Livro de resumos
Editor: LNEG - Laboratório Nacional de Energia e Geologia
Título: Livro de Resumos do X Congresso Ibérico de Geoquímica/XVIII Semana de Geoquímica
Autor: Vários
Suporte: Multimédia ISBN: 978-989-675-039-8
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Evaluation of the mining potential of the São Domingos mine wastes,
Iberian Pyrite Belt
Avaliação do potencial mineiro das escombreiras da mina de São
Domingos, Faixa Piritosa Ibérica
Vieira, A.1*, Matos, J. X.2, Lopes, L.1, Martins, R.1
1
Dep. Geociências, Universidade de Évora, R. Romão Ramalho 59, 7000.671 Évora, Portugal* alexanderv@live.com.pt
2
Laboratório Nacional de Energia e Geologia, I.P., R. Frei Amador Arrais 39rc. Ap. 104 - 7801.902 Beja, Portugal
Abstract
Located in the northern sector of the Iberian Pyrite Belt (IPB), near the border with Spain, the outcropping São Domingos deposit was mined since Roman times. Between 1854 and 1966 the Mason & Barry Company developed open pit excavation and underground mining up to depths of 120 m 420 m, respectively. The São Domingos subvertical deposit is associated with felsic volcanics and black shales of the IPB Volcano-Sedimentary Complex and is represented by massive sulphide and stockwork ore (py, cpy, sph, ga, tt, aspy) and related supergene enrichment ore (hematite gossan and covellite/chalcocite). Different mine waste classes were mapped around the old open pit: gossan (W1), felsic volcanic and shales (W2), shales (W3) and mine landfill (W4). Considering the LNEG database (CONASA mining waste characterization based in 162 shafts and 160 reverse circulation drillholes), new mine wastes resources are presented. Using block modelling (SURPAC software) a total mineral resource of 2.38 Mt @ 0.77 g/t Au and 8.26 g/t Ag is considered in non-conditioned volumes of waste. Considering all evaluated wastes, including urban areas, a resource of 4.0 Mt @ 0.64 g/t Au and 7.30 g/t Ag is presented, corresponding to a metal content of 82,878 oz/t Au and 955,753 oz/t Ag.
Keywords: São Domingos mine, mining wastes resources, Iberian Pyrite Belt
Resumo
Situado no setor setentrional da Faixa Piritosa Ibérica (FPI), o jazigo de São Domingos foi explorado na época Romana, e, entre 1854 e 1966, pela Mason & Barry, tendo sido abertas uma c orta e galerias, respetivamente até 120m e 420m de profundidade. O jazigo é formado por uma massa subvertical associada a rochas vulcânicas félsicas e xistos negros do Complexo Vulcano-Sedimentar da FPI, sendo a mineralização primária representada por pirite, calcopirite, esfalerite, galena, sulfossais e arsenopirite e a mineralização secundária por hematite, limonite, covelite e calcocite. Apresentam-se novas estimativas de reservas para os resíduos mineiros, baseadas na modelação 3D de blocos (software Surpac) a partir de dados do LNEG, referentes à campanha de avaliação CONASA-1991, 162 poços+160 sondagens de circulação inversa sobre diferentes materiais: chapéu de ferro (W1), vulcanitos e xistos (W2), xistos (W3) e aterros (W4). Indicam-se um total de 2,38 Mt @ 0,77 g/t Au e de 8,26 g/t Ag para resíduos livres de condicionamentos. Considerando os valores totais, incluindo volumes com construção urbana no topo, inferem-se recursos de 4,0 Mt @ 0,64 g/t Au e 7,30 g/t Ag, correspondentes a uma quantidade de metal de 82.878 oz/t Au e 955.753 oz/t Ag.
1. Introduction
Discovered in 1854 and exploited between 1857 and 1966 by the Mason & Barry Company, the São Domingos mine is an example of old exploitation of a IPB pyrite-rich massive sulphide deposit. The orebody is associated with black shales and felsic rocks of the IPB Volcano-Sedimentary Complex (Strunian to Upper Visean age; Matos et al., 2006). These rocks are surrounded by thrust faults verging SW, by shales and quartzites of the Phyllite-Quartzite Formation of Famennian age. The mineralization is represented by massive sulphide and stockwork ore (py, cpy, sph, ga, tt, aspy and sulfossalts). Intense oxidation and supergene enrichment occur related with the deposit erosion and represented by hematite and limonite gossan and covellite/chalcocite zone. The São Domingos subvertical E-W ore-body (537 m length, variable thickness 45 m to 70 m) was exploited in open pit up to a depth of 120 m and in underground mining (chambers and pillars, cut and fill) up to a depth of 420 m (Webb 1958, Matos et al. 2006). Between 1867 and 1880, 3M m3 of rock were extracted from the open pit. The mine products were pyrite, sulphur and copper. Native copper was obtained by cementation evolving extensive ore leaching at Moitinha plateaus (Matos et al. 2006, 2011). Related with the mining process intense acid mine drainage occurs and large areas are occupied by different mine wastes (Matos 2004, Quental et al., 2003). The mining activity affected a total area of 3,076,900 m2, from the São Domingos village to the Chumbeiro dam, located 11 km downstream. A total of 14.7 Mm3 of mining wastes are estimated presenting heterogeneous dimensions (between >14 m and <1 m) (Matos 2004, Alvarez-Valero et al. 2008). The present work is focused in the evaluation of the northern sector of the São Domingos mine wastes, considered to have economic potential and characterized by gossan, volcanic and sedimentary rocks. These wastes were mapped in detail by Matos (2004) and previously evaluated by the
Compañia Nacional de Piritas, S.A. (CONASA) (Malavé and Mora 1991).
2. Data used for resource estimation and resource estimation methodolgy The study was based on a LNEG data set, from an historical evaluation program carried by CONASA (Malavé and Mora 1991) constituted by 160 vertical reverse circulation (RC) drillholes (total of 1,884 m) and 162 pits (total of 1,307 m), totalling 1148 samples. The drillhole and pit distribution pattern is irregular with an average sampling spacing of 30 m. Different waste classes were considered: gossan (W1), felsic volcanics and shales (W2), shales (W3); and landfill (W4), see Fig. 1. Waste mapping (Matos 2004) was updated using field work and Bing Maps aerial photographs. Considering the presence of urban areas on top of the mining wastes (São Domingos village) two different scenarios were considered: total resources and conditioned resources. The RC drillholes were sampled in fixed length intervals (2 m), and the pits in varying lengths, providing 1 to 3 samples at successively deeper levels (maximum 12.5 m deep). The pit samples were sorted into 4 size fractions of >40 mm, 40-9 mm, 40-9-5 mm, and <2 mm, and assayed for Au and Ag. A statistical analysis of the data set was conducted using the raw assay data grouped by waste pile, material class and size fraction. Assay statistics were calculated with GEOVIA Surpac 6.6 using the “basic statistics window”, and size fraction assay statistics with R language. The volume modelling was performed by defining a group of boundaries for each waste pile: a) the present terrain surface, extracted from a digital elevation model (MINEO project, Quental et al., 2003); b) the original terrain surface; c) a lateral pile boundary. The lower boundary (b) was interpolated in ArcMap 10.2 from the coordinate points of the bottom of the pits. In areas where the waste thickness was greater than the excavator reach (12.5 m), a local study was perfomed to obtain the best depth estimation. The quality of the modeled
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surface (b) was verified by comparing with the Ribeiro 1857 São Domingos map, perfomed before the modern mining (Custódio 1996, Matos et al., 2011). The lateral boundaries of the waste piles (c) were created from the geological map (Matos 2004).
Fig. 1 - Study area: location of the mining wastes and landfill areas assayed for gold and silver. W1 – gossan; W2 – felsic volcanic and shales; W3 – shales; W4 – landfill areas for resource estimation; W – other mining wastes; L - landfills P – Paleozoic basement. Adapted from Matos (2004).
Considering the actual and future mine waste management, conditioned by the presence of urban areas and legally protected industrial and geological heritage sites (e.g. roman galleries and slags and mining infrastructures and open pit geossite) a scenario of conditioned exploitation was considered. Detailed mine site analysis and field work permitted the identification of a particular set of polygons, corresponding to the non-conditioned part of the areas where waste remobilization will be certainly possible. Separate block models, constrained by the defined boundaries, were created for each of the 23 waste piles and landfill bodies. Block sizes were chosen to take into account the sample spacing, a quarter of the average sample spacing (Sinclair and Blackwell 2004). The average block model thickness varies between 7 and 12 m, with maximum thickness of 16 m. The volume for all the block models is 2,615,850 m3, and 2,152,559 m3 if considered only the non-conditioned portion. Grades for Au and Ag were interpolated into the models by the inverse distance squared method
(Fig. 2), using between 2 to 12 samples to estimate block grades. The size of the search neighbourhoods was set accordingly to the average spacing between samples, assuring that all the blocks reached the minimum samples required. Bulk density values between 1.83 and 1.30 were applied to all blocks within the models, considering the average specific gravity of the main lithological constituents and a void ratio of 0.30. The quality of the estimates was evaluated by the comparison of the resource models statistics with those of the original assay data.
Fig. 2 - Distribution of gold grades in one of the block models from a felsic volcanics and shales (class W2) waste pile. Grid line intervals: 10 m. Coordinates in local projected coordinate system (HG D73) in meters.
As this resource estimates are for mining waste piles no cut-off grade was applied to the reported resource, as any practical recovery will probably require processing of all material in the piles. However, in order to determine the amount of material with reasonable prospects for economic extraction, only the piles with an average gold grade above 0.50 g/t were considered for reporting (Table 1). Due to the lack of bulk density data from the historical drilling campaign, poor spatial distribution of the boreholes, and high variability Au and Ag grades within the wastes, it is deemed appropriate to consider all modelled blocks as Inferred resources.
Table 1. Resource estimates in the non-conditioned
volumes of the models with an average gold grade above 0.50 g/t.
Quantity (t) 613,208 667,150 1,105,121 2,385,479
Grade (g/t)
Au 1.07 0.71 0.65 0.77
Ag 10.01 8.61 7.08 8.26
Contained metal (oz t)
Au 21,145 15,266 23,078 59,489
Ag 197,443 184,581 251,464 633,488
4. Conclusions
The São Domingos Au- and Ag-rich mining wastes present in the northern sector of the mine were produced by the former mine owner, Mason & Barry Company, and are characterized by gossan, volcanic and sedimentary rocks. These tailings were mapped in detail by Matos (2004) and previously evaluated by CONASA (Malavé and Mora 1991). The metal distribution study shows a mineral resource of 2.38 Mt in the non-conditioned volumes of the waste piles and landfill models, with an average grade of 0.77 g/t Au and 8.26 g/t Ag in the 11 waste piles and 6 landfill bodies with an average Au grade above 0.5 g/t, totalling a metal content of 59,489 oz t Au and 633,488 oz t Ag. If the conditioned part of the volumes were to be considered, the mineral resource increases to 2.94 Mt with an average grade of 0,77 g/t Au and 8,27 g/t Ag, corresponding to 72,871 oz/t Au and 781,531 oz/t Ag. If all the 17 waste piles and 6 landfill volumes modelled in this investigation were to be considered, including the portions with improbable exploitation (e.g. urban areas), the mineral resource could be increased to 4.0 Mt with an average grade of 0.64 g/t Au and 7.30 g/t Ag, corresponding to a metal content of 82,878 oz/t Au and 955,753 oz/t Ag. The size fraction analysis revealed that the highest Au grade particles are in all of the > 40 mm waste classes, indicating that is possible to concentrate the highest grade materials by screening the coarser fragments. However, the weight
percentages for each fraction are unknown, because there is no information about these values in the CONASA historical drillhole database. Considering the presence of urban structures (São Domingos village) on top of several mine wastes, two different scenarios must be considered in the mine site planning: total resources and conditioned resources. More detailed knowledge is needed, complementary to the previous CONASA exploration and evaluation work for accurate reserves calculation. The characterization of the fine vertical zonation of the waste piles and their material calibration will be essential to define high grade levels. In future land use plans of the São Domingos mining area, the balance between economic mine profit and negative impacts of the waste exploitation must be considered and properly evaluated.
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