Exploring and Mapping Marine Placers in Vigo Estuary Shoreline

Using GIS Cartographic Tools

Wai L. Ng-Cutipa

1,4 a

, Ana Lobato

, Francisco J. González

, Georgios P. Georgalas

Irene Zananiri

, Joana Cardoso-Fernandes

and Ana C. Teodoro

Geological Survey of Spain (IGME-CSIC), Rios Rosas 28, Madrid, Spain

Hellenic Survey of Geology & Mineral Exploration (HSGME), Spirou Loui 1, Athens, Greece

Institute of Earth Sciences, Faculty of Sciences, University of Porto, Rua Campo Alegre s/n, Porto, Portugal

Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, José Antonio Novais 12, Madrid, Spain

Keywords: Marine Placer, Integration, GIS, Heavy Minerals, Cartography.

Abstract: Marine placer deposits are accumulations of heavy minerals in coastal areas, both on beaches and in shallow

water, usually consisting of ilmenite, rutile and zircon, and less commonly rare earth minerals (REEs) such

as monazite and xenotime. This study investigates marine placer deposits in the Vigo Estuary (NW Spain),

focusing on integrating diverse on and offshore cartographic data (geology, mineral resources, drainage,

bathymetry, tides, Earth observation and others) for exploration. Six hundred two information points of

marine placers have been analysed, 379 of them from shallow water, where Thiessen polygons have been

spatially calculated. Our results, integrating regional cartographies in a Geographic Information System (GIS),

shown great potential of placer minerals on the Santa Marta and Vao beaches (with presence of garnet,

ilmenite, zircon, monazite and, locally, xenotime). This work 1) highlights the importance of collecting and

analysing different previous information for marine placer exploration in integrated digital cartographies, 2)

allows to program new activities to investigate local areas, 3) remark the remote sensing applications (cheap,

easy and non-invasive tool), better applied to inaccessible areas, and 4) contribute to allow a sustainable

resource exploration and coastal management.

1 INTRODUCTION

Marine placer deposits are accumulations of heavy

minerals found on beaches and shallow water near the

coast (Rona, 2008; Van Gosen et al., 2014; Hou et al.,

2017). Some minerals of economic interest include

ilmenite and rutile (Ti), zircon (Zr), cassiterite (Sn),

monazite and xenotime (Rare Earth Elements- REE),

gold (Au), diamonds, and gems, which accumulate

due to their higher density compared to other

minerals. These deposits are distributed globally on

most coasts (Rona, 2008). In particular, marine placer

https://orcid.org/0000-0003-2431-0604

https://orcid.org/0000-0002-8620-6506

https://orcid.org/0000-0002-6311-1950

https://orcid.org/0009-0003-1180-2519

https://orcid.org/0000-0002-3440-4316

https://orcid.org/0000-0001-8265-3897

https://orcid.org/0000-0002-8043-6431

deposits with REE content are located on the coasts

of South Carolina and Florida (USA), Australia,

Brazil, India, Sri Lanka, China, Thailand and

Malaysia (Segupta and Van Gosen, 2016).

Marine placers contain several critical raw

materials (CRMs) that can contribute to the supply of

the European Union (EU) (Grohol and Veeh, 2023)

and other countries, thereby supporting the ecological

transition. Indeed, the European platforms EMODnet

Geology (https://emodnet.ec.europa.eu/en/geology),

GeoERA-MINDeSEA and EGDI provide an

overview of marine mineral resources in their reports

Ng-Cutipa, W. L., Lobato, A., González, F. J., Georgalas, G. P., Zananiri, I., Cardoso-Fernandes, J. and Teodoro, A. C.

Exploring and Mapping Marine Placers in Vigo Estuary Shoreline Using GIS Cartographic Tools.

DOI: 10.5220/0013496900003935

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 11th International Conference on Geographical Information Systems Theory, Applications and Management (GISTAM 2025), pages 339-345

ISBN: 978-989-758-741-2; ISSN: 2184-500X

339

and viewer, including marine placer deposits and

occurrences (Zananiri et al., 2021; González et al.,

2023; GSEU, 2024).

Multiple authors have described these deposits

and proposed their exploration from different

approaches, such as the commodity approach (e.g.,

focused on Ti according to Force (1991)), geological,

geochemical and geophysical techniques (Van Gosen

et al., 2014), and specialized mapping (GIS), aerial

photography and topography, remote sensing

techniques, geological methods, drilling, and 3D

modelling (Hou et al., 2017). Recently, and along

with advances in Earth observation (EO), remote

sensing applications have been initiated for placer

exploration (e.g., Rejith et al., 2020 and 2021;

Cardoso-Fernandes et al., 2023; Ng-Cutipa et al.,

2024). This work proposes a methodology for

integrating land-sea information in a geographic

information system (GIS), aimed at: 1) analysing

existing information layers to select local pilot areas,

and 2) applying field exploration and EO tools

focused on the study of marine placers in NW Spain.

2 REGIONAL SETTING

The Vigo Estuary, one of the Rías Baixas estuaries, is

located in NW Spain. The area's geology comprises

intrusive and metamorphic rocks ranging in age from

the Paleoproterozoic to the Jurassic (González and

Vicente, 2004). The study area is situated within the

MAGNA 50k map sheets of Cíes (0222) and Vigo

(0223), where the estuary is located, with an

approximate NE-SW orientation. In the outer part of

the estuary, the Cíes Islands are located, which are

oriented N-S and act as a natural geographic barrier

that protects the inner area of the estuary (Figure 1).

The main tributary river of the estuary is the Verdugo

River; however, there are numerous smaller tributary

drainages, with those located on each side of the

central axis of the estuary being much shorter.

3 DATA AND GIS INTEGRATION

Previous studies have synthesized and grouped

exploration techniques for marine placer deposits

based on geological, geophysical, remote sensing

conditions, etc. (e.g., Van Gosen et al., 2014; Hou et

al., 2017). Within the S34i project, we have focused

on geological information at regional and local scales,

which can contribute to the successful search for

marine placer occurrences in beach and shallow-

water sands. In general, we have grouped the

available data by their geographic environment with

respect to their coverage: on-shore, off-shore and

intertidal zones. To this, we have added a fourth

group, "EO data" (Figure 2), that, depending on the

characteristics of the image, can be used to obtain

information from the three zones described above.

We have also integrated a fifth group called

"restrictive areas", which are areas of natural

protection, economic activities or other

specifications.

Figure 1. Location of Vigo estuary.

3.1 On-Shore Data

The analysed datasets correspond to:

-Topography from the National Geographic

Institute (IGN) of Spain

(https://www.ign.es/web/ign/portal/inicio), which

constitutes the base layer with information on

toponymy and hydrography. The sheets used are 0222

(Cíes) and 0223 (Vigo).

-Basins and drainages from MITECO

(https://www.miteco.gob.es/es/cartografia-y-

sig/ide/descargas/agua.html) at a scale of 1:25,000 (©

Ministerio para la Transición Ecológica y el Reto

Demográfico), which are located within the map

sheets 0222 and 0223.

-Continuous digital geological cartography at a

scale of 1:50,000 (GEODE survey,

https://info.igme.es/cartografiadigital/geologica/geo

de.aspx) from the IGME-CSIC (© CN Instituto

Geológico y Minero de España (IGME), with

geological and structural information of the Z1200

Galicia Tras-Os-Montes area. Around the estuary, a

clear predominance of intrusive and metamorphic

rocks with an approximate N-S orientation is

observed. Quartz dikes can be secondary sources of

heavy minerals of economic interest, such as

tourmaline, cassiterite and gold; however, at a scale

S34I 2025 - Special Session on S34I - From the Sky to the Soil

340

of 1:50,000, they are scarce around the estuary,

according to GEODE.

-Mineral resources database (BDMIN,

http://info.igme.es/catalogo/resource.aspx?portal=1

&catalog=3&ctt=1&lang=spa&dlang=eng&llt=drop

down&master=infoigme&resource=23) from the

IGME-CSIC (© CN Instituto Geológico y Minero de

España (IGME)), is a database that integrates

geological-mining information on evidence, deposits

and exploitation of rocks and minerals in Spain.

Figure 2. Integration methodology for marine placer deposit

exploration in the NW Spain.

3.2 Off-Shore Data

Data related to:

-Bathymetry and seabed substrate data are from

EMODNet Geology

(https://emodnet.ec.europa.eu/en/geology), a

European portal that provides harmonized data. These

allow us to know the morphology and type of seabed.

In the study area, the seabed substrate with Folk 7

classification is available.

-The mineral exploration studies of the

continental shelf of the FOMAR project (IGME, 1976

and 1979) have samples of beach and shallow-water

and semi-quantitative analyses of heavy mineral

contents in Rias Baixas. The most abundant minerals

are generally garnets, ilmenite, zircon and monazite.

-Thiessen polygons were calculated using ArcGIS

from the samples collected on the shallow water in

the FOMAR project, providing an approximate area

of similar heavy mineral content. Ng-Cutipa et al.

(2023) classified the heavy mineral content into five

classes: <1 g, 1-5 g, 5-50 g, >50 g and absence.

-EMODNet-Geology marine resources

(https://emodnet.ec.europa.eu/en/geology) provide us

with marine mineral occurrences in point and polygon

format.

-Other publications, such as these articles and

conferences, include various data related to marine

placer occurrences( Manso, 2001; Ng-Cutipa et al.,

2024).

3.3 Intertidal Zone Data

Low and high tide data from the Hydrographic

Institute (IHM) of Spain provide a regional overview

of the intertidal area, which can influence the surface

occurrence of placer deposits. As mentioned before,

the NW area of Spain has a mesotidal regime, with

variations of 2 to 4 m in height in the intertidal zone.

3.4 EO Data

National Aerial Orthophotography Plan (PNOA) and

Light Detection and Ranging (LiDAR) surveys from

IGN (https://pnoa.ign.es/web/portal/inicio), consist

of orthorectified visible and near-infrared imagery,

and digital elevation information at a very high local

resolution. The PNOA images (PNOA 2020 CC-BY

4.0 ign.es) have a spatial resolution of 15 cm per pixel

in the visible and 25 cm per pixel in the near-infrared.

For geological mapping, the most current images in

the area (year 2020) have been used; the near-infrared

has been used to highlight faults and lineaments (Ng-

Cutipa et al., 2024). The most recent LIDAR flight

(PNOA LiDAR 2020 CC-BY 4.0 ign.es) is from

2015, with a minimum point density of 0.5 - 2

points/m².

Additionally, images were collected through a

UAV survey using a DJI Mini Pro drone for specific

beach research. In March 2024, drone flights were

carried out at 80 and 50 m altitudes, obtaining a

spatial resolution of < 3 cm in the visible range. The

images were taken on the Limens, Santa Marta, Vao,

Fontaiña and Fechiño beaches. EO data provide the

map beach morphology and better identify rock

outcrops, dykes, faults and lineaments, and

delineation of modern placer boundaries.

3.5 Restrictive Areas

This level of information includes the on- and off-

shore areas with access restrictions, areas of local

interest, economic activities, environmental

Exploring and Mapping Marine Placers in Vigo Estuary Shoreline Using GIS Cartographic Tools

341

conservation, and those related to navigation and port

activities of the Vigo Estuary (Figure 2).

4 RESULTS

4.1 Thiessen Polygons and Heavy

Mineral Occurrences

In the Vigo Estuary, 602 information points from the

FOMAR project (223 from the beach and 379 from

the shallow water) have been identified, obtaining

379 Thiessen polygons on the shallow platform,

maintaining the same semi-quantitative values of

heavy mineral concentration (garnet, ilmenite, zircon,

monazite and xenotime; Figure 3).

On the beach, occurrences of garnet of Class 4

(>50 g) are predominant compared to Class 3 (5-50 g,

Figure 3). These are found in the middle-outer part of

the estuary. Ilmenite (brown lines in Figure 3) are

mainly associated with the same points where Class 4

(>50 g) garnets are located, except for the middle part

of the estuary (E of Cangas), where there is an

occurrence of ilmenite and zircon, both of Class 3 (5-

50 g). In addition, zircon with concentrations of Class

4 (>50 g) is only related to Class 4 samples of garnets

and ilmenite on the Santa Marta (W of Cangas) and

Vao-Fuchiños (SW of Vigo) beaches.

Figure 3. Thiessen polygons in shallow water and heavy

mineral content classes 4 and 3.

To a lesser extent, there are also occurrences of

REE minerals, such as monazite and xenotime.

Monazite (green dots in Figure 3) is related to the

Class 4 (>50 g) points described above, with abundant

garnets, ilmenite and zircon (Santa Marta and Vao-

Fuchiños). Monazite is also found to the E of Cangas,

as well as ilmenite and zircon, all Class 3. Xenotime

is only identified with Class 1 (<1 g) in 3 samples

from the same Cesantes Beach, in the inner part of the

estuary (green X in Figure 3). Also, on the beaches

around the Cíes Islands, garnets and ilmenite

predominate.

On the shallow platform, there are no

concentrations of class 4 or 3 in the estuary,

indicating that there are no areas of preferential

concentration in these classes.

4.2 Regional Integrated Map

The geological environment of the Vigo Estuary is

dominated by granites, where outcrops of

orthogneisses, schists and amphibolites mainly

alternate, and to a lesser extent, quartzites and para-

gneisses. On the surrounding seabed, the rocky

substrate and boulders continue from the middle to

the outer part of the estuary, including the Cíes

Islands. The interior of the estuary has a finer

sediment (mud) that changes to mixed mainly. The

other grain size transitions of the seabed appear more

isolated towards the outside of the estuary. The faults

and lineaments on land are generally NW-SE, NNW-

SSE and NE-SW (Figure 4).

Mineral resources onshore are reported as

indications of hematite (Fe, Mn, Ti) and feldspar

(industrial mineral) in the inner part of the estuary

around Redondela. There are several active and, to a

lesser extent, abandoned exploitations of ornamental

rocks (mainly granites). In addition, there are three

zones with minerals hosted in rocks, pegmatites or

dikes and one zone of marine placer occurrences.

These last ones have identified garnets, ilmenite,

zircon, monazite and xenotime, and other heavy

minerals on the beach. As we have seen before in 4.1,

these heavy minerals are less abundant on the shallow

platform.

The areas of human activity and current restrictive

areas have helped us to better plan our field trips and

explore the occurrences of marine placers, previously

identified in the 70s. Some areas of the estuary are no

longer accessible due to the creation of new restricted

areas.

S34I 2025 - Special Session on S34I - From the Sky to the Soil

342

Figure 4. Integrated cartography for placer deposit

exploration in Vigo Estuary, NW Spain.

4.3 Validation: Local Pilots'

Exploration

With the information from 4.2 and the flowchart in

Figure 2, we proceeded to schedule different local

reconnaissance activities to validate the

methodology, starting with an initial visit to confirm

the presence of marine placers on beaches. Upon

finding evidence of heavy minerals, we scheduled

subsequent exploration activities to delve deeper into

the study of marine placers, mainly on the Santa

Marta and Vao beaches. For example, we carried out:

detailed mapping using PNOA and UAV flights (for

outcrops, beach extensions and areas with placers,

Figure 5), identification of faults and dikes (in the

field and with PNOA and UAV images), analysis of

intertidal variation, pits to observe heavy minerals in

depth, ground-penetrating radar exploration, among

others.

Figure 5. Cartography from the UAV survey showing

placer mineral (red polygons) in coastal areas of Santa

Marta Beach (Ng-Cutipa et al., 2024).

The campaigns in different seasons have allowed

to identify the variation of heavy mineral

concentrations and the variation of small streams that

contribute to the supply of these minerals from the

onshore. The important role of tides in the

redistribution and deposition of placers has also been

observed. Likewise, in some areas, such as Vao, wind

action plays an important role in accumulating heavy

minerals at the base of the dune face where the wind

has action (Figure 6).

Figure 6. Accumulation of reddish heavy minerals on the

base of the coastal dune at Vao Beach, Vigo Estuary

(November 2024).

Sand samples have also been taken from different

beaches and shallow water, with and without placers,

to conduct chemical and mineralogical analyses, and

reflectance measurements for their correlation and

use in multi- and hyperspectral images.

5 DISCUSSIONS AND

CONCLUSIONS

Thiessen polygons offer a primary spatial distribution

of occurrence of heavy mineral in shallow-water, that

were perfomed using semiquantitative data. Placer

occurrence on beach show greater accumulation than

shallow-water areas using the same semi-quantitative

data.

Regarding to regional integration map, the

exploration of marine placer deposits, as evidenced

by Van Gosen et al. (2014) and Hou et al. (2017),

highlights the importance of collecting and analysing

different previous information. These depend on the

state of advancement of the region and on the

knowledge desired to acquire about marine placer

deposits. With this, it is possible to propose

complementary studies at the regional level and

others to deepen research at the local level of these

placer occurrences. On the other hand, existing

geological maps at a scale of 1:50,000 offer a great

approximation to select areas of greater interest to

explore these deposits. It is true that, in the case of

Exploring and Mapping Marine Placers in Vigo Estuary Shoreline Using GIS Cartographic Tools

343

NW Spain, the occurrences of marine placers of

FOMAR (IGME, 1976 and 1979) and the occurrences

of heavy minerals previously identified in the

MAGNA project carried out by IGME

(https://info.igme.es/cartografiadigital/geologica/Ma

gna50.aspx) have facilitated the visualization of the

aforementioned areas of interest.

Due to the same difficulty of access and nature of

each area, geological maps and information are more

detailed and updated on-shore than off-shore.

In the local pilots, UAVs offer a high spatial

resolution image where surface placers have been

identified around the shoreline. This survey depends

on the season and the stage of development of the

ephemeral drainages. It is very likely that the large

exposure of surface placers is mainly due to the large

drainage combined with wave and tidal action.

This work highlights the importance of 1)

integrating on- and off-shore information and the

intertidal zone, and 2) knowing the characteristics of

marine placer occurrences in NW Spain, providing

very good results. They have also allowed for the

increase of knowledge of the coastal "white band"

(land-sea transition). Similarly, EO surveys offer a

cheap, easy and non-invasive tool that can be better

applied to inaccessible areas, allowing for sustainable

resource exploration and coastal management.

Finally, in these last 2 years, improvements have been

developed in the knowledge of placers at the local

level that include integrated and improved geological

mapping, new geochemical and mineralogical data of

marine placers, and EO applications and development

of algorithms for images of different satellites

(Sentinel 2, WorldView, EnMap, others).

ACKNOWLEDGEMENTS

This study is funded by the European Union under

grant agreement no. 101091616

(https://doi.org/10.3030/101091616), Project S34I –

SECURE AND SUSTAINABLE SUPPLYOF RAW

MATERIALS FOR EU INDUSTRY.

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