A mineralogical-geomorphological terrain analysis of hotspot volcanic islands

The missing link between carbonatite- and pegmatite Nb-F-Zr-Li-Be-bearing REE deposits and new tools for their exploration (Canary Islands Archipelago, Spain)

verfasst von

Harald G. Dill, Andrei Buzatu, Sorin Ionut Balaban, Kurt A. Rüsenberg

Abstract

Two geoscientific disciplines (mineralogy, geomorphology) which are placed far apart from each other are selected to forge a genetic tie line between carbonatite- and agpaitic pegmatite-hosted REE deposits, likewise far away from each other. The Canary Islands Archipelago, Spain, (=CIA) is known as a hotspot hosting REE among other rare elements (28 Ma to the Recent) in volcanic landforms. This rare metal metallogenic province is connected to a carbonatite-pegmatite REE province in NNW Africa. It is a binary mineralization with one hub covering SW Tenerife and NE La Gomera that is characterized by an alkali-magmatite-related REE system with pegmatitic syenites at depth and another hub covering Central - and NW Fuerteventura where (meta) carbonatites are exposed and form a bridge to the REE-bearing carbonatites and pegmatites on mainland Africa, Morocco. The REE concentration took place in four stages which differ from each as with regard to their landscape and mineralogy: (1) magmatic calcic-carbonatites with REE-bearing apatite (28 to 24 Ma) (2) calcic-carbonatites, alkali magmatites with REE phosphate and carbonates (contact-metamorphic - metasomatic, epithermal) (19 to 4 Ma) (3) alkali magmatites with hydroxyl-group-bearing REE phosphates, REE oxides, Fe oxide, Fe-Cu sulfides, kaolinite-halloysite, and REE-APS minerals (epithermal) (5 to 0.3 Ma) (4) pyroclastic and volcanic rocks with smectite, kaolinite, REE-bearing silicates and oxides (auto-metasomatic) and a variegated series of more than 100 REE-, Nb-, F-, Zr-, Li- and Be minerals which are diagnostic for carbonatite- and/or alkali-magmatites pegmatitic systems at depth (<0.6 Ma). This REE mineralization of the CIA extending into on-shore Morocco is akin to the REE mineralization recorded from titanomagnetite–magnetite–apatite-REE- deposits in carbonatite-alkaline complexes in N Europe with the reference catena-deposits Kiruna-Sokli-Chibina (Sweden, Finland, Russia). The NNW African REE metallogenic province shows from E to W four geodynamic zones: (1) Atlas Mts. REE - Fe Province (thick continental crust of the mainland), (2) Fuerteventura (thick continental shelf pinching out to the W), (3) Gran Canaria – Tenerife-La Gomera (thin continental shelf to oceanic crust), (4) El Hierro – La Palma (barren zone as to REE oceanic crust).The REE mineralization starts off with syn- to post-kinematic en-echelon strike-slip faulting followed by a clock-wise rotation of its morpho-structural elements and wanes with a reactivation of NE-SW morpho-structural elements as normal faults attesting to a decoupling from the Atlas Mts. deformation and intensification of an individual hotspot mineralization. The land-forming processes and the resultant volcanic landscape are genetically linked to the REE-, Nb-, F-, Zr-, Li-, and Be mineralization: mass wasting (MW) > fluvial drainage (FD) > lacustrine (L), ≥ coastal marine (CM) ≫ aeolian (A). Each of the land-forming processes provides marker landforms indicative processes relevant for rare element accumulations: MW: thrusting, exhumation of the level of primary REE mineralization, zones of structural weakness, FD: morphostratigraphic tracing of fertile bedrocks, demarcation of ring dyke complexes and zones of contact-metamorphic and contact-metasomatism, zones of ground- and surface water stagnancy, zones of strong argillitization and hematitization, CM: exhumation, providing marker heavy minerals and gravel lithoclasts, L: rifting structures, zones of volcanic and epithermal activity, A: pathfinder heavy minerals. The methodological approach taken throughout this composite study (field- and laboratory-based research work combined with a meticulous review of the mineralogical, chronological and geological regional literature) is that of a mineralogical-geomorphological terrain analysis. Numerical geomorphological and sedimentological parameters are of assistance during remote sensing and ground follow-up mapping: (1) elevated degree of sinuosity and argillitization, (2) numerical classification of the terrain based on dip of slope plus altitude, (3) gravel analysis based on granulometry, morphology and situmetry (GMS technique) for provenance and environment analysis. The morphodynamic elements have a high preservation potential and are not blurred by morphoclimatic ones so that the mineralogical terrain analysis can be taken as a trusted exploration tool for REE. Geodynamic and volcanic activity are the driving forces and prevail over the climate impact in shaping the landscape. Therefore, landforms in combination with compositional data from mineralogy can shed some light on the origin of REE-Nb-F-Zr-Li- Be mineral associations on hotspot volcanic islands. This holistic approach can also be taken to different mineral commodities elsewhere in the world given a low maturity of the landscape in the target area. It can also be used for submarine landforms in and around an archipelago simply by minero-stratigraphic correlation from land to sea. The mineralogical terrain analysis as applied in the current study is not a fixed methodological construct but open also for replacements and supplements by e.g. geophysics.

Organisationseinheit(en)

Gottfried Wilhelm Leibniz Universität Hannover

Externe Organisation(en)

Al. I. Cuza University
Birkbeck University of London

Typ

Artikel

Journal

Ore geology reviews

Band

163

Anzahl der Seiten

60

ISSN

0169-1368

Publikationsdatum

12.2023

Publikationsstatus

Veröffentlicht

Peer-reviewed

Ja

ASJC Scopus Sachgebiete

Geologie, Geochemie und Petrologie, Ökonomische Geologie

Ziele für nachhaltige Entwicklung

SDG 14 – Lebensraum Wasser

Elektronische Version(en)

https://doi.org/10.1016/j.oregeorev.2023.105702 (Zugang: Offen)