Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/209 Comunicações Geológicas (2016) 103, 1, 143-147 ISSN: 0873-948X; e-ISSN: 1647-581X
Magnetic susceptibility and δ
18O characterization of Variscan
granites related to W-(Mo) and Sn-(W) mineralizations:
Lamas de Olo Pluton case study
Caracterização da suscetibilidade magnética e δ
18O de granitos
Variscos relacionados com mineralizações de W-(Mo) e Sn-(W):
Exemplo do Plutão de Lamas de Olo
C. Cruz
1,2*, H. Sant’Ovaia
1,2, F. Noronha
2© 2016 LNEG – Laboratório Nacional de Energia e Geologia IP
Abstract: The “Iberian Sn-W metallogenic province”, located in
central-northern region of Portugal is characterized by the presence of various W, W-(Sn), W-(Mo) and Sn deposits related with synorogenic Variscan granites, mainly emplaced during the post-collisional stage (321 to 290 Ma). The granites can be classified into magnetite- or ilmenite-type granites according to their magnetic susceptibility. These two types of granites result from different oxygen fugacities during the granitic magma genesis, and have also distinctive whole-rock δ18O values. Thus, higher values (between 12.3 ‰ and 13.5 ‰) indicate a crustal origin, while the low δ18O (between 8.9 ‰ and 12.2 ‰) suggests a mantle origin
and/or source regions dominated by mafic and meta-igneous rocks. Major W-(Sn) and Sn ore deposits are more related to ilmenite-type granites with high δ18O, while W-(Mo) deposits are related with magnetite-type granite with lower δ18O values.
Keywords: magnetic susceptibility, post-tectonic Variscan granites,
magnetite-type granites, W-(Mo) and Sn-(W) mineralizations.
Resumo: A “Província metalogénica de W-Sn” situada no norte e centro
de Portugal é caracterizada pela presença de vários jazigos de W, W-(Sn), W-(Mo) e Sn, espacialmente associados a granitos sinorogénicos Variscos que se instalaram maioritariamente durante a etapa pós-colisional (321 a 290 Ma). Os granitos podem ser classificados em granitos tipo-magnetite ou tipo-ilmenite, consoante a sua suscetibilidade magnética. Estes dois tipos de granitos resultam ainda de diferentes condições de fugacidade de oxigénio durante a génese dos magmas graníticos e possuem ainda valores distintos de δ18O em rocha-total. Deste modo, valores elevados de δ18O (entre 12,3 ‰ e 13,5 ‰) indicam origem crustal para os magmas graníticos, enquanto valores baixos de δ18O (entre
8,9 ‰ e 12,2 ‰) sugerem uma origem mais mantélica e/ou com contribuição de rochas máficas e meta-ígneas. A maioria dos jazigos de W-(Sn) e Sn estão relacionados com granitos tipo-ilmenite com valores de δ18O altos, enquanto as ocorrências de W-(Mo) tendem a estar relacionadas com granitos tipo-magnetite e com baixos valores de δ18O. Palavras-chave: suscetibilidade magnética, granitos Variscos
pós-tectónicos, granitos tipo-magnetite, mineralizações de W-(Mo) e Sn-(W).
1
Faculty of Sciences, University of Porto, Department of Geosciences, Environment and Spatial Planning, Rua do Campo Alegre, 4169 -007 Porto, Portugal.
2
Institute of Earth Sciences – Porto Pole, Rua do Campo Alegre, 4169 -007 Porto, Portugal.
* Corresponding author / Autor correspondente: claudiacruz@fc.up.pt
1. Geological setting
The Lamas de Olo Pluton (LOP) is a post-tectonic Variscan body and constitutes a small outcrop with rhombus shape. This pluton is located in the northern part of Central-Iberian Zone in Iberian sector from Variscan belt, near the limit with Galiza Trás-os-Montes Zone (Farias et al., 1987; Ribeiro et al., 1990).
The emplacement of the LOP was controlled by NNW-SSE offsets conjugated with the NNE-SSW Verin-Régua-Penacova fault (VRPF). The LOP has distinct biotite facies: 1) Lamas de Olo (LO), the more representative facies, is a medium to coarse-grained porphyritic granite; 2) Alto dos Cabeços (AC), a medium to fine-grained porphyritic granite and 3) Barragem (BA), a leucocratic fine-grained and slightly porphyritic granite (Pereira, 1989; Fernandes et al., 2013). The LOP granites intrude the Douro Group metasediments (Precambrian to Cambrian in age) and two-mica syntectonic granites from Vila Real Massif (Fig. 1) (Pereira, 1989). The studied area belongs to the “Northern mineralogenic province” and W-Mo-(Sn) occurrences are present mostly in N80ºE sub-vertical quartz veins cutting LOP granites (Thadeu, 1965; Pereira, 1989).
2. Materials and methods
2.1. Sampling
At each site of sampling, three, four or five oriented cores (25 mm in diameter and 60 to 70 mm in length) were collected in situ with a portable drill machine. Then, in laboratory each core was sawed in order to obtain around 22 mm long specimens. At least three, or more, specimens per station were obtained for magnetic susceptibility measurements.
This study was based on 292 specimens from 36 sampling sites, roughly distributed in LOP granites: 30 sites in LO facies, 3 in BA and 3 in AC facies (Fig. 1).
2.2. Magnetic susceptibility measurements
The relationship between the magnetization induced in a material M and the external field H is defined as M=KH, where K is the magnetic susceptibility. The K values are dimensionless because M (per unit volume) and H have the same units (e.g. Hrouda,
Artigo original Original article
144 Cruz et al. / Comunicações Geológicas (2016) 103, 1, 143-147
Figure 1. Simplified geological map of the Lamas de Olo Pluton and sampling sites (modified: Pereira et al., 1987).
Figura 1. Mapa geológico simplificado do Plutão de Lamas de Olo e localização dos locais de amostragem (modificado: Pereira et al., 1987)
1982; Bouchez, 1997). The K reflects the mineral composition of the whole-rock (W.R.), comprising the diamagnetic (e.g. quartz, feldspar), paramagnetic (e.g. biotite, muscovite or hornblende) and ferromagnetic minerals (e.g. magnetite or hematite). K measurements were performed using a KLY-4S Kappabridge susceptometer Agico model (Czech Republic) from the Institute of Earth Sciences – Porto Pole. Measurements were undertaken in a 300 A/m field. For each site, the ANISOFT 4.2 program package (www.agico.com) enabled us to calculate the mean susceptibility Km, which is the mean of the individual arithmetic means (Kmax + Kint + Kmin)/3.
The Km values can distinguish two kinds of granites: magnetite- and ilmenite-type granites. The first group has Km values above 3.0 × 10–3 SI and in the second, Km is lower than 3.0 × 10–3 SI (Ishihara, 1977).
The magnetic susceptibility measurements were taken on all 292 samples from the 36 sampling sites (Fig. 1).
2.3. Oxygen fugacities and whole-rock δ18O
The granite rocks reflect redox states of their corresponding magmas, and the presence of magnetite or ilmenite as accessory minerals represents oxidized- and reduced-type respectively. These two types of granites also result from exposure of different oxygen fugacity during the magma genesis. The magnetite-type is generated at great depth where no carbonaceous material
exists, whereas the ilmenite-type is originated at a shallow level where small amounts of crustal carbon are present (Ishihara, 1977; Sheppard, 1977; Ellwood and Wenner, 1981).
In nature, the oxygen occurs mainly in the form of the 16O isotope, but it can also occur in very small quantities in the stable 18
O isotope form. The abundance of the 18O isotope is expressed as a percentage of the 18O isotope relative to 16O (indicated by R in the equation: R = 18O / 16O × 100). However, when the differences in the abundance of both isotopes, on the rocks, are very small, the quantity of the isotope is expressed as its ratio in the sample relative to an international accepted standard, normally the VSMOW standard (Vienna Standard Mean Ocean Water). This relative portion in rock samples, represented as 18O, is multiplied by 1000 and is expressed by the equation: δ18O = [(Rsample/ Rstandard) – 1] × 1000 ‰ (Morrill and Koch, 2002).
The highest δ18O values indicate a crustal origin, through anatexis of metasedimentary protoliths, and the low values suggest a mantle origin and/or source regions dominated by mafic and meta-igneous rocks (Ishihara, 1977; Ellwood and Wenner, 1981).
Oxygen isotope data measurements were performed on 4 samples from LOP at the Stable Isotopic Laboratory of Salamanca. Oxygen was extracted from rocks by laser fluorination techniques, quantitatively converted to CO2 by the reaction with a heated carbon rod and analyzed for 18O/16O ratio with a dual inlet VG SIRA-II Mass Spectrometer.
3. Results and discussion
3.1. Magnetic susceptibility evidences
The K measurements were examined in order to deduce the redox conditions of magma systems as well as the distribution of magnetite- and ilmenite-type granites over the pluton. The high Km values (>10-3 SI) indicate a ferromagnetic behavior, due to the presence of iron minerals, such as magnetite and the low Km values (~10-6 SI) indicate the presence of paramagnetic minerals, like ilmenite and biotite (Ishihara, 1977).
Km values are heterogeneous and range between 24 × 10-6 and 44382 × 10-6 SI for all LOP granites. The Km values range between: 65 × 10-6 and 44382 × 10-6 SI in LO facies, 24 × 10-6 and 34 × 10-6 SI in BA, and 167 × 10-6 and 804 × 10-6 SI in AC (Table 1). The mean values by facies are 2447 × 10-6 SI in LO, 30 × 10-6 SI in BA and 403 × 10-6 SI in AC facies.
Table 1. Magnetic susceptibility data for the sampling sites (Km: mean magnetic susceptibility in 10-6 SI).
Tabela 1. Valores de suscetibilidade magnética por locais de amostragem (Km: suscetibilidade magnética média em 10-6 SI).
Sample LOP facies Km × 10-6 SI
LM 1 LO 174 LM 2 LO 1848 LM 3 LO 2475 LM 3V LO 316 LM 4 LO 65 LM 5 LO 2873 LM 6 LO 229 LM 8 LO 1677 LM 13 LO 44382 LM 14 LO 2179 LM 16 LO 67 LM 17 LO 77 LM 18 LO 79 LM 19 LO 1869 LM 20 LO 2760 LM 21 LO 233 LM 22 LO 606 LM 23 LO 254 LM 27 LO 69 LM 28 LO 2065 LM 29 LO 1147 LM 30 LO 2385 LM 7 BA 34 LM 15 BA 24 LM 32 LO 2031 LM 33 LO 1261 LM 34 LO 100 LM 35 LO 205 LM 36 LO 242 LM 37 LO 101 LM 38 LO 1559 LM 39 LO 81 LM 31 BA 32 LM 24 AC 239 LM 25 AC 167 LM 26 AC 804
Figure 2 presents the frequency of mean magnetic susceptibility values for all the LOP granites. Concerning the susceptibility, the LO is the more heterogeneous facies, with Km varying between, Km < 10-4 SI to Km > 10-3 SI.
The BA and AC facies have only Km < 10-4 SI, but some AC Km values were relatively higher for ilmenite-type granites. These values show that the two types of magnetic behavior are present in the same pluton. The interpolation of Km data, using
Figure 2. Frequency histogram of mean magnetic susceptibility values for the Lamas de Olo Pluton facies (LO – Lamas de Olo; AC – Alto dos Cabeços; BA – Barragem). Figura 2. Histograma de frequência da suscetibilidade magnética média das fácies do Plutão de Lamas de Olo (LO – Lamas de Olo; AC – Alto dos Cabeços; BA – Barragem).
the software ArcGIS, shows that magnetite is present in locals where fracturing is well pronounced, namely in the eastern part of the LOP and along an E-W direction (Fig. 3).
Figure 3. Map of the magnetic susceptibility (Km) values in Lamas de Olo Pluton (interpolation made by Inverse Distance Weighted in the ArcGIS software). Figura 3. Mapa de distribuição dos valores de suscetibilidade magnética (Km) do Plutão de Lamas de Olo (interpolação realizada no software ArcGIS utilizando o Inverso da Distância).
3.2. Oxygen-isotope data
The W.R. δ18O data were performed on 4 samples from LOP: 3 from LO facies, being 2 of magnetite-type and 1 of ilmenite-type and 1 sample of BA ilmenite-type granite.
The Km versus W.R. δ18O diagram (Ellwood and Wenner, 1981) permit to compare LOP with other Variscan Portuguese Massifs, in order to establish a relationship between granites and mineralizations.
The studied Variscan granites (Fig. 4) have Km values between 28 × 10–6 and 92100 × 10–6 SI, however the majority of the granites vary from 20 × 10–6 to 300 × 10–6 SI which corresponding to reduced, ilmenite-type granites. The W.R. δ18O values from some Variscan granites, obtained from bibliography (Antunes et al., 2008; Martins et al., 2009; Neiva et al., 2009 and Sant’Ovaia et al., 2012), shows that studied granites have heterogeneous 18O values ranging from 8.9 ‰ to 13.5 ‰.
146 Cruz et al. / Comunicações Geológicas (2016) 103, 1, 143-147
Figure 4. Distribution of the studied variscan granitoids in northern Portugal (modified: Ferreira et al., 1987).
Figura 4. Distribuição dos granitóides variscos estudados no norte de Portugal (modificado: Ferreira et al., 1987).
The oxidized or magnetite-type granites are scarce and represented by post-tectonic granites with Km values ranging from 1 × 10–3 to 92 × 10–3 SI units and low δ18O values ranging from 8.9 ‰ to 10.4 ‰.
Figure 5 points out that major W-(Sn) ore deposits are related to ilmenite-bearing granites with high δ18O values, while W-(Mo) deposits are related with magnetite-type granite with lower δ18O values.
4. Final Remarks
The magnetic susceptibility and δ18O data allow to establish a difference between ilmenite-type and magnetite-type granites and relate each granite type to a particular metal association (Fig. 5) (Takagi and Tsukimura, 1977; Kumar, 2010). The presence of magnetite reflects melt oxidized conditions, on deeper crustal conditions, which played an important role on the granite genesis related to W-Mo mineralizations (Ishihara, 1977).
Our results confirm the presence in the LOP of ilmenite-type and magnetite-type granites. However, in facies exhibiting post-magmatic alterations the magnetite alteration can change K values of the granites because implies the formation of new minerals like hematite (Lagoeiro, 1998). This fact can explain K mean values around 403 × 10-6 SI which are lower than the expected to magnetite-type granites in AC facies.
The association between granites with different magnetic behavior in the same pluton is not present in other similar post-tectonic plutons, from northern Portugal, such as Vila Pouca de Aguiar and Águas Frias-Chaves (Fig. 4) (Martins et al., 2009). These plutons are very similar to LOP, either in geochemistry or in geochronology characteristics, but they are ilmenite-type granites.
We assume that W-(Mo) mineralizations are present in LOP because magnetite-type granites are present. This relationship between magnetite-type granites and W-(Mo) mineralizations was also found in the post-tectonic (280 Ma) Gerês pluton (Fig. 4) (Noronha et al., 2013). The abundant W-(Sn) and Sn mineralizations present in the area can be related with ilmenite-type granites, namely the two-mica granites.
Acknowledgments
The first author is financially supported by SFRH/BD/109693/2015 (FCT Portugal); The authors acknowledge the funding by COMPETE 2020 through the ICT (Institute of Earth Sciences) project (UID / GEO / 04683/2013) with POCI-01-0145 reference - FEDER-007690 and NewOreS Project (ERA-MIN / 0002/2014).
Figure 5. Semi-log plot of Km (in SI units) versus δ18O for the studied granitoids, considering the data in Antunes et al. (2008); Martins et al. (2009); Neiva et al. (2009) and Sant’Ovaia et al. (2012).
Figura 5. Gráfico semi-log da Km (unidades em SI) versus δ18O para os granitóides estudados, considerando os dados reportados em Antunes et al. (2008); Martins et al. (2009); Neiva et al. (2009) and Sant’Ovaia et al. (2011).
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