Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, Especial I, 489-493 IX CNG/2º CoGePLiP, Porto 2014 ISSN: 0873-948X; e-ISSN: 1647-581X

High resolution stratigraphy of the Phyllite-Quartzite Group in the northwest region of the Iberian Pyrite Belt, Portugal Estratigrafia de alta resolução do Grupo Filito-Quartzítico na região NW da Faixa Piritosa Ibérica, Portugal J. X. Matos1*, Z. Pereira2, C. J. P. Rosa3, J. T. Oliveira4

© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: Recent data from regional geological mapping in the NW sector of the Iberian Pyrite Belt complemented with detailed mapping in the Lousal and Caveira old mine sites performed by LNEG was combined with palynostratigraphic data from exploration boreholes of the Lousal, Caveira and São Francisco da Serra antiforms. This work allowed the division of the former Corona Formation of the Phyllite Quartzite Group (PQG) into two informal units, the Upper and Lower Corona units. The Upper Corona Unit is composed of phyllites and quartzites of Strunian age, as shown by miospores assigned to the LN Biozone. The Lower Corona Unit comprises homogenous black phyllites with sparse siliceous nodules and minor quartzite lenses (<1m thick) that yield a miospore assemblage containing Geminosporalemurata, Emphanisporitesannulatus, Grandisporaprotea, Verrucosisporitespremnus and V. scurrus which is assigned to the upper part of AD miospore Biozone, subzone Lem, of lower Givetian age (Middle Devonian). A stratigraphic hiatus encompassing the Upper Givetian, the Frasnian and the Famenian ages, separates these two informal units. The origin of this hiatus is tentatively related to the morphology of the basin, considered to be controlled by extensional faults and associated elevated blocks subject to erosion and later tectonic erosion. Drill core from the hinge zone of the São Francisco da Serra antiform showed also phyllites and thinly bedded quarzites of lower Givetian age correlated with the Lousal-Caveira stratigraphic sequence. The upper part of the PQG was not dated at S. Francisco and must be investigated in the future.

Keywords: Lower Givetian (Middle Devonian), Corona Unit, Phyllite-Quartzite Group, Iberian Pyrite Belt. Resumo: Cartografia geológica e estudos palinoestratigráficos de sondagens efetuadas na região NW da Faixa Piritosa Ibérica, permitiram a divisão informal da Formação de Corona do Grupo Filito-Quartzítico (GFQ) em duas unidades. A unidade Corona Superior é constituída por uma sequência de filitos e quartzitos de idade Estruniano, obtida pela identificação de miosporos da Biozona LN. A unidade Corona Inferior é formada por filitos negros, com nódulos siliciosos esparsos, localmente com lentículas de quartzito, apresentando os miosporos Geminosporalemurata, Emphanisporitesannulatus, Grandisporaprotea, Verrucosisporitespremnuse V. Scurrus da parte superior da Biozona AD, subzona Lem de idade Givetiano inferior. Em Lousal e Caveira constata-se a existência de uma lacuna estratigráfica que inclui o Givetiano superior, Frasniano e Famenniano. A sua origem não é ainda bem compreendida, podendo estar relacionada com falhas distensivas, que geraram blocos elevados, sujeitos a erosão. Numa sondagem efetuada no núcleo do antiforma de São Francisco da Serra foram também identificados filitos e quartzitos de idade Givetiano inferior, sugerindo uma correlação com as estruturas de Lousal e Caveira.

Palavras-chave: Givetiano inferior (Devónico Médio), Unidade de Corona, Grupo Filito-Quartzítico, Faixa Piritosa Ibérica.

1LNEG, Ap. 104, 7801-902 Beja, Portugal 2LNEG, 4465-965 S. Mamede Infesta, Portugal 3EDM, 1070-248, Lisboa, Portugal 4LNEG, Ap. 7586, 2611-901 Amadora, Portugal. *Corresponding author / Autor correspondente: joao.matos@lneg.pt

1. Introduction and geological setting

The NW region of the Iberian Pyrite Belt (IPB) comprises the Lousal-Caveira, Cidrão and São Francisco da Serra antiforms which are part of the IPB south branch (Oliveira et al., 2013). The stratigraphic sequence is composed by the basal IPB unit, the Phyllite-Quartzite Group (PQG), followed by the Volcano-Sedimentary Complex (VSC) and the turbidites of the Mértola Formation of the Baixo Alentejo Flysch Group. The PQG crops out in the core of the Lousal, Caveira and São Francisco da Serra structures and is characterized by dark phyllites, metasiltstones and quartzites. The quartzite beds are dominant at the upper part of the lithological sequence and promote structural reliefs related with differential erosion at Sra. Do Livramento chapel and Caveira mine hills (Pereira et al., 2008, 2010). This lithological succession was formerly named as Corona Formation in the Lousal-Caveira area (Strauss, 1970; Strauss & Madel, 1974; Schermerhorn et al., 1987) and renamed as Phyllite-Quartzite Formation (Oliveira et al., 2013) due to the similarities with the PQG rocks in many other IPB sections. Within the scope of the present work, we decided to maintain the designation as Corona Unit. The age of the PQG, based on palynostratigraphic data, ranges from the lower Givetian (Middle Devonian) to the late Famennian and Strunian (Upper Devonian) (Pereira et al., 2008). Palynomorph assemblages include acritarchs and prasinophytes, substantiating a shallow marine depositional environment for these rocks. The VSC occurs in all the mentioned structures and is represented by acid and basic volcanic rocks, black, grey, green and purple shales and volcanogenic sediments. At Lousal and Caveira mines the

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lower VSC black shales that enclose part of the massive sulphide mineralization were dated from the LN Biozone, Strunian age (Matos et al., 2011). The Strunian black shales represent a key IPB exploration guide horizon since they typically host massive sulphide deposits.

The study of drill-core from mineral exploration campaigns combined with regional and detail geological

mapping (Matos, 2006; Matos & Relvas, 2006; Pereira et al., 2008; Relvas et al., 2012; Oliveira et al., 2013) allowed a better understanding of the above mentioned antiforms as well as the Corona Unit in terms of its lithology and age. The studied drill-holes reached ca. 360 m depth at São Francisco da Serra, ca. 700 m at Cidrão, ca. 500 m at Caveira and ca. 900 m at Lousal (Fig. 1).

Fig. 1. Geological map of the Iberian Pyrite Belt NW region, with studied boreholes and cross-sections. Geology ad.: LNEG 1:50 000 42A (Grândola), 42B (Azinheira de Barros) and 42D (Aljustrel) and Fl. 7 1:200 000 maps. Fig. 1. Mapa geológico do setor NW da Faixa Piritosa Ibérica, com a localização das sondagens e cortes geológicos estudados. Base geológica ad. de cartografia LNEG 1:50 000 42A (Grândola), 42B (Azinheira de Barros) e 42D (Aljustrel) e Fl. 7 1:200 000. 2. Methodology

Shale samples for palynostartigraphic dating were collected at selected outcrops and from exploration boreholes housed at LNEG (Fig. 1). Rock samples were prepared at São Mamede Infesta LNEG facilities where all samples, residues and slides are curated. The miospore biozonal scheme used follows the standard Western Europe Miospore zonations.

3. São Francisco da Serra structure

The São Francisco da Serra structure consists of a NW-SE trending antiform that plunges to SE. The stratigraphic succession consists of PQG phyllites and quartzites, the latter becoming thicker bedded, amalgamated and lenticular towards the top of the lithological succession (e.g. Senhora do Livramento). This unit is overlain by the VSC, characterized by siliceous shales, volcanogenic sediments, minor felsic volcanic rocks, purple shales and dark carbonaceous shales (Pereira et al., 2010). The palynostratigraphic study of borehole M1 (vertical and 367 m deep, by SMRA), which is

located in the core of the antiform (Figs. 1, 2A) shows a highly fractured sequence of dark phyllites locally interbedded with thinly-bedded metasiltstones and quartzites. This succession yielded a moderately preserved miospore assemblage assigned to the upper part of AD miospore Biozone, subzone Lem, of lower Givetian age, based on the presence of Geminosporalemurata. Reworked miospores of Lower Devonian age were also identified in this sequence. The top of the studied section is situated 200m below the upper PQG quartzite thick beds which means that the age of this unit may reach younger Devonian ages.

4. Caveira mine region

The geology of the Caveira region is dominated by a complex NNW-SSE trending antiform, with phyllites and quartzites of the PQG (Corona Unit) in the nucleus, surrounded by felsic volcanic rocks, volcanogenic sediments and dark shales of the VSC (Pereira et al., 2008; Oliveira et al., 2013). Several pyrite ore lenses occur near the PQG/VSC contact forming two main ore horizons: the Helena Shaft (west) and the Luísa Shaft (east). The

Phyllite-Quartzite Group in the NW sector of the IPB 491

borehole CAV2, located in the eastern sector of the antiform (301 min depth, 60º dip to Az. 270º, by SMRA) was studied for palynostratigraphy. From top to base, the borehole (Fig. 2B) intersected black phyllites and thin-bedded siltstones and quartzites ascribed to the Upper Corona Unit, followed by VSC felsic porphyritic volcanic rocks and black shales with massive sulphide intercalations (associated with the Luisa Shaft orebody). Both the Upper Corona phyllites and the lower VSC black shales show moderately preserved miospore associations of the LN Miospore Biozone, of late Strunian age (Retisporalepidophyta together with the rare specimens of the guide specie Verrucosisporitesniditus).Below the VSC lithologies black phyllites interbedded with quartzites occur which contain miospores indicating the AD Lem Interval Zone based on the first occurrence of Geminosporalemurata, of lower Givetian age. Older reworked miospores, of Lower Devonian age complete the assemblage. This section of the borehole, here provisionally designated as Lower Corona Unit, is similar in terms of lithology and age, to that of the M1 São Francisco da Serra borehole and to the Lousal boreholes (see discussion below).

The CAV2 upper Corona Unit lithologies are thrusted over the VSC acid volcanic rocks and the black shales with massive sulphides are limited by faults. Between the lithologies of the Upper and the Lower Corona Units there is a stratigraphic hiatus that encompasses the Famennian, the Frasnian and part of the Givetian age. The origin of this hiatus is not well understood and may result from a combination of an erosional hiatus (subsequent to extensional faults?) and superimposed thrusting. The data recovered from the borehole were complemented with surface sampling based on detail mapping. A black shale sample assigned to the Corona Formation collected in the Caveira main road (see Fig. 1) (543510, 4219629, WGS84), in the north central area of the antiform, yielded a lower Givetian age (date age by palynomorphs) comparable to that of the Lower Corona Unit

5. Lousal mine region

The stratigraphic sequence at the Lousal region is defined from base to top by black phyllites with sparse siliceous nodules and minor quartzite lenses of the Corona Unit (PQG) followed by a VSC sequence represented by black shales, several rhyolitic and dacitic lava flows. The massive sulphide and stockwork mineralization is associated with rhyolitic lavas. Stratigraphically above this sequence, the upper VSC sequence consists of black and siliceous shales with nodules, that hosts two thick basaltic units, locally with doleritic texture and pillow lavas, and green and purple shales (Rosa et al., 2010; Matos et al., 2011; Relvas et al., 2012; Oliveira et al., 2013). The antiform at Lousal strikes N45ºW and plunges to NW and the axial plan is sub vertical to 80ºSW. The studied boreholes LS08001-2 (966 min depth, 71º dip to Azimuth 57º, by AGC) and LS08002 (840 min depth, 65º dip to azimuth 60º, by AGC), see figure 2C, intersected the

Lousal antiform structure and a large part of the stratigraphic sequence. The Corona Unit was intersected at the core of the antiform and provided a palynomorph assemblage of the AD Lem interval zone, and is therefore comparable with that of Lower Corona Unit identified at the Caveira and São Francisco da Serra. Both boreholes intersected black shales that host rhyolitic volcanic rocks with hydrothermal alteration and massive sulphide lenses. These black shales provided miospores of the LN biozone, of Strunian age. This sequence shows some similarities with the lithostratigraphic sequence of the Neves Corvo mine (Oliveira et al., 2013). A well exposed section located at the left bank of the Corona River (Fig.1) (550690, 4209766, WGS84) shows a succession predominantly composed of compact black shales with sparse siliceous nodules and minor disrupted quartzite beds (<1 m thick). A Lower Givetianmiospore assemblage (presence of Geminosporalemurata, Emphanisporitesannulatus, Grandispora protea, Verrucosisporitespremnus and V. scurrus) was recovered from the black shales, indicating that they have the same age as the Lower Corona Unit, but with distinct lithological composition. These shales contact, through faults, with VSC siliceous shales dated of middle-upper Visean age (NM Miospore Biozone).

6. Discussion and conclusions

The stratigraphic succession of the IPB NW region is represented by the PQG that forms the detritic basement overlain by the VSC. Recent LNEG geological mapping complemented with the stratigraphic studies of exploration boreholes shed light on the PQG lithostratigraphic succession. In the Lousal and Caveira antiforms, in spite of the tectonic complexity, it has been possible to recognize two main informal units. The Upper Corona Unit, composed of phyllites, metasiltstones and thick bedded quartzites, of Strunian age based onmiospores assigned to the LN zone. The Lower Corona Unit consists of phyllites, thin bedded metasiltstones and quartzites, that yelded miospores of the AD Lem interval of lower Givetian age. Compact black shales with siliceous nodules, that crop out in a section limited by faults at the left bank of the Corona River also provided miospores of lower Givetian age. This section stresses the presence of lateral facies changes within the Lower Corona Unit. In the northern sector of the Caveira antiform, black shales of the Lower Corona Unit were also dated as lower Givetian age.

At the core of the São Francisco da Serra antiform, the PQG intersected by M1 borehole is mostly composed of dark phyllites and thin bedded quartzites of lower Givetian age, as indicated by miospores of the AD Lem interval zone, i.e., this interval has the same age of the Lower Corona Unit. However, the upper part of the PQG has not been dated in this sector and younger Devonian ages for this part of the succession may occur but confirmation is needed.

The existence of a stratigraphic hiatus embracing the upper Givetian, the Frasnian and the Famennian age in the Lousal and Caveira antiforms is difficult to explain

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due to faults that disrupt the stratigraphic sequence of the Corona Formation. It is suggested that extensional faults may have generated elevated blocks in the basin, which were subjected to erosion. Post depositional tectonic erosion may have also played an important role. Stratigraphic hiatus were identified in the Devonian

sequence of the Neves Corvo mine region, however with much less time extend. These gaps may be related to sea level changes in the Devonian siliciclastic platform. More stratigraphic research needs to be performed across the IPB, in order to achieve a better understand the PQG basin development.

Fig. 2. Simplified logs and sediment age dating of the follow boreholes: 2A. M1 (Quinta do Poço, S. Francisco da Serra), 2B. CAV2 (Caveira mine), 2C. LS08001-2 and LS08002 (Lousal mine). Fig. 2. Logs simplificados das sondagens estudadas e datação de sedimentos: M1 (Quinta do Poço, S. Francisco da Serra), CAV2 (mina da Caveira), LS08001-2 e LS08002 (mina do Lousal). References

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