Geology of giant quartz veins and their host rocks from the Eastern Pyrenees (Southwest Europe)

Abstract

Giant Quartz Veins (GQVs) are ubiquitous in different tectonic settings and, besides being often related to hydrothermal ore deposits, also represent large-scale fingerprints of the structural and geochemical history of the rocks in which they are hosted. Here we present detailed geological maps and interpretations of three key areas of the Eastern Pyrenees where GQVs are well exposed. The studied rocks record different styles of deformation and are representative of common settings of the Pyrenees where GQVs are present: pre-Variscan metasedimentary and metavolcanic rocks, late Variscan granitoids, and Mesozoic and Cenozoic sedimentary rocks. GQVs in the study areas formed along pre-existing brittle and ductile structures or at locations with lithological heterogeneities, and have alteration haloes of silicified host rocks. The geological maps and interpretations presented here contribute to gain insights into the formation mechanisms of GQVs and into the structural constraints on fluid flow and mineral reactions at different depths of the Earth's crust.

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Geology of giant quartz veins and their host rocks
from the Eastern Pyrenees (Southwest Europe)
Eloi González-Esvertit, Àngels Canals, Paul D. Bons, Henrique Murta, Josep
Maria Casas & Enrique Gomez-Rivas
To cite this article: Eloi González-Esvertit, Àngels Canals, Paul D. Bons, Henrique Murta,
Josep Maria Casas & Enrique Gomez-Rivas (2022): Geology of giant quartz veins and
their host rocks from the Eastern Pyrenees (Southwest Europe), Journal of Maps, DOI:
10.1080/17445647.2022.2133642
To link to this article: https://doi.org/10.1080/17445647.2022.2133642
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UK Limited, trading as Taylor & Francis
Group.
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Geology of giant quartz veins and their host rocks from the Eastern Pyrenees
(Southwest Europe)
Eloi González-Esvertit
a
, Àngels Canals
a
, Paul D. Bons
b,c
, Henrique Murta
d
, Josep Maria Casas
e
and
Enrique Gomez-Rivas
a
a
Departament de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Ciències de la Terra , Universitat de Barcelona, Barcelona, Spain;
b
China University of Geosciences, Beijing, People’s Republic of China;
c
Department of Geosciences, Tübingen University Tübingen,
Germany;
d
Deep Nature Photography, Belo Horizonte, Brasil;
e
Departament de Dinàmica de la Terra i l’Oceà, Facultat de Ciències de la
Terra, Universitat de Barcelona, Barcelona, Spain
ABSTRACT
Giant Quartz Veins (GQVs) are ubiquitous in different tectonic settings and, besides being often
related to hydrothermal ore deposits, also represent large-scale fingerprints of the structural
and geochemical history of the rocks in which they are hosted. Here we present detailed
geological maps and interpretations of three key areas of the Eastern Pyrenees where GQVs
are well exposed. The studied rocks record different styles of deformation and are
representative of common settings of the Pyrenees where GQVs are present: pre-Variscan
metasedimentary and metavolcanic rocks, late Variscan granitoids, and Mesozoic and
Cenozoic sedimentary rocks. GQVs in the study areas formed along pre-existing brittle and
ductile structures or at locations with lithological heterogeneities, and have alteration haloes
of silicified host rocks. The geological maps and interpretations presented here contribute to
gain insights into the formation mechanisms of GQVs and into the structural constraints on
fluid flow and mineral reactions at different depths of the Earth’s crust.
ARTICLE HISTORY
Received 13 July 2022
Revised 31 August 2022
Accepted 3 October 2022
KEYWORDS
Giant quartz veins;
geological mapping; Eastern
Pyrenees
1. Introduction
Quartz veins with widths that range from metres to
hundreds of metres and lengths from tens of metres
to kilometres, hereafter Giant Quartz Veins (GQVs),
are widespread in various tectonic settings (Main
Map, Map A; Figure 1)(Bons, 2001;Jia & Kerrich,
2000;Lemarchand et al., 2012;Slabunov & Singh,
2022;Yilmaz et al., 2014). These structures can act
as either conduits or barriers to heat and mass
transfer within the Earth’s crust, can be associated
with hydrothermal ore deposits, and reveal infor-
mation about the deformational and geochemical
history of their host rocks (Amanda et al., 2022;
Bons et al., 2012;Groves et al., 2018;Sharp et al.,
2005;Wagner et al., 2010). However, there still
are several open questions about the origin and sig-
nificance of GQVs: e.g. about the sources of such
large amounts of silica and the tectonic, lithological
and geochemical control(s) on their emplacement.
The decrease in silica solubility and quartz precipi-
tation have been classically linked to temperature
and pressure variations during fluid flow (Bons,
2001 and references therein) and, thus,
understanding what drives that fluid flow may
reveal the window’(Tannock et al., 2020).
In the Alpine fold-and-thrust belt of the Pyrenees
(SW Europe), at least 741 GQVs mappable at the
1:25,000 scale are exposed at different structural levels
and emplaced along brittle and ductile structures
(Figure 2)(González-Esvertit et al., 2022a). Here we
present detailed geological maps, descriptions, and
cross-sections of three areas of the Pyrenees in
which GQVs crop out in different rock types (Figure
2B, C, D): (1) Upper Neoproterozoic –Early Palaeo-
zoic metasediments at the Ger-Gréixer sector (La Cer-
danya area), (2) Lower Permian igneous rocks and
Upper Neoproterozoic –Early Palaeozoic metasedi-
ments in the Roses area (Cap de Creus Massif), and
(3) Upper Cretaceous sedimentary rocks at the
Masarac-Vilarnadal area (Roc de Frausa Massif). For
each area, we first describe the main stratigraphical
and structural features of the host rocks (Figures 3–
5) and then examine the occurrence of GQVs based
on their macrostructure, texture, and deformation
structures, in order to address the relationship
between GQVs, host rocks and regional structures
(Figures 6 and 7). The overarching aim of this work
© 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrest-
ricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
CONTACT Eloi González-Esvertit e.gonzalez-esvertit@ub.edu Departament de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Ciències de
la Terra, Universitat de Barcelona, C/Martí i Franquès s/n, Barcelona 08028, Spain
Supplemental data for this article can be accessed online at https://doi.org/10.1080/17445647.2022.2133642.
JOURNAL OF MAPS
https://doi.org/10.1080/17445647.2022.2133642

is to reveal the different structural controls on GQV
emplacement, as well as to update the geology of
three sectors that are key to understand the tectonic
evolution of the Pyrenees.
2. Geological setting
The Pyrenees are an E-W-trending asymmetric
double-verging Alpine fold-and-thrust belt located
at the northern boundary of Iberia (Figure 2A). It
extends from Cap de Creus in the East to the Bay
of Biscay in the West, whilst northwards and south-
wards it is bounded by the Aquitanian and Ebro
foreland basins, respectively. The formation of the
Pyrenees resulted from the collision between the
Iberian and the Eurasian plates from the Late Cre-
taceous to the Miocene (Muñoz, 1992). This collision
produced the exhumation of a pre-Variscan metase-
dimentary succession, late-Neoproterozoic to Car-
boniferous in age, that extensively crops out in the
backbone of the cordillera (Axial Zone) (Figure 2).
In the Eastern Pyrenees, these rocks record Sardic
(Ordovician), Variscan (Carboniferous-Permian)
and Alpine deformational events, host igneous
bodies of different age, extent, and composition,
and were affected by Variscan regional metamorph-
ism (Casas, 2010;Guitard, 1970;Navidad et al.,
2018;Zwart, 1986). The Ger-Gréixer (La Cerdanya;
Figure 2B) and Roses (Cap de Creus; Figure 2D)
study areas are located within this pre-Variscan
basement (Figure 2)(Druguet et al., 2014;Padel
et al., 2018). In contrast, the Mesozoic and Cenozoic
sedimentary rocks that crop out in the South Pyre-
nean Zone (Figure 2A), and in several small areas
of the Axial Zone (as the Masarac-Vilarnadal area
in the Roc de Frausa massif; Figure 2C), have only
been affected by post-Variscan tectonics (Muñoz,
2019;Pujadas et al., 1989).
3. Methods
The study areas were selected considering two pre-
mises: (1) access to outcrops, to ensure that each
area is mapped by fieldwork rather than by extrapol-
ation or remote sensing-based interpretations, and
(2) host rock variability, to make sure that each area
is representative of a key tectonic setting in which
GQVs form. The geology of the study areas was
characterised in the field at the 1:2,500 scale. More
than 2,000 orientation measurements of GQVs, host
rock bedding and cleavage/s, fault structures, fold
axes, and joint sets were collected and georeferenced
in 3D.
In the final layout, a world map with the location of
GQVs previously reported in the literature (Main
Map, Figure A) is presented together with a geological
Figure 1. Examples of Giant Quartz Veins cropping out in different tectonic settings: (A) the Esquerdes de Rojà vein in the Canigó
Massif, pre-Variscan basement of the Pyrenees, SW Europe; (B) the Poolamacca veins in the Broken Hill Inlier, western New South
Wales, Australia (from Bons, 2001); (C) the China Wall vein in the Halls Creek Belt, north Western Australia, Australia; (D) the Castle
Ruin Weissenstein vein in the Bavarian Phal Zone, Bohemian Massif, Central Europe (from the Naturpark Bayerischer Wald admin-
istration); and (E) the Heyuan vein in the Shaowu-Heyuan Fault zone, South China block (from Tannock et al., 2020).
2E. GONZÁLEZ-ESVERTIT ET AL.

sketch map of the Eastern Pyrenees (Main Map, Figure
B), to provide a general overview of the research topic
and the geological framework of the Pyrenees. The
1:7,500 geological maps of the Ger-Gréixer
(6.5 km
2
), Roses (5 km
2
), and Masarac-Vilarnadal
(4.2 km
2
) sectors are presented over a multidirectional
hillshade (Figures C, D and E of the Main Map).
4. Geology of the study areas
4.1. Ger-Gréixer (La Cerdanya)
The Ger-Gréixer sector is characterised by a low-
grade pre-Variscan metasedimentary succession in
which three stratigraphical units can be distinguished
(Figure 3A–D): the Lower Cambrian Err Fm., the
Cambrian-Lower Ordovician Serdinya Fm. (Jujols
Group), and the Upper Ordovician Cava Fm. (see
Main Map, Figure C) (Cavet, 1957;Hartevelt, 1970;
Laumonier, 1988;Padel et al., 2018). These units can
be correlated with those of other sectors of the Eastern
Pyrenees and other regions of the eastern Variscan
Ibero-Armorican Arc, i.e. Montagne Noire and
Sardinia (e.g. Padel et al., 2018).
At the northern sector, the Err Fm. is mainly com-
posed of a rhythmic alternation of mm –to cm-thick
dark-blueish shale layers (Figure 3A). Coarse-grained
levels of greywacke and arkose can be found inter-
bedded within monotonous shale-dominant packages.
At the top of the Err Fm., a thin (ca. 25 cm thick) dis-
continuous key level of black shales (Figure 3B), inter-
preted as the Xatard Mb. (Padel et al., 2018), can be
recognised as the boundary with the Serdinya Fm.
The overlying Serdinya Fm. is composed of a rhythmic
alternation of greenish sandstone and shale layers, ca.
0.1–5 cm-thick (Figure 3C). Coarse-grained cm to
dm-thick sandstone levels are characteristic of this for-
mation. A ca. 10 m-thick package of mm –to cm-thick
quartzite layers, attributed to the Font Frède Mb.
(Padel et al., 2018), has also been identified in the
southern sector of the study area (Figure 3D).
An unconformable, scarcely outcropping contact
defines the limit between Serdinya and the overlying
Cava Fm. This intra-Ordovician unconformity was
first recognised in other sectors of SW Europe
Figure 2. (A) Geological sketch map of the Pyrenees showing the distribution of giant quartz veins and the location of the study
areas located in La Cerdanya (B), Cap de Creus (C), and Roc de Frausa Massifs (D) (orthophotographs from the Institut Cartogràfici
Geològic de Catalunya –ICGC).
JOURNAL OF MAPS 3

Figure 3. Field photographs of the host rocks from the Ger-Gréixer sector: (A) Typical aspect of the dark-blueish shale layers of the
Err Fm. showing a marked L
S2/S0
intersection lineation; (B) Discontinuous level of black shales that represents the boundary
between the Olette and Serdinya Fms.; (C) Typical example of the greenish sandstone and shale layers of the Serdinya Fm.;
(D) Quartzite layers of the Font Frède Mb. strongly folded in the southern (reverse) limb of a major anticline fold; (E) Cut and
polished sample of the Err Fm. showing the strongly folded bedding surfaces (S
0
) and a poorly developed slaty cleavage (S
1
);
(F) Host rock silicification area linked to one of the south-directed thrusts faults present in the study area, which postdate the
development of the main cleavage S
2
;(G) Apophysis-like texture (white arrow) and host rock silicification aureole ca. 2 m
away from the giant quartz vein located at the northern sector of the study area; (H) Detail of the GQV boundary showing highly
silicified adjacent host rocks of the Serdinya Fm. (I) Detail of the vein located at the southern sector of the study area, showing host
rock fragments that preserve the original trend of the main S
2
surfaces and are crosscut by cm-wide veins.
4E. GONZÁLEZ-ESVERTIT ET AL.

(Teichmüller, 1931), and has been and interpreted as
a result of the Sardic Phase (e.g. Casas, 2010), a Mid
–Late Ordovician deformation episode (see review
in Puddu et al., 2019). The overlying Cava Fm. is
composed of feldspar-rich conglomerates and
coarse-grained sandstones with scarce levels of varie-
gated shales and fine-grained sandstones. Neogene
detrital deposits unconformably lie on top of the
pre-Variscan rocks at the southern edge of the
study area.
Slaty cleavage (S
1
) has only been observed in scarce
outcrops of the Err Fm. as a poorly developed axial pla-
nar cleavage of micro-folds that affect the bedding sur-
faces (Figure 3E). Other deformation mesostructures
associated with this cleavage have not been identified.
A well-developed NW-SE-trending and roughly mod-
erately-to-subvertical NE-dipping cleavage (S
2
) is the
most recognisable structure in the pre-Variscan rocks
(Figure 3A, D, F). Bedding (S
0
) planes and S
2
surfaces
define a well-developed intersection lineation (L
S2/S0
)
(Figure 3A, C). S
0
and S
2
dip attitudes define an anti-
form-synform fold sequence in the northern sector of
the study area (Figure 7A). These folds present SSW-
verging axial planes parallel to the S
2
surfaces, are
open to tight, and have a wavelength ranging between
metres and hundreds of metres. S
0
-S
2
relationships
indicate repetitive variations of the rocks, from reverse
to normal arrangement fold limbs (Figure 7A). Further
south, S
0
surfaces dip moderately towards the NNE and
form the normal limb of a meso-scale overturned anti-
cline located in the central sector of the study area
(Figure 7A). The reverse limb of this fold is character-
ised by sub-vertical NNE-dipping S
0
surfaces that
become moderately NNE-dipping southwards. The
contact between the Err Fm. and the Serdinya Fm.
crops out along this southern reverse fold limb,
where S
0
surfaces exhibit a moderate to subvertical
NNE dip (Figure 7A). Cm –to m-scale second-order
folds, with axial planes parallel to the S
2
cleavage, are
also common in the study area (Figure 3D). They
verge to the SSW with a ‘Z’asymmetry when located
in reverse fold limbs and to the NNE with an ‘S’asym-
metry when located in normal fold limbs. A set of five
E-W-trending south-directed thrusts was identified
(Figure 3F). They postdate folding and the S
2
cleavage
and are spatially related to silicification areas (Figure
3F, G, H). Southwards displacement cannot be quan-
tified due to the lithological similarities between the
various rock units involved. An Alpine age has been
proposed for the southernmost thrust on the basis of
their geometry and orientation (González-Esvertit
et al., 2022b).
Two thrust-related parallel GQVs are present in
the study area (Figures 6A and 7A). They are mainly
composed of massive milky quartz with variable
amounts of silicified host rock fragments (5–35%),
0.5–25 cm in size (Figure 3H, I). The number of
these fragments increases notably (up to 80%)
towards the edges of the main bodies, defining a pro-
gressive and diffuse vein-host rock contact. S
0
and S
2
surfaces observed within these host rock fragments
show, as in the Roc de Frausa and Cap de Creus
areas (see sections 4.2 and 4.3), the same orientation
as in the rocks located in the outer part of the
GQVs. This fact is difficult to explain by fluid-assisted
fracturing and entrapment of host rock fragments
within the main quartz mass (e.g. by hydraulic brec-
ciation), which would involve transport (and hence
misorientation) of the host rock fragments (Figures
3I, 4I, J, 5H, I, J). Silicification ‘aureoles’of ca. 5–
30 m width can be observed in those rocks adjacent
to the GQVs (Figures 3G, H and 7A). The GQVs, sili-
cification ‘aureoles’, and undisturbed host rocks are
crosscut by anastomosing 1–10 cm thick quartz vein-
lets (Figure 3C, G, H), that occasionally have void
cavities of ca.1–4cm
3
partially filled by euhedral pris-
matic quartz crystals.
4.2. Roses (Cap de Creus massif)
In the Roses sector, pre-Variscan fine –to coarse-
grained low-grade metasedimentary and metavolcanic
rocks, as well as the late-Variscan Roses granodiorite,
constitute the main host rocks of GQVs (see Main
Map, Figure D; Figure 4A–E). Fine-grained metasedi-
mentary rocks, muscovite-rich greyish-blueish slate
and alternating shale layers (Figure 4A) are the strati-
graphically lowermost unit. A ca. 80 m-thick level of
black slates is found at the top of this unit. The fine-
grained unit is attributed to the Upper Neoproterozoic
Lower Series (i.e. Cadaqués and Montjoi Series) (e.g.
Druguet & Carreras, 2019), which crops out along
the central and northern areas of the Cap de Creus
Massif. A ca. 30 m-thick unit of acid volcanic rocks
(Figure 4B) is interbedded within this unit (Mas de
la Torre acid metatuffs, 558 ± 3 Ma; Casas et al.,
2015). Volcanic rocks show porphyritic textures on
which mm –to sub-mm-sized plagioclase and, predo-
minantly, potassium feldspar crystals are embedded in
afine-grained matrix (Figure 4B). The overlying
coarse-grained unit is made up of cm-thick layers of
dark greenish-yellowish sandstone and greywacke
(Figure 4C, D) and is attributed to the Lower Cam-
brian Upper Series (i.e. Norfeu Series). The uppermost
part of this unit is composed of greyish-blackish
banded limestones that crop out in the northern sector
of the study area. The sheet-shaped Roses granodiorite
(290.8 ± 2.9 Ma; Druguet et al., 2014)(Figure 4E) con-
stitutes a ca. 3 × 4 km late-Variscan igneous body
mainly composed of feldspar, quartz, biotite, and
hornblende. This intrusion produced a contact meta-
morphic aureole of highly variable width (from 5 to
100 m) recognisable as spotted phyllites and horn-
felses (Carreras et al., 2004;Carreras & Losantos,
JOURNAL OF MAPS 5

1982). Aplite and leucogranite dykes, 1 cm to 2 m
wide, often crosscut the granodiorite.
Bedding surfaces of the pre-Variscan metasedimen-
tary rocks (S
0
) are often difficult to recognise in the
Roses area due to the complex deformational history.
When present, S
0
surfaces are moderately to steeply
dipping and show a highly variable trend due to late
folding (Figures 4A, C, D). A large NW-SE-trending
Figure 4. Field photographs of the host rocks from the Roses sector: (A) Strongly folded bedding surfaces of the fine-grained
metasedimentary rocks; (B) metavolcanic rocks intercalated within fine-grained metasediments crosscut by pinch-and-swell
(occasionally boudinated) quartz veins; (D) Coarse-grained metasediments showing a NW-SE-oriented main cleavage (S
1
) and
a SW-NE-oriented crenulation cleavage (S
2
); (E) Roses granodiorite showing 2 cm-wide shear bands parallel to the main cleavage
(S
2
; gneissic foliation); (F) Zone of major deformation within the silicification area of the NW segment of the Roses giant quartz
vein, showing deformed quartz veins that crosscut the coarse-grained metasediments; (G) Silicified and sheared quartz-schist
band located a few metres away from the NW segment of the Roses giant quartz vein; (H) Giant quartz vein-host rock contact
zone showing cm-wide quartz veins with apophysis-like textures; (I) and (J) Host rock fragments within the main quartz mass
preserving the orientation of the main cleavage (S
2
); (K) Vein-parallel stylolite networks (red arrows) and highly-silicified host
rocks at the contact zone between the SE segment of the Roses vein and the fine-grained metasediments; (L) Detail of the SE
segment of the Roses vein showing a network of cm-wide crack-seal veins crosscutting a blueish-greyish quartz mass that records
traces of the original fabric of the metasedimentary host rocks.
6E. GONZÁLEZ-ESVERTIT ET AL.

antiform-synform pair was identified from the map
analysis (Figure 7B). A bedding-parallel cleavage (S
1
)
is the most recognisable structure within the metasedi-
ments (Figures 4A, C, D). S
1
surfaces consist of 0.2–
3 cm thick shallow-dipping layers, broadly N-S –to
NW-SE-trending, that are defined by the orientation
of fine-grained phyllosilicates. They can be defined
as a slaty cleavage when present in the fine-grained
unit, or as an anastomosing spaced cleavage when
identified in the coarse-grained unit. S
2
surfaces con-
sist of a NE-SW-trending and moderately SE –or
NW-dipping crenulation cleavage (Figures 4C, D). S
2
is heterogeneously developed through the study area
and show mm –to cm-thick layers interpreted as the
axial planar cleavage of cm-scale folds affecting the
S
0
/S
1
surfaces (Figure 4C). In the Roses granodiorite
(Figure 4D), a NW-SE-trending and mostly SE-dip-
ping gneissic foliation defined by biotite crystals ana-
stomosing around mm-cm sized feldspar crystals
(S
3
) is present (Figures 4E and 7B). Deformation local-
isation along cm-wide mylonitic bands that follow the
main trend of the S
3
surfaces can occasionally be
identified (Figure 4E). According to regional compari-
sons (e.g. Druguet, 1997;Llorens et al., 2013), S
1
struc-
tures correspond to the oldest ‘D
1
’deformation phase
that occurred prior to the metamorphic peak. The S
2
crenulation cleavage can be attributed to the hetero-
geneous D
2
deformation event that led to the folding
of S
0
and S
1
layers, whilst S
3
surfaces can be attributed
to the multiphase post-magmatic ‘D
3
’deformation
event widely recognised in the Cap de Creus area,
which is responsible for the formation of the main
shear zones in this massif.
In addition, the study area exhibits high-strain
zones where a strong NW-SE-oriented and moder-
ately-to-strongly SW-dipping mylonitic/phyllonitic
foliation developed (Figures 4F, G and 7B). These
zones mostly affect both the Roses GQV and the sili-
cified quartz-schists that occur close to it in the NW
sector of the study area (Figure 4G). A second NE-
SW-oriented sub-horizontal or slightly NW-dipping
mylonitic foliation has also been identified in the
Roses granodiorite and the GQV (Figure 7B). It
defines cm –to m-wide high-strain bands that post-
date both the formation of the GQV and the develop-
ment of the NW-SE mylonitic/phyllonitic foliation.
‘Late’quartz veins, with a width of 1–5 cm, are
often emplaced parallel to these mylonitic/phyllonitic
foliation, showing pinch-and-swell (occasionally bou-
dinage) structures (Figure 4B).
The Roses GQV (Figures 4H–Land 6B, C) is
defined by several NW-SE-trending aligned discon-
tinuous quartz bodies with heterogeneous internal
structure in terms of grain size, crystal transparency
and finite strain. Two main outcropping domains
can be distinguished according to the GQV main
trend and the deformation style: the NW (Figures
6B and 7B) and SE (Figure 6C) domains. The
main quartz bodies exhibit a gentle strike variation,
from NW-SE (ca. N135°) in the NW segment to
WNW-ESE (ca. N120°) in the SE segment. On
the NW segment (Figure 6B) finite strain is higher
and the quartz bodies follow the main trend of the
silicified sheared quartz-schist with variable contents
of muscovite and chlorite (Figures 4G and 7B).
Contrarily, quartz bodies of the SE segment of the
GQV (Figure 6C) are emplaced following a
WNW-ESE-trending and NNE-directed thrust.
This fault separates the volcanic breccias, in the
hanging wall, and the black slates that are at the
top of the fine-grained unit.
The host rocks immediately adjacent to the GQV
have a 2–20 m wide silicification halo. Within the
main quartz bodies, greyish quartz aggregates with
variable phyllosilicate and oxide content and highly
silicified fragments of the metasedimentary rocks
are common (Figure 4H–K). In some cases, clea-
vage surfaces present in host rock fragments iso-
lated within the main quartz bodies show the
same structural attitude as those rocks located
immediately outside the GQV (Figure 4I, J). As in
the La Cerdanya area (see section 4.1), this suggests
that host rock silicification by replacement was an
important quartz precipitation mechanism during
the formation of this GQV, which left remnants
of the original fabric and the unreplaced host
rock fragments in their original position. Beyond
silicification and replacement, quartz fabrics also
point to other formation processes. For example,
cm-thick veins that crosscut and postdate the
highly-silicified host rocks exhibit vein-parallel sty-
lolites suggesting host rock pressure-solution related
to the opening of cm-wide veins (Figure 4K). The
aggregation of many of these smaller veins, simi-
larly to a crack-seal system (Ramsay, 1980), could
also give rise to substantial accumulations of quartz
(Figure 4H, L).
4.3. Vilarnadal-Masarac (Roc de Frausa Massif)
The Vilarnadal-Masarac study area represents the
southeasternmost portion of the late-Variscan Sant
Llorenç-La Jonquera pluton, which is unconformably
overlain by Triassic, Cretaceous, and Palaeocene sedi-
mentary rocks. The Sant Llorenç-La Jonquera pluton
crops out extensively along the NW sector of the
study area and consists of a biotite and hornblende
rich granodiorite (Figure 5A) crosscut by leucogra-
nite and granite porphyry dykes and cm-wide quartz
veins (see Main Map, Figure E) (Autran et al., 1970;
Liesa, 1988). Occasionally, the granodiorite is altered
into an orange-pink quartz-feldspar rock, from which
biotite and hornblende crystals have been completely
removed (Figure 5B). This alteration style is similar
JOURNAL OF MAPS 7

to the quartz-feldspar gneisses that formed by biotite
dehydration in South Australia (see Weisheit et al.,
2013). The granodiorite is unconformably overlain
by Lower Triassic Buntsandstein facies, consisting
of SW-NE-trending beds, ca. 80 m-thick, of an
alternation of reddish claystone mm –to cm –
thick beds and fine-grained sandstone. Occasionally,
2–10 cm thick conglomerate levels with a reddish
matrix were found within this unit. Above, a ca.
50 m-thick greyish unit of laminated limestones,
Figure 5. Field photographs of the host rocks from the Masarac-Vilarnadal sector: (A) unaltered granodiorite from the St. Llorenç-
La Jonquera pluton; (B) altered granodiorite that is crosscut by 2 cm-wide quartz veins; (C) micritic limestone of the Mid Triassic
lower Muschelkalk facies; (D) calcite vein networks close to the hinge zone of a NW-SE-oriented anticline in the Mid Triassic lower
Muschelkalk facies; (E) SW-NE-oriented bedding surface of the Upper Cretaceous conglomerates showing a NW-SE-oriented sub-
vertical cleavage; (F) silicification area with cm-wide quartz veins within the Upper Cretaceous-Paleocene lower Garumnian facies;
(G) Syncline-anticline NW-SE-oriented folds affecting the Upper Cretaceous ochre limestones; (H) Embryonic replacement textures
of host rocks in the Upper Cretaceous conglomerates close to the giant quartz vein (see location on Figure 6D); (I) and (J) detail of
a giant quartz vein with visible remnants of the replaced conglomerate fabric (see location on J).
8E. GONZÁLEZ-ESVERTIT ET AL.

marly limestones and dolostones, corresponding to
the Mid Triassic lower Muschelkalk facies, crops
out with the same SW-NE trend (Figure 5C). Lami-
nated limestones show 0.2–2 cm-thick micritic layers
and are the most abundant lithology. Calcite vein net-
works linked to high deformation zones are abundant
in (and restricted to) this carbonate unit (Figure 5D).
Triassic rocks are unconformably overlain by a ca.
60 m-thick Upper Cretaceous unit composed of
ochre limestones interbedded with siltstone levels
(Figure 5G), fine –to coarse-grained yellowish-red-
dish sandstones, and matrix-supported, polymictic
and heterometric conglomerates (Figure 5E, H).
Ochre limestones and siltstones are only present at
the base of this unit and form ca. 2–10 m-thick dis-
continuous levels (Figure 5G), whilst sandstone layers
of variable thickness (1–15 m) are the most abundant
rock type within this unit. Furthermore, a ca. 10–
15 m-thick conglomerate key level that follows the
main trend of the post-Variscan rocks can be ident-
ified at the map scale (Figure 5E). The conformably
overlying Cenozoic succession crops out in the SE
sector of the study area and is attributed to the
Upper Cretaceous –Lower Paleocene Garumnian
facies (Cirés et al., 1994). The lowermost unit of
this succession is composed of a ca. 30 m-thick
monotonous alternation of mm-cm-thick reddish
claystone layers, occasionally crosscut by cm-wide
quartz veins (Figure 5F). It is conformably topped
by an intermediate package of greyish-blueish micri-
tic limestones, ca. 50 m-thick, and an uppermost unit
of reddish and ochre claystones intercalated with
some small lenses (0.5–1 m thick) of conglomerates
and coarse-grained sandstones.
A map-scale interference between two fold systems
is the most striking feature of the Vilarnadal-Masarac
area. Together with the Sant Climent (ca. 3 km north-
wards) and Montpedrós (ca. 2 km southeastwards)
areas, they represent three km-scale isolated remnants
of synform-shaped post-Variscan rocks (e.g. Cirés
et al., 1994). Part of the Montpedrós syncline structure
can be observed in the Garumnian succession crop-
ping out at the southeasternmost sector of the study
area. The major synclines in the Sant Climent and
Montpedrós areas are NW-SE-oriented (calculated
fold axes of ca. 36/134), whilst the arrangement of
the post-Variscan rocks in the Vilarnadal-Masarac
area depicts a SW-NE-oriented syncline (calculated
Figure 6. (A) The giant quartz vein located at the northern sector of the Ger-Gréixer area, emplaced along a ca. 50° N-dipping
thrust fault; (B) UAV (Unnamed Aerial Vehicle) aerial photograph of the NW segment of the Roses vein, where it was emplaced
along a ca. 50–70° SW-dipping silicified and sheared quartz-schist band; (C) The SE segment of the Roses vein, where it was
emplaced along a ca. 40–55° SW-dipping thrust fault; (D) Digital Elevation Model coupled with aerial photographs acquired
using an UAV of a giant quartz vein (white arrows) from the Vilarnadal-Masarac area.
JOURNAL OF MAPS 9

fold axis of ca. 040/20). Furthermore, a well-exposed
anticline-syncline sequence of NW-SE-oriented
minor folds can be identified at both the outcrop
and map scales (Figure 5G) in the Vilarnadal-Masarac
area. These folds are related to the major NW-SE-
oriented major synclines of the Sant Climent and
Montpedrós areas and form an angle of ca. 90° to
the main trend of the SW-NE syncline (measured
fold axes from Figure 5G of ca. 42/124). Anticlines
are tight, regularly spaced, and their limbs dip moder-
ately-to-strongly towards the SW –and NE (occasion-
ally in a reverse way) in the immediately adjacent areas
(ca. 5–10 m) of their axial plane (Figure 5G). Folds
have only been identified affecting Triassic and Cre-
taceous rocks in the NW limb of the major syncline,
although a well-developed and sub-vertical dipping
axial-planar cleavage is present throughout the rest
of the study area following the main fold trends
(Figure 5E). In the central sector of the study area, a
SW-NE-oriented normal fault defines the contact
between the granodiorite and the moderately SE-
dipping Buntsandstein, Muschelkalk and Upper
Cretaceous successions (Figure 7C). This sequence
progressively dips less towards the SE and becomes
moderately-to-strongly NW-dipping in the southern-
most sector of the study area, where it is truncated
by a WSW-ENE-oriented normal fault that
defines the southern contact with the granodiorite
(Figure 7C).
GQVs of the Masarac-Vilarnadal area are hosted
either in the granodiorite or in the Upper Cretaceous
conglomerates and sandstones (Figures 5I, J and 6D).
They are in all cases associated with intense silicifica-
tion, and no previous fractures along which the
GQVs could have been emplaced have been identified.
Irregular and diffuse boundaries and the presence of
remnants of the replaced fabric within the veins
(Figure 5H, I), suggest that the main formation mech-
anism of these GQVs was the replacement of the pre-
existing rocks rather than filling of fracture porosity
with quartz precipitates. For example, when hosted
in Upper Cretaceous sandstones and conglomerates,
the GQVs record the former clast distribution patterns
and show embryonic replacement textures at their
boundaries (Figure 5H, I) that become dominant
towards their core zone (Figure 5J). Silicification
areas not related to GQVs have also been identified
(Figure 5F). The orientation of GQVs ranges from
NW-SE near the Vilarnadal village to SW-NE close
to the Masarac village, where they follow the main
trend of an Upper Cretaceous conglomerate key
level. Contrarily, GQVs located at the southern contact
between the Upper Cretaceous rocks and the Sant Llor-
enç-La Jonquera granodiorite (Figure 6D) are W-E –to
Figure 7. Representative cross sections of the Ger-Gréixer (A), Roses (B) and Vilarnadal-Masarac (C) sectors. See Main Map, Figures
C, D and E for location.
10 E. GONZÁLEZ-ESVERTIT ET AL.

WSW-ENE-oriented (Figure 7C). These latter veins
show the most intense wall rock alteration.
5. Concluding remarks
The geological maps presented in this work are
representative of the three key tectonic settings in
the Pyrenees in which GQVs are present. In the
Ger-Gréixer sector, GQVs follow south-directed
thrusts, probably Alpine, that were emplaced along
major fold limbs that postdate the development of
the main S
2
cleavage. In the Roses area, the age of
the GQVs is not constrained and could be related
to either Variscan or Alpine deformational events,
although they postdate the development of S
1
,S
2
,
and S
3
cleavages and predate the NW-SE mylonitic/
phyllonitic foliation. Quartz bodies that crop out in
the SE sector of the Roses area were emplaced
along a NE-directed thrust, whilst the NW segment
of the Roses GQV follows the main trend of a
sheared quartz-schist. GQVs of the Vilarnadal-
Masarac area are post-Variscan and mostly hosted
in Upper Cretaceous sedimentary rocks, following
the main trend of an Upper Cretaceous conglomerate
level and the fold limbs of a major SW-NE-oriented
syncline. Rocks of this area show an Alpine fold
interference pattern and a NW-SE-oriented fold-
related Alpine cleavage.
Despite being hosted in different host rocks and
emplaced along distinct structures, the GQVs share
various strikingly similar features (Figures 3–7): (1)
the development of silicification ‘halos’in either sedi-
mentary, igneous, or metamorphic host rocks; (2)
evidences of different mechanisms of quartz growth
through host rock replacement, host rock pressure-
dissolution and stylolite formation simultaneous to
the opening of cm-wide veins, and aggregation of
multiple ‘late’crack-seal veins with different orien-
tations; and (3) a strong structural control rep-
resented by zones of localised deformation (e.g.
normal faults, thrusts, shear zones, and fold axes or
fold axial planes) or lithological contrast levels (e.g.
conglomerate units) that vary even within hundreds
of metres.
Software
Fieldwork was carried out using the FieldMove® (Pet-
roleum Experts) application running on a high-per-
formance tablet connected to a Garmin® GPSMAP
66ST device. Accuracy of positioning (±3 to ±8 m)
was continuously checked with high-resolution ortho-
photographs and LiDAR-derived Digital Elevation
Models from ICGC (Institut Cartogràfic i Geològic
de Catalunya; http://icgc.cat). Geological boundaries
and dip data collected on the field were imported to
the 3D software MOVE® (Petroleum Experts), where
representative cross-sections of the study areas were
built by projecting orientation data through calculated
vectors into the cross-section lines.
When necessary, aerial photographs and elevation
models were acquired using UAVs (Unnamed Aerial
Vehicles) DJI Mavic Air 2 and DJI Mini Pro equipped
with a 12 MP camera. The mapping accuracy of geo-
logical boundaries was further improved using high-
resolution orthophotographs from PNOA-IGN (Plan
Nacional de Ortofotografía Aérea –Instituto Geo-
gráfico Nacional; http://ign.es) and the ICGC (Institut
Cartogràfic i Geològic de Catalunya; http://icgc.cat).
The Main Map was designed with the software QGIS
(https://qgis.org) using a 1:50,000 scale geological
map base from the ICGC. Dip symbols were designed
according to the standard geological representation in
an SVG editor and exported as an SVG library. Accu-
racy and truthfulness of the cross-sections were
checked through extra fieldwork in each study area.
Open Scholarship
Acknowledgements
We are grateful to Heike Apps (Geoscience Australia), Leo
Afraneo Hartmann (Universidade Federal do Rio Grande do
Sul) and Carlos Galé (Universidad de Zaragoza) for their use-
ful and constructive revisions, and to Jordi Cirés for introdu-
cing us to some outcrops of the Vilarnadal-Masarac area.
Claudia Prieto-Torrell and Daniel Martí Tubau are gratefully
acknowledged for their fieldwork assistance. EGE acknowl-
edges the funding provided by the Geological Society of
London (GSL) Student Research Grants 2022.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
This work is a contribution to the ‘Geologia Sedimentaria’
(2017SGR-824) Research Group and to research projects
DGICYT CGL2017-87631-P, PGC2018-093903-B-C22 and
PID2020-118999GB-I00, funded by the Spanish Ministry
of Science and Innovation (MCIN)/State Research Agency
of Spain (AEI)/10.13039/501100011033. EGE acknowledges
the funding provided by the Geological Society of London
(GSL) Student Research Grants 2022. The PhD grant of
EGE is funded by the Generalitat de Catalunya and the
European Social Fund (2021 FI_B 00165 and 2022 FI_B1
00043). EGR acknowledges the ‘Ramón y Cajal’fellowship
RYC2018-026335-I, funded by the Spanish Ministry of
Science and Innovation (MCIN)/State Research Agency of
Spain (AEI)/European Regional Development Fund
(ERDF)/10.13039/501100011033; Ministerio de Ciencia,
Innovación y UniversidadesAgència de Gestió d’Ajuts Uni-
versitaris i de Recerca.
JOURNAL OF MAPS 11

DATA availability statement
The authors confirm that the data supporting the findings of
this study are available within the article and its supplemen-
tary materials.
ORCID
Eloi González-Esvertit http://orcid.org/0000-0002-1168-
2532
Àngels Canals http://orcid.org/0000-0002-5544-0201
Paul D. Bons http://orcid.org/0000-0002-6469-3526
Josep Maria Casas http://orcid.org/0000-0001-7760-7028
Enrique Gomez-Rivas http://orcid.org/0000-0002-1317-
6289
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