Fábio Marques (1), João Xavier Matos (1), João Carvalho (2), Pedro Sousa (1), Maria João Batista (2), Patrícia

Represas (2), Pedro Gonçalves (1), Igor Morais (1), Luís Albardeiro (1), Elsa Ramalho (2), Vitor Araújo (3) and Nelson Pacheco (3)(1) Laboratório Nacional de Energia e Geologia, Apartado 14, 7600-909 Aljustrel, Portugal, (2) Laboratório Nacional Energia e Geologia, Apartado 7586, 2721-866 Amadora, Portugal, (3)

Somincor/Lundin Mining, Stª Bárbara dos Padrões, 7780-409 Castro Verde, Portugal, fabio.marques@lneg.pt

1 Geological setting

2 Gravity data

3 Magnetic data

4 Radiometric data 5 Conclusions

1.

6 References

Figure 1 – Geological map of the Rosário - Neves Corvo antiform(Oliveira et al. 2013).

The Neves-Corvo mine is located in the southeast region of the geologicalstructure Rosário - Neves-Corvo (fig. 1), characterized by a complex antiform,conditioned by the presence of thrust faults well visible in drilling holes andmine galleries (Oliveira et al., 2013). The regional geology is characterized bythree major stratigraphic units, from base to top, Phyllite-Quartzite Formation(PQ), Volcano-Sedimentary Complex (VSC) and Mértola Formation (Mt), thelower unit of the Baixo Alentejo Flysch Group. PQ is late Devonian age andconsists of quartz sandstones, shales and rare conglomerates deposited in apossible shallow water environment (West & Penney, 2017). VSC is lateDevonian to early Carboniferous age and locally at Neves Corvo it is up to 700m thick (Oliveira et al., 2013), includes two sequences: Lower VSC - from baseto top, mafic volcanic rocks, felsic volcanic rocks and dark shales and UpperVSC - composed of the Grandaços Formation (dark shales), the “Borra deVinho” Formation (purple shales), the Godinho Formation (siliceous shalesand felsic volcaniclastic rocks), and the Brancanes Formation (dark shales andthinbedded greywackes). The massive sulphide orebodies are associated to aLower VSC black shale unit, called Neves Formation (Leca et al. 1983, Oliveiraet al. 2013). The Neves-Corvo includes in the top the Mértola Formation, athick turbidite sequence composed by greywackes and shales from upperVisean age.

LNEG gravimetric land database was leveled in the 90's (Represaset al., 2016) and presented high-frequency noise and someleveling artifacts were anticipated. Therefore, each survey wascarefully revisited and its contours were plotted over the Bougueranomaly map and possible artifacts were identified by comparisonwith the original individual maps. After data verification, severalinterpolations of all data points were attempted in order toproduce a better quality Bouguer anomaly map. Afterwards, a 2nd

degree polynomial regional anomaly was extracted using theleast-squares approach (Dobrin and Savit, 1988). In order to studythe deep structure and identify the major structural lineaments,the regional field was analyzed and derivative maps wereproduced from the Bouguer anomaly map (residual, 1st and 2nd

degrees vertical derivatives, horizontal derivatives and upwardand downward continuations of the field).

Figure 2 – Residual anomaly map of the Neves-Corvo region.

Figure 3 – Gravity modeling of the geological cross-section through the Corvo andGraça deposits with an adjustment error of 0.069 mGal.

Figure 4 – Magnetic field reduced to pole map, of the Neves-Corvo region. Figure 6 – 3D model of apparent magnetic susceptibilities , calculated by Hilbert solution.

Formation Minimum Maximum Average Stdev.p nMértola 2.02 2.86 2.72 0.13 1858Brancanes 2.57 2.99 2.76 0.06 231Godinho 2.39 2.83 2.71 0.05 477Borra de Vinho 2.4 2.85 2.75 0.06 178Grandaços 2.58 3.01 2.76 0.05 799Graça 2.72 2.8 2.77 0.03 11Felsic volcanic rocks 2.57 3.11 2.74 0.06 1970Neves 2.68 2.87 2.8 0.03 63Mafic volcanic rocks 2.61 2.93 2.8 0.05 134Phyllite-Quartzite 2.56 3.22 2.79 0.07 428Massive sulphide (Corvo) 2.4 5.6 4.53 0.28 13288Massive sulphide (Graça) 2.77 5.43 4.49 0.27 4178Massive sulphide (Lombador) 2.47 5.3 4.59 0.28 4260Massive sulphide (Neves) 2.66 5.41 4.54 0.29 15340Massive sulphide (Zambujal) 2.5 5.39 4.64 0.25 5800Stockwork (felsic volcanic rocks) 2 4.97 3.07 0.39 59200Stockwork (shales) 2.1 4.89 3.08 0.36 33900

Figure 7 – Radiometric map (total count), of the Neves-Corvo region.

Table 1 – Densities of the Neves - Corvo region, obtained from drill-holes .

In the construction of the magnetic anomaly map, after the subtraction of theIGRF, the reduction to pole (RTP), filter was applied (fig. 4) and the 1st degreevertical derivative was calculated based on the values of the total magneticfield (Roest et al., 1992, Represas et al., 2016). The Neves-Corvo magnetic mapshows NW-SE trends related with VSC basic rocks present mostly in the upperVSC sequence. Over the deposit there is a spotty anomaly reflecting the mineore plant.

A 3D model of apparent magnetic susceptibilities was constructed by inversion (fig. 6),through depth to basement technique. The position, dip and susceptibility of thebodies with a magnetic source were determined using the Hilbert solution (Nabighian1972). Several NW-SE lineament can be correlated with the VSC structures define bybasic volcanic rocks. The method assumes only the induced magnetization and the factthat in areas with negative anomalies appear bodies with magnetic susceptibility canbe interpreted as remnant magnetization.

In conclusion, the gravity, magnetic and radiometric maps highlight the NE-SW Variscan trendof alternating synform and antiform strcutures aligned by NW-SE oriented thrust faults andnappes that resulted from Variscan compression and were later affected by tardi-variscan NE-SW strike-slip faults. From the point of view of mining prospecting, the information extractedfrom the different charts and models made it possible to identify some gravimetric targets inseveral sectors, in the continuity and proximity of Neves-Corvo mine. Characterizing theNeves-Corvo gravity anomaly, a proven target with confirmed existence of mineralizations,this is a large anomaly with an amplitude of 2 mgal, observed in the residual gravity field andis the result of the densities contrast from geological structures, particularly of the orebodiesand the antiform Rosário - Neves-Corvo (Fig. 2). It is also visible a disturbance in the gravityfield over the mineral masses Semblana and Monte Branco.

The map of the natural gamma radiation (total count), showing that chemical elements with radioactive isotopes reflect mostly rock and hydrothermalalteration in the Neves-Corvo NW-SE structure (fig. 7). This is mostly the result of potassium gamma ray and thorium and uranium equivalent gammaray measurements reflecting the VSC felsic volcanic rocks with high presence of hydrothermal sericite mica. Natural gamma ray also allow a cleardistinction between Mértola and Mira formations, units of the Baixo Alentejo Flysch Group and shale rich units in the Mértola Formation. The airbornespectrometer survey was carried out at low altitude and with high along line sampling (Batista et al., 2016), though the radiometric chart showed non-geological effects caused by long wavelength noise along the survey lines. This leveling problem was eliminated by applying a decorrugation filter inthe Fourier transform domain. In selected sectors, a series of 2D lines were constructed (fig. 8), radiometric, magnetic and gravimetric, in order toshow the values of a given "grid“, as well as 2D gravimetric sections, in order to show areas of higher density.

Batista, M.J., Torres, L., Leote, J., Prazeres, C., Saraiva, J., Carvalho, J., Matos, J.X., 2016. Carta Radiométrica (Contagem Total) da Zona SulPortuguesa, Faixa Piritosa Ibérica, esc. 1/400 000. Edição LNEG/URMG, Lisboa, ISBN: 978-989-675-045-9.Dobrin, M.B. and Savit, C.H. [1988]. Introduction to Geophysical Prospecting. 4th Edition, McGraw-Hill, New York, 867.Nabighian, M. N. [1972]. The Analytic Signal of Two-Dimensional Magnetic Bodies with Polygonal Cross-Section: Its Properties and Use forAutomated Anomaly Interpretation: Geophysics, 37, (3), 507-517.Oliveira, J.T., Rosa, C., Rosa, D., Pereira, Z., Matos, J.X., Inverno, C., Andersen, T. [2013]. Geology of the Neves-Corvo antiform, IberianPyrite Belt, Portugal: New insights from physical volcanology, palynostratigraphy and isotope geochronology studies. Mineralium Deposita48, 749-766.Oliveira J., Pereira Z., Carvalho P., Pacheco N., Korn D. [2004]. Stratigraphy of the tectonically imbricated lithological succession of theNeves-Corvo mine area, IPB, Portugal. Miner Deposita, 39, 422-436.Pereira, Z. [2017]. Nota técnica sobre o subandar Estruniano. Notas do LNEG.Represas, P., Sousa, P., Carvalho, J., Torres, L., Matos, J.X., 2016. Carta Magnética (Campo Total Reduzido do IGRF) da Zona Sul Portuguesa,Faixa Piritosa Ibérica, esc. 1/400 000. Edição LNEG/URMG, Lisboa, ISBN: 978-989-675-047-3.Represas, P., Sousa, P., Torres, L., Bengala, M., Matos, J.X., 2016. Carta Gravimétrica (anomalia de Bouguer, densidade 2,6) da Zona SulPortuguesa, Faixa Piritosa Ibérica, esc. 1/400 000. Edição LNEG/URMG, Lisboa, ISBN:978-989-675-046-6.West, D. and Penney, M. [2017]. Brownfields and Beyond - Undercover at Neves Corvo, Portugal. Proceedings of Exploration 17, 291-304.

The analytical signal (fig. 5), was also calculated based on the values of the totalmagnetic field (Roest et al., 1992). The analytic signal is the square root of thesum of the squares of the derivatives in the x, y, and z directions. This map isuseful in locating the edges of magnetic source bodies, particularly whereremanence and/or low magnetic latitude complicates interpretation.

Figure 5 – Analytic signal map, of the Neves-Corvo region.

Figure 8 – 2D radiometric, magnetic and gravimetric lines on the surfaceof the Semblana massive sulphide orebodie.