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
Artigo Curto
Short Article
© 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 LousalCaveira 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 FilitoQuartzítico (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.
1
LNEG, Ap. 104, 7801-902 Beja, Portugal
LNEG, 4465-965 S. Mamede Infesta, Portugal
3
EDM, 1070-248, Lisboa, Portugal
4
LNEG, Ap. 7586, 2611-901 Amadora, Portugal.
*
Corresponding author / Autor correspondente: joao.matos@lneg.pt
2
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
490
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
J. X. Matos et al. / Comunicações Geológicas (2014) 101, Especial I, 489-493
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
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 thinbedded 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
491
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
492
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
J. X. Matos et al. / Comunicações Geológicas (2014) 101, Especial I, 489-493
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).
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