Spatial analysis of geological structures favorability for the occurrence of Sn and W mineral deposits in Bejanca Mine (Viseu - Portugal)

Spatial analysis of geological structures favorability for the occurrence of Sn and W mineral deposits in Bejanca Mine (Viseu – Portugal) Lima, A. & Santos, P. [email protected] Centro de Geologia da Universidade do Porto Abstract The geographic information systems (GIS) are very useful tools for compilation and management of information of different types and sources. These may be used to produce potential or predictivity maps for the definition of areas of high potential for occurrence, but that are not yet known deposits. The integration of different data in the same referential may allow a better understanding of the parameters that control the metallogeny of the region. In order to better understand the structural influence in Bejanca Tin and Tungsten Ore Deposit, we chose to evaluate the spatial correlation between the known occurrences of tin and tungsten and fracturing. These correlations can be very useful in the future development of a predictability map for tin and tungsten deposits. The maximum distance (d max) used in the study was 250 meters. This was determined graphically on a cumulative frequency histogram of mineral deposits on the distance to the closest faults (Fig.5). In this diagram it was found that about 88% of ore occurrences are located within 250 meters of a fault, so this was the area of influence considered in our study. Note that the same deposit can be included in the area of influence of various faults . 0 10 20 30 40 50 60 70 80 90 100 250 500 750 100012501500 175020002250 250027503000 32503500 Distance Spatial Relations All Points (%) Deposits (%) Fig 5.- Spatial Relation of distance between mineral depósits and faults. Fig 4.- Diagram that ilustrates assignement criteria for each fault. Fig 6.- Most common fault directions Introduction Bejanca Mine is located in the district of Viseu, more specifically in Queirã, Vouzela. This Mine begun cassiterite and wolframite exploitation in 1917. This region consists of greisenized masses included by porphyritic two mica granite, with dominant biotite. Some quartz veins cut the greisen in different directions. This granite contains tourmaline and is traversed by small greisenized veins, oriented NW-SE. Faults are often filled by quartz, cassiterite, wolframite and clay. The mine area is crossed by a fault system N30 ⁰ W, there is a fault system oriented N20 ⁰ whose box failure is filled with clay, as well as other well-marked direction N55 ⁰ E and N70 ⁰ E (Cotelo Neiva, 1944). The numerous Sn-W mineralizations located in the study area are arranged roughly in the peripheral borders of granitic batholith (Viseu). It is envisaged that the transfer may have occurred from the region adjacent to batholith and was thermally induced by the granitic intrusion. Matter would be obtained from the leaching of the moscovitic-biotitic granites and eventual primary Sn-W mineralization that they contained. The concentration of tin and tungsten occurrences are bounded laterally by two faults on the west by the Ribama Fault approximate N-S direction and by a fault of this approximate direction NW-SE. These mineralizations are agglomerated in a region of dense fracturing. Regional fracturation is organized in families arranged mainly according to the following directions: N4560° and N300-310°, corresponding to the most common directions, followed organized by frequency N20-35°; N335-355° and N80-90° (Ferreira et al, 2010). The highest concentration of mineralization is located on NW of the Geological Map 17 A – Viseu (Fig. 3), in a region composed by the Abraveses granite. It allows us to suspect of a strong structural control, since this tin and tungsten occurrences concentration are laterally bounded by two faults. Note that the same deposit can be included in the area of influence of various faults . Values were assigned to failure criteria according to its direction, using class intervals of 10 ⁰, ranging from 0 ⁰ to 180 ⁰. The weight was assigned (ni) of failures of category i was calculated from the favorability of the line that is the sum of weights of each fault belonging to each category. The calculated favorability FAVI it is the relative index weight quotient (Pri=weight of each fault category, ni, over the total weight of all the categories, Ntot), weighted by the relative length (Lri =length of family i, or Length, over the total length of all the faults in the area studied, Length tot), (Eqs. (1), (2), (3), and Table 1). Classes Faults N. Length Mean Lri=Lengthi/ Lengthtotal% ni Pri=ni/Ntotal% Favi Favorability 0⁰-10⁰ 170 71106 418,2706 5,699633042 31 31,63265306 0,744289 0,744289 10⁰-20⁰ 197 86266 437,8985 6,914810902 10 10,20408163 0,168994 0,168994 20⁰-30⁰ 271 121348 447,7786 9,726873546 11 11,2244898 0,062193 0,062193 30⁰-40⁰ 299 127647 426,913 10,23178155 4 4,081632653 -0,39912 0 40⁰-50⁰ 280 110091 393,1821 8,824547875 6 6,12244898 -0,15877 0 50⁰-60⁰ 186 81762 439,5806 6,553784445 23 23,46938776 0,55401 0,55401 60⁰-70⁰ 220 108364 492,5636 8,686116994 12 12,24489796 0,149129 0,149129 70⁰-80⁰ 144 60367 419,2153 4,83882862 4 4,081632653 -0,07391 0 80⁰-90⁰ 156 65875 422,2756 5,280332555 9 9,183673469 0,240355 0,240355 90⁰-100⁰ 161 60590 376,3354 4,856703598 16 16,32653061 0,526552 0,526552 100⁰-110⁰ 66 23745 359,7727 1,903324425 13 13,26530612 0,843204 0,843204 110⁰-120⁰ 106 42688 402,717 3,421735652 10 10,20408163 0,474527 0,474527 120⁰-130⁰ 196 77446 395,1327 6,207827477 5 5,102040816 -0,0852 0 130⁰-140⁰ 65 25681 395,0923 2,058508089 17 17,34693878 0,92567 0,92567 140⁰-150⁰ 49 18238 372,2041 1,461900647 21 21,42857143 1,166075 1,166075 150⁰-160⁰ 95 30546 321,5368 2,448471168 18 18,36734694 0,875151 0,875151 160⁰-170⁰ 151 57376 379,9735 4,599079479 10 10,20408163 0,346103 0,346103 170⁰-180⁰ 163 78418 481,092 6,285739936 19 19,3877551 0,489171 0,489171 Total 2975 1247554 419 , 3459 100 98 100 ----------- ----------- 0 0,2 0,4 0,6 0,8 1 1,2 1,4 Favorability for Each Fault Class Results Fig 1.- Ancient Bejanca Mine. Fig 2.- Actual areal image view were we can identify the lake (resulting from the open pit), the mine tailings and the dense vegetation coverage surrounding the mine.. The favorable values for mineral deposits occurrence for each Fault Class are summarized in the graph (Fig.7) as well as in the table1. Legend falhas Alluvium Sandy-clay arkosic deposits Fluvial Terraces Ü Methodology Conclusion Delfim de Carvalho (1977), attempted to correlate spatial distribution of endogenic deposits known in Portugal with the main lineaments patterns. He found that a large part of the deposits are located on or near lineaments intersections, suggesting structural control as a factor in their location. For the geological structures spatial analysis, we used the algebraic method (Knox-Robinson and Groves, 1997) to quantify the criteria associated with different fault classes. Our set of data points correspond to deposits as well as linear data correspond to fault structures, identified in the geological map of Viseu 17A at scale 1/50000 (LNEG, Portuguese Geological Survey). A value of 1 is assigned to deposits (points), regardless of their size. Placing a value on the line consists of looking for deposits located inside a band parallel to the line, so all deposits within a certain maximum distance (dmax) of a fault was assigned to the line (Fig.4). The spatial relations analysis between the fault and the known occurrences of tin and tungsten in the region revealed that, despite the most common fractures being N45⁰-60 ° and N300⁰-310⁰, followed ordered in terms of frequency, N20 ⁰ -35 °; N335⁰-355⁰ and N80 °-90 °, the fractures more favorable for Sn and W deposit occurrence are those having N130⁰-160⁰, N100⁰-110⁰ and N0⁰-10⁰ directions. The results obtained by spatial analyses can result in great importance for future deposits unknown occurrences predictive analysis. References Total 2975 1247554 419 , 3459 100 98 100 ----------- ----------- DELFIM DE CARVALHO, (1977). Lineament Patterns and Hypogene Mineralization in Portugal. Estudos, Notas e Trabalhos, Vol. XXIII, fasc. 3-4, p. 91-106. FERREIRA, N., CASTRO P., GODINHO M. M., NEVES L., PEREIRA A., SEQUEIRA A., MEIRELES C., BENTO DOS SANTOS T. (2010). Carta Geológica de Portugal, folha nº 17-A Viseu, LNEG – Laboratório de Geologia e Minas , Lisboa KNOX-ROBINSON, C. M., GROVES, D. I., (1997). Gold prospectivity mapping using Geographic Information System (GIS) with example from Yilgarn block of Western Australia. Chronique de la recherche minière 529, 127-138. NEIVA, J. M. C. (1944). Jazigos portugueses de cassiterite e volframite. Com. Serv. Geol. Portugal, T. XXV, 251 pág. Fig 3.- Geological context of Bejanca Mine (Extract of Geological Map of Viseu 17-A at scale 1/50000 ). Table 1.- Favorability assignment. Fig 7.- Favorability estimated for each fault class. Fluvial Terraces Arkosic sandstones and clayey depósits - Beira Alta Group Nelas Unit: metapelites e metapsammites, with metagreywackes and metaquartzwackes intercalations Casinha Derrubada Unit: micashists with clorite, biotite, biotite and garnet, staurolite and sillimanite Ponte Chinchela Formation - Superior Member : metapelites with limestone and calc-silicated rocks Ponte Chinchela Formation - Inferior Member: metapelites and metagreywackes Rosmaninhal Formation: micashists,phyllitites and metapsamites with metagreywackes Beiras Group: micashists, metagreywackes and migmatites near granites Monzogranite moscovitic-biotitic, medium grained, with porphyritic tendency (Granito de Alto de Vila Garcia) Monzogranite biotitic-moscovitic, porphyritic, medium grained (Granito de Nesperide) Monzogranite biotitic-moscovitic, medium grained with porphyritic tendency (Granito de Fagilde) Monzogranite biotitic, porphyritc, medium to coarse grained (Granito de Mangualde) Monzogranite biotitic-moscovitic, porphyritic, medium grained (Granito de Alcafache) Monzogranite biotitic-moscovitic, porphyritic, medium grained (Granito de Alcafache);"stocks" de grão fino Monzogranite biotitic - moscovitic, porphyritic, very coarse grained (Granito de Farminhão e S. João de Lourosa) Monzogranite biotitic - moscovitic, porphyritic, very coarse grained, stocks" with very thin grain (Granito de Guimarães) Rocks with basic to intermediate composition Moscovitic granite, with rare biotite,medium to thin grained, with globular quartz (Granito de Salgueiral e "stocks" dispersos) Granite moscovitic-biotitic medium grained, with tourmaline (Granito de Abraveses) Moscovitic granite with deformation (Granito de Vila Pouca) Granite thin grained with globular quartz (Granito de Portela) Sr.ª do Castro: granite biotitic-moscovitic, porphyritic, medium grained (Granito de S. Martinho) Moscovitic granite medium to thin grained Granite thin grained (Granito da Sr.ª do Castro) Granodiorite biotitic, porphyritic, medium grained (Granito de Fráguas) Granite thin to medium grained, biotitic-moscovitc (Granito de Folgosa) Granite moscovitic-biotitic, coarse grained with porphyritic tendency (Granito de Vilar) Two mica granite, porphyritic, coarse grained (Granito de Silvares) Two mica granite, porphyritic, coarse grained with globular quartz (Granito de Eira dos Picos) Two mica granite, medium to coarse grained, with porphyritic tendency (Granito de Vouzela) Granite biotitic-moscovitic thin to medium grained, with porphyritic tendency (Granito de Fataunços) Granite biotitic-moscovitic medium grained, with porphyritic tendency (Granito de S. Pedro do Sul) Granite biotitic-moscovitic, medium grained, with silimanite (Granito de Cavernães) Diorites and amphybolitic granodiorites (Granodiorito de Fragueira) Quartz veins and masses Aplite veins and masses Microgranites veins and masses Pegmatite veins and masses Basic rocks amphybolitic veins and masses Metadioritic amphybolitic veins and masses 0 5.0002.500 Meters Ü