Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial II, 747-752
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
3D predictive modelling using drill-hole geochemistry and gravity inversion. First approach using data from Neves Corvo mining area, Iberian Pyrite Belt Modelação 3D preditiva utilizando geoquímica de sondagens e inversão da gravimetria. Ensaio com dados da área mineira de Neves Corvo, Faixa Piritosa Ibérica M. J. Batista1*, P. Represas1, J. X. Matos1, C. Inverno1
Artigo Curto Short Article
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: Drill-hole rock geochemical analyses were compiled from previous Neves Corvo Iberian Pyrite Belt (IPB) exploration campaigns, having selected only those with minor interpolation distance and with weight % concentrations. Considering the different IPB geological formations the exploration drill-hole data show a variable distribution of the Baixo Alentejo Flysch Group metasedimentary rocks, the Volcano-Sedimentary Complex formations (VSC) and the Phyllite-Quartzite Group formations (PQ). Because different formations have different densities and the massive sulphides usually have higher density than the host VSC and PQ rocks, densities were inverted relative to a restricted area near the Neves Corvo mine. A 3D geochemical grid was built and a 3D density model was calculated by applying an inversion methodology to the superficial gravity data. The gravity dataset allowed a resolution grid of 100 m, given that this was the average distance between data points, and the geochemical data were interpolated to a resolution grid of 50 m. Also at this stage, several drill-hole geochemical data were discarded due to the distance of interpolation. Even though the geochemistry and density models were calculated using very different approaches, it is noteworthy that places with high Cu concentration are in general coincident with high density anomalies, showing a positive exploration indication. Therefore, the other density anomalies are worth being investigated in the Neves Corvo IPB sector. Keywords: D modelling, Gravity inversion, Geochemistry, Neves Corvo. Resumo: Com base em análises químicas de Cu (%) em várias sondagens efetuadas no sector de Neves Corvo da Faixa Piritosa Ibérica, foi efetuado um estudo de distribuição de teores e densidades litológicas. Na seleção das sondagens prevaleceu o critério de escolha daqueles que se encontravam mais próximos entre si, de modo a diminuir o erro de interpolação. Entre este grupo de sondagens foram escolhidas apenas aquelas onde o elemento Cu se encontrava indicado em percentagem. Os perfis das sondagens selecionadas mostram uma distribuição variável de Cu e das densidades. Estas sondagens atravessam as formações do Grupo do Flysch do Baixo Alentejo, do Complexo Vulcano-Sedimentar (CVS) e do Grupo Filito-Quartzítico (PQ). As mineralizações de sulfuretos apresentam densidades mais elevadas do que as respetivas rochas encaixantes (CVS e PQ). No tratamento dos dados de densidades de rochas foi selecionada uma área de levantamentos coincidente com o setor abrangido pelas sondagens consideradas no estudo geoquímico de Cu. Foram então interpolados modelos por krigagem 3D com malha de 100 m para as densidades e de 50 m para a litogeoquímica. O modelo 3D de densidade foi calculado através da aplicação de uma metodologia de inversão para os dados gravimétricos. Os modelos
3D de geoquímica e de densidade foram calculados utilizando abordagens muito diferentes, no entanto, pode observar-se que a anomalias de densidades elevadas correspondem grosso modo os setores com mais elevadas concentrações de cobre. Esta constatação é favorável à prospeção mineral. A metodologia utilizada sugere que outras anomalias de densidade elevada, observadas na região de Neves Corvo, devem merecer especial atenção e estudo de detalhe. Palavras-chave: Modelação Geoquímica, Neves Corvo.
3D,
Inversão
da
gravimetria,
1
Unidade de Recursos Minerais e Geofísica, Laboratório Nacional de Energia e Geologia, Apartado 7586, Bairro do Zambujal, 2610-999 Amadora. * Corresponding author / Autor correspondente:
[email protected]
1. Introduction Available exploration data from previous surveys is interpreted as a first approach before targeted exploration studies. Superficial data is not always sufficient as a first diagnosis when buried ore deposits are to be explored. Superficial geophysics can be used for subsurface exploration by modelling and inversion techniques, providing more information for exploration purposes (Phillips et al., 2001). Gravity data provides information related to transitions between geological formations of different density, or the presence of higher density ore deposits capable of producing anomalies in the gravitational field (Morgan, 2012). Its sensitivity to higher density mineralized bodies is the key to its well-known effectiveness in the Iberian Pyrite Belt exploration (Rocha Gomes & Silva, 1955). Density derived from wide area gravity surveys may enable the identification of large-scale tectonic structures, or reveal anomalies that exhibit high density associated with mineralisation. At a more detailed scale, core physical property measurements, such as core density, are used to constrain inversion results. The inclusion of data from a single drill-hole is shown to significantly enhance the detailed density distribution and produces models that correlate well with mineralization. Gravity inversion is a powerful generator of 3D density
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models that can be combined with other 3D models. One example of data to combine with 3D density models is drill-hole geochemistry (Gibson et al., 2007). Geochemical data provide direct information of grade and volume of mineralization, when it is present, and is an essential predictive tool to reveal the deposition conditions and to preview the volcanogenic massive sulphide host rocks. Chemostratigraphy is used to follow chemically defined geological units along the profile. In the exploration phase, companies sample for geochemistry purposes only a small percentage of the total drill-holes . Therefore, effective 3D geophysics gives a significant input to the predictive models of a target area. Moreover, physical properties data are more frequently obtained for the entire depth of the drill-hole, whereas geochemistry data are from specific layers of the geological profile which makes predictive models more difficult. Geological formations logging, location of ore deposits already known and even magnetic susceptibility also helps to provide effective 3D predictive models. Such studies were attempted in a first approach during PROMINE– “Nano-particle products from new mineral resources in Europe”, a research and technological development project co-funded by the European Commission’s Seventh Framework Programme within Theme 4: NMPNanosciences, Nanotechnologies, Materials and new Production Technologies. The objective of the present study is to check if drill-hole geochemistry from old exploration studies and inverted gravity data show coincident anomalies at depth. 2. Geology Regional geology and the location of the studied drillholes are depicted in figure 1. The Neves Corvo IPB region is characterized by one of the most important European mining centers related with the occurrence of the Neves Corvo giant deposit, represented by the massive sulphide lenses of Neves, Corvo, Graça, Zambujal, Lombador, Semblana and Monte Branco (Relvas et al., 2006, Oliveira et al., 2013b, Lundin Mining website 2013). A complex antiform is present at Neves Corvo, with a tectonic structure characterized by several thrusts imbricated with SW vergence, related with Variscan deformation. From base to top the local stratigraphy is represented by the Phyllite-Quartzite Group (Famennian to Strunian), with phyllites, siltstones and quartzites, the Volcano-Sedimentary Complex (VSC) (Strunian to upper Visean), with siliceous shales, green and purple shales, black shales, jaspers and several felsic volcanic rocks and the Baixo Alentejo Flysch Group Mértola Formation (upper Visean), characterized by shales and greywackes (Fig. 2). The VSC comprises a Lower Sequence of Strunian age and an Upper Sequence of upper Visean age (Oliveira et al., 2006, Pereira et al., 2008). The Neves Corvo mineralization is associated with the Neves Formation black shales, of Strunian age (Albouy et al., 1981; Pereira et al., 2008, Matos et al., 2011, Oliveira et al., 2013a).
3. Methods 3.1. Geochemical data and 3D modelling The geochemical information was compiled from drillhole rock geochemical analyses from previous exploration campaigns. Different methods of lithogeochemical analyses were used by different companies. Therefore it is necessary to find criteria to choose the appropriate information to combine them. In many of the explorations and especially in the reserves evaluation stage, only Cu, Pb, Zn were analysed. The information was selected using only drill-holes where Cu results were reported in weight %. A 3D grid was built and, also at this stage, several drillholes were discarded due to the distance of interpolation. The 3D voxel is composed by cubic cells whose values were determined using linear interpolation between the values of the eight voxel points which form the corners of the cube. The size of the edges of the cubes is 50 m to obtain a continuous image. 3.2. Gravity data and inversion As most of the area is covered by the thick flysch of the Mértola Fm., it is appropriate to go over the data information and location available relative to VSC/PQ underneath that formation. The exploration drill-hole data show a variable distribution of the VSC/PQ, controlled by Late Variscan faults and regional folds. Because different formations have different densities, and the massive sulphides usually have higher density than the host rock, densities at depth were investigated pertaining to a restricted area around Neves Corvo mine. These data were collected over several geophysical prospecting campaigns performed by LNEG (former SFM/IGM/INETI; e.g., Rocha Gomes & Silva, 1955) and a few private companies and consortiums. Later all was levelled and integrated in a single dataset. The distance between stations in this particular area allowed a grid spacing of 100 m. The dataset was gridded by application of the kriging method. A 3D density contrast model was calculated by applying an inversion methodology to the gravity data. A 3D density contrast model was estimated from the Bouguer anomaly field by means of a stabilized non-linear inversion methodology developed by Camacho et al. (2002). This inversion technique aims to determine the geometry of the sources of the observed gravity field, upon the adjustment of a three dimensional model of prismatic cells which adopt a priori values of density contrast (positive and negative). The algorithm looks for anomalous sources by a 3D aggregation of the parallelepiped cells, which are filled by prescribed positive or negative density contrasts. For each step of the growth process one cell changes its property. By the kth step, k cells have been filled with one of the prescribed density contrast values. For each density contrast essayed the residual of the models response is tested against the observed gravity anomaly (ϕd), weighted by a corresponding scale factor (λ) and conditioned by smoothness constraints (ϕm). The objective function to minimize is ϕ=ϕd+λϕm.The used algorithm requires that both a positive and negative a priory values for density contrast are chosen.
3D predictive modelling of Neves Corvo mining area data
Fig. 1. Regional geological map representing the Rosário-Neves Corvo PQ - VS structure and the Flysch sedimentary rocks, ad. Oliveira et al., 2013a. Fig. 1. Mapa geológico regional da estrutura PQ-VS de Rosário-Neves Corvo e metassedimentos do Flysch, ad. Oliveira et al., 2013a.
Fig. 2. Geological profile of the Neves Corvo area, ad. Relvas et al., 2006. Fig. 2. Perfil geológico da área de Neves Corvo, ad. Relvas et al., 2006.
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4. Results and discussion Copper concentrations were determined between 250 m and 800 m deep with the highest concentrations obtained in SF8 and SE20 drill-holes located in the Corvo area where Cu reached more than 25% Cu and also SK2(NS1) in Senhora da Graça area and SF16 in Corvo area where the higher concentrations in both drill-holes were over 20% (Fig. 3).
Fig. 3. Graphical representation of Cu concentrations and depth of sampling in the drill-holes. Fig. 3. Diagrama das concentrações de Cu e da profundidade das sondagens.
The exception was NN28 drill-hole located in Lombador, where mineralisation occurs deeper than in the other known orebodies; there, lithogeochemical samples were collected below 1200 m, and concentrations were below 5% Cu. The Bouguer anomaly map reflects the Neves Corvo geology showing the NW-SE trend of the Paleozoic structure and the near surface massive sulphide deposits of Neves, Corvo and Zambujal. Negative gravity gradients are present to north-eastward and southwestward representing an increase of the least dense rocks of the Mértola Formation, formed by shales and greywacke turbidites (Fig. 4). The density values of each formation were analysed and it was determined that the maximum density contrast between the ambient and the anomalous mass would be around 0.3 g/cm3, given that the host rocks present medium values of around 2,75–2,85 g/cm3 and the anomalous mass, rich in high density massive sulphides, shows values of ≥ 3,0 g/cm3. This estimate was confirmed by the analysis of the results obtained using this and other density contrast values. Due to the absence of significant minimums in the gravity field, and because the known geology gives no indications otherwise, it was assumed that there are no negative contrasts and the minimum value was set to −0,01 g/cm3 to meet the requirements of the inversion tool. This method has been applied in several situations with good results (e.g. Montesinos et al., 2003; Represas et al., 2012, 2013). The resulting model is presented in figure 5b using a voxel of the calculated density contrasts produced by 3D kriging. The resulting calculation observed in both 3D maps indicates a trend of Cu open in both sides in the geochemistry map and in the same area a gravity anomaly was also observed (Fig. 5). It is not discussed yet and not sufficiently understood if both anomalies can be related but the location coincidence needs further investigation.
Fig. 4. Bouguer anomaly map of the Neves Corvo area. Fig. 4. Mapa da anomalia de Bouguer da área de Neves Corvo.
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Fig. 5. Geochemical and gravity inversion 3D interpolation maps. Fig. 5. Mapas geoquímico de Cu e gravimétrico de interpolação 3D.
5. Conclusions Even though the geochemistry and density models were calculated using very different approaches, it can be stated that the places where high density and high Cu concentration coincide, are places where the predictability values are considered to be the highest. This corresponds to one of the known massive sulphide orebodies of Neves Corvo mining site. Therefore, the other anomalies, observed further away are worth being investigated in the Neves Corvo mine restricted area. Furthermore, it is relevant to consider that gravity inversion modelling of exploration data is to be applied in the remaining areas where this kind of information is available. Acknowledgments The work was developed within the project FP7-NMP2008-LARGE-2- PROMINE Nano-particle products from new mineral resources in Europe (2009-2013). The authors thank Zélia Pereira, João Carvalho and Tomás Oliveira (LNEG), Carlos Rosa (EDM) and Lundin Mining/Somincor geologists for the discussion of the Neves Corvo model. References Albouy, L., Conde, L.N., Fogliierini, F., Leca, X., Morikis, A., 1981. Le gisement de sulfures massifs polymétalliques de Neves-Corvo (Baixo Alentejo, Sud Portugal). Chronique Recherche Minière, 460, 5-27. Camacho, A.G., Montesinos, F.G., Vieira, R., 2002. A 3-D gravity inversion tool based on exploration of model possibilities. Computers & Geosciences, 28, 191–204. Gibson, H.L., Allen, R.L., Riverin, G., Lane, T.E., 2007. The VMS Model: Advances and Application to Exploration Targeting. In: B. Milkereit, (Ed.). Proceedings of Exploration 07: Fifth Decennial International Conference on Mineral Exploration, 713-730.
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