible of the general methodology of investigation of granites made to date by IGME during the development of several works, realised with the aim of understanding the potential of natural stones in Spain.

The previous works of IGME were bereft of necessary details for directly applying the criteria of making cubic reserves. Neither did the geological-mineral information collected in the work of cubication, provide the necessary details for the estimation of reserves adequate to the current interests of the company. That is why IGME decided to do a lithological and detailed structural cartography, and a cartography of industrial qualities, which is what was considered adequate for the objectives of the project, working for it at a scale of 1:1000.

For the entire concession a geological cartography at a scale of 1:10000 was done. Given the existence of practically continuous reaching till the surface in the area of current exploitation, due to the intense actual exploitation, a detailed observation was done of the fronts and working platforms. The results are represented on a geological map, in which the existing lithologies stand out on a scale of 1:3500, the fractures, the veins, some significant changes in colour in the common fronts of granite, as well as some areas with greater presence and size of enclaves. Special attention was paid in the establishment of the limit that separates the front Rosa Porriño porphyric equigranular to the inequigranular, from the porphyric front Rosa Porriño porphyric, which the quarriers assimilate, to the commercial variety Rosa Dante. Also the areas of sand-alteration of granite were marked out, as well as the areas covered by the deposits of Quaternary.