Conference PaperPDF Available

Indexation, evaluation and interpretation of regional geological structures using geographic information systems: the giant quartz veins of the Pyrenees (SW Europe) as a case study

Authors:
  • Spanish National Research Council - GEO3BCN-CSIC
  • Andorra Research + Innovation

Abstract

Surface and sub-surface geological structures are common targets in both academic and industry research. For example, regional faults and shear zones are used to define the evolution of orogens and sedimentary basins worldwide, whereas quartz veins, unconformities and diagenetic geobodies are of special interest for the mining sector. As a result, vast amounts of information about different types of structures have been generated by earth scientists, geological surveys and mining and hydrocarbon exploration companies during the last decades. This data can be open access and potentially reusable for further studies, but commonly scattered in different reports, maps and databases. Inventorying different types of geological structures and organizing their related information in thematic and interactive databases is thus key for the growth of geoscience knowledge. However, a well-established and well-defined workflow to achieve this goal is currently missing. Here we propose a workflow for systematically compiling and characterizing geological structures from a Geographic Information System (GIS)-based approach. The method is demonstrated by indexing quartz veins outcropping in the fold and thrust belt of the Pyrenees (SW Europe), although it can be applied to other geological structures and replicated in other tectonic settings. Quartz veins have been classically used to grasp the mechanisms of fluid flow, vein cement precipitation and quartz deformation, as well as to unravel paleo-stress fields, deformation kinematics and the geochemical history of their host rocks (e.g., Bons et al., 2012). Fieldwork-based small-scale analyses of the orientation and distribution of centimetre to metre-scale veins are common and can provide information about their formation mechanisms, relative stress and fluid pressure evolution (e.g., Mondal and Mamtani, 2013). However, quartz veins can also reach hectometric thicknesses and kilometric lengths, being thus mappable at 1:25,000 scale and referred to as "Giant" Quartz Veins (GQVs). Because of their exceptionally large sizes, these impressive structures can be addressed by macroscale remote sensing and other computer-assisted methods. GIS environments provide suitable methods for these purposes, since they allow to systematically acquire their orientation, distribution and host rock variability, gaining insights into the structural controls on vein emplacement at the large scale. However, such a GIS-based overall assessment of GQVs and their host rocks has not been performed yet in any world region. Here we close this gap by mining, curating and enlarging the open access datasets of the French, Spanish, Catalan and Andorran geological surveys (Figure 1). By this method, the main features of 743 GQVs and their host rocks are presented as a dynamic and interactive tool: the GIVEPY database (GIant quartz VEins of the PYrenees), which follows the FAIR principles of database management (Findable, Accessible, Interoperable and Reusable). Vector-format geological maps were used as a starting point after applying topology rules to detect and fix overlaps and gaps between polygons. Two duplicated copies of each corrected layer were initially required (Figure 1). The first layer duplicates were filtered by polygon attributes to only extract the shape and properties of the GQVs. Inversely, the second layer duplicates were filtered to only extract the shape and properties host rocks. Layer copies containing GQVs were semi-automatically unified and merged to obtain a unique vector file where the GQVs of the Pyrenees are mapped as individual features. Contrarily, layer copies containing the host rock properties had untidied data and outdated or heterogeneous names of stratigraphical units. Thus, host rock layer duplicates required a step-by-step homogenization following the latest stratigraphical definitions and ages of magmatism from the Pyrenees.
XXI Congreso Geológico Argentino
Puerto Madryn, Chubut - Marzo de 2022
Simposio/Sesión técnica XX
INDEXATION, EVALUATION AND INTERPRETATION OF REGIONAL GEOLOGICAL
STRUCTURES USING GEOGRAPHIC INFORMATION SYSTEMS:
THE GIANT QUARTZ VEINS OF THE PYRENEES (SW EUROPE) AS A CASE STUDY
Eloi González-Esvertit(1), Enrique Gómez-Rivas(1), Àngels Canals(1), Aina Margalef(2), Maria-Gema Llorens(3)
and Josep Maria Casas(4)
(1)Departament de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Ciències de la Terra, Universitat de Barcelona.
C/Martí i Franquès s/n, 08028, Barcelona, Spain. Email: e.gonzalez-esvertit@ub.edu
(2) Andorra Recerca + Innovació, Av. Rocafort 21-23, AD600, Sant Julià de Lòria, Principat d’Andorra.
(3) Geosciences Barcelona - CSIC, 08028, Barcelona, Spain.
(4)Departament de Dinàmica de la Terra i l’Oceà, Facultat de Ciències de la Terra, Universitat de Barcelona. C/Martí i
Franquès s/n, 08028, Barcelona, Spain.
Surface and sub-surface geological structures are common targets in both academic and industry research. For
example, regional faults and shear zones are used to define the evolution of orogens and sedimentary basins
worldwide, whereas quartz veins, unconformities and diagenetic geobodies are of special interest for the mining
sector. As a result, vast amounts of information about different types of structures have been generated by earth
scientists, geological surveys and mining and hydrocarbon exploration companies during the last decades. This
data can be open access and potentially reusable for further studies, but commonly scattered in different reports,
maps and databases. Inventorying different types of geological structures and organizing their related
information in thematic and interactive databases is thus key for the growth of geoscience knowledge. However,
a well-established and well-defined workflow to achieve this goal is currently missing.
Here we propose a workflow for systematically compiling and characterizing geological structures from a
Geographic Information System (GIS)-based approach. The method is demonstrated by indexing quartz veins
outcropping in the fold and thrust belt of the Pyrenees (SW Europe), although it can be applied to other
geological structures and replicated in other tectonic settings. Quartz veins have been classically used to grasp
the mechanisms of fluid flow, vein cement precipitation and quartz deformation, as well as to unravel paleo-
stress fields, deformation kinematics and the geochemical history of their host rocks (e.g., Bons et al., 2012).
Fieldwork-based small-scale analyses of the orientation and distribution of centimetre to metre-scale veins are
common and can provide information about their formation mechanisms, relative stress and fluid pressure
evolution (e.g., Mondal and Mamtani, 2013). However, quartz veins can also reach hectometric thicknesses and
kilometric lengths, being thus mappable at 1:25,000 scale and referred to as “Giant” Quartz Veins (GQVs).
Because of their exceptionally large sizes, these impressive structures can be addressed by macroscale remote
sensing and other computer-assisted methods. GIS environments provide suitable methods for these purposes,
since they allow to systematically acquire their orientation, distribution and host rock variability, gaining insights
into the structural controls on vein emplacement at the large scale. However, such a GIS-based overall
assessment of GQVs and their host rocks has not been performed yet in any world region. Here we close this gap
by mining, curating and enlarging the open access datasets of the French, Spanish, Catalan and Andorran
geological surveys (Figure 1). By this method, the main features of 743 GQVs and their host rocks are presented
as a dynamic and interactive tool: the GIVEPY database (GIant quartz VEins of the PYrenees), which follows
the FAIR principles of database management (Findable, Accessible, Interoperable and Reusable).
Vector-format geological maps were used as a starting point after applying topology rules to detect and fix
overlaps and gaps between polygons. Two duplicated copies of each corrected layer were initially required
(Figure 1). The first layer duplicates were filtered by polygon attributes to only extract the shape and properties
of the GQVs. Inversely, the second layer duplicates were filtered to only extract the shape and properties host
rocks. Layer copies containing GQVs were semi-automatically unified and merged to obtain a unique vector file
where the GQVs of the Pyrenees are mapped as individual features. Contrarily, layer copies containing the host
rock properties had untidied data and outdated or heterogeneous names of stratigraphical units. Thus, host rock
layer duplicates required a step-by-step homogenization following the latest stratigraphical definitions and ages
of magmatism from the Pyrenees.
XXI Congreso Geológico Argentino
Puerto Madryn, Chubut - Marzo de 2022
Simposio/Sesión técnica XX
The cartographic accuracy of GQVs shapes was improved manually on the basis of two main sources (Figure 1):
(1) remote-sensing images, including high-resolution orthophotographs and satellite images, and (2) published
data and geological maps. Furthermore, LiDAR-derived high resolution DEMs were used as cost rasters through
a least-cost path approach to perform a semi-automatic (supervised) mapping of some GQVs, using the plugin
GeoTrace. This allowed to improve algorithmically the trace of some GQVs, when DEM resolution and vein
relief were high enough that veins appeared as topographic ridges. After this shape correction, geometry
calculations were applied to the GQVs to derive their outcropping area, width, length and mean azimuth,
obtaining the GQVs Final Layer (Figure 1). The attributes included in both the GQVs Final Layer and the Host
Rock Final Layer were joined by location to derive de GIVEPY database, where the 743 GQVs of the Pyrenees
are indexed as spatial features and include, as individual attributes, geological and geospatial information about
their geometry, host rocks and regional geological setting (Figure 1).
Results obtained allowed to carry out a statistical-geometrical approach of the GQVs size, orientation and
distribution through the FracPaQ toolbox integrated in MATLABTM (Healy et al. 2017). The GIVEPY database
aims to be a useful resource for academic and industry research, i.e., representing a “targeting tool” for location
choice before carrying out structural or geochemical investigations on GQVs, their host rocks or their related
(macro to micro) structures. Furthermore, the GIVEPY database can also be used for structural geology teaching,
as it provides information of exceptional outcrops of quartz veins. This work is a contribution to the 2017SGR-
824 Research Group and to projects RyC-2018-026335-I and PGC2018-093903-B-C22 (MCIU / AEI / FEDR /
UE). PhD of EGE is supported by Generalitat de Catalunya and European Social Fund (2021 FI_B 00165).
Figure 1. Schematic workflow of the GIVEPY database building process, its contents and the future updates planned for
2022-2023. 1Bureau de Recherches Géologiques et Minières; 2Institut Cartogràfic i Geològic de Catalunya; 3Instituto
Geológico y Minero de España; 4Institut d’Estudis Andorrans 5Instituto Geográfico Nacional; 6Copernicus Open Access Hub.
Bons, P.D., Elburg, M.A. & Gomez-Rivas, E., 2012. A review of the formation of tectonic veins and their microstructures.
Journal of Structural Geology 43: 33–62.
Healy, D., Rizzo, R. E., Cornwell, D. G., Farrell, N. J., Watkins, H., Timms, N. E., Gómez-Rivas, E. & Smith, M. (2017).
FracPaQ: A MATLAB™ toolbox for the quantification of fracture patterns. Journal of Structural Geology 95: 1-16.
Mondal, T.K. & Mamtani, M.A., 2013. 3-D Mohr circle construction using vein orientation data from Gadag (southern India)
Implications to recognize fluid pressure fluctuation. Journal of Structural Geology 56: 45–56.
ResearchGate has not been able to resolve any citations for this publication.
Article
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Article
In this paper orientations of quartz veins from the Archaean age lode-gold bearing region of Gadag (southern India) are used to determine the relative stress and fluid pressure (Pf) conditions by constructing 3-D Mohr circle. Anisotropy of magnetic susceptibility (AMS) analysis of the host massive metabasalt reveals that the magnetic foliation is NW–SE striking, which is related to early NE–SW compression (D1/D2 deformation) that affected the region. The quartz veins have a wide range of orientations, with NW–SE striking veins (steep northeasterly dips) being the most prominent. Vein emplacement is inferred to have taken place under NW–SE compression that is known to have caused late deformation (D3) in the region. It is argued that the NW–SE fabric defined the pre-existing anisotropy and channelized fluid flow during D3. The permeability was initially low, which resulted in high Pf (>σ2). 3-D Mohr circle analysis indicates that the driving pressure ratio (R′) was 0.94, a condition that favoured fracturing and reactivation of fabric elements (foliations and fractures) having a wide range of orientations. This led to an increase in permeability and fluid flowed (burped) into the fractures. Resulting vein emplacement and sealing of fractures led to a reduction of Pf (<σ2). It is argued that at this low Pf, NW–SE oriented structures continued to remain susceptible to reactivation and vein emplacement, while fractures of all other orientations were inactive and remained sealed. As a consequence, the study area has a cluster of NW–SE oriented veins. R′ is calculated to be 0.07 from 3-D Mohr circle analysis at low Pf, when fractures with NW–SE orientation only were susceptible to dilation. However, it is envisaged that any emplacement of veins in these fractures would have sealed them, thus reducing the permeability and initiating the next cycle of rise in Pf (>σ2). Thus, it is concluded that the quartz veins in the Gadag region are a consequence of an interplay between conditions that fluctuated from Pf > σ2 to Pf < σ2.
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Veins are common features in rocks and extremely useful structures to determine stress, strain, pressure, temperature, fluid composition and fluid origin during their formation. Here we provide an overview of the origin and terminology of veins. Contrary to the classical tripartite division of veins into syntaxial (inward growth), antitaxial (outward growth) and stretching veins (no consistent growth direction), we emphasise a continuum between syntaxial and stretching veins that form from the crack-seal process, as opposed to antitaxial veins that grow without the presence of an open fracture during growth. Through an overview of geochemical methods that can be applied to veins we also address the potential, but so far little-investigated link between microstructure and geochemistry. There are basically four mechanisms with increasing transport rates and concomitant decreasing fluid–rock interaction: (1) diffusion of dissolved matter through stagnant pore fluid; (2) flow of fluid with dissolved matter through pores; (3) flow of fluid with dissolved matter through fractures and (4) movement of fractures together with the contained fluid and dissolved matter (mobile hydrofractures). A vein system is rarely the product of a single transport and mineral precipitation mechanism, as these vary strongly both in space and time within a single system.