Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial II, 821-823
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Classification of rare earth deposit and occurrence
types: what’s new?
Classificação de depósitos e ocorrências de terras raras:
uma nova perspectiva
D. Oliveira1*, C. Inverno1
Artigo Curto
Short Article
.
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The wealth of minerals and rocks which form the building
blocks of mineral deposits has very early sparked attempts among
geoscientists to classify mineral deposits and refine their
terminology. Traditionally, the rare earth deposits and occurrences
have been placed under broad umbrellas but this trend is changing as
new information comes to light fueled largely in Europe by the
recognition that rare earth supply is officially regarded as critical.
New information remains the keystone in updating or refining
existing classification schemes and deposits types that fall outside
these schemes must be added. Two such examples in Portugal are
Vale de Cavalos (Portalegre) and Monfortinho (C. Branco).
Keywords: Rare earths, Deposits, Occurrences, Types,
Classification.
Resumo: A riqueza de minerais e rochas que formam os blocos de
edificação dos depósitos minerais desde muito cedo levou a
tentativas entre os geocientistas para classificar os depósitos minerais
e refinar a sua terminologia. Tradicionalmente, os depósitos e
ocorrências de terras raras foram colocados sob grandes chapéus
classificativos mas esta tendência está a mudar à medida que novas
informações vêm à luz e que é alimentada em grande parte pelo
reconhecimento de que a oferta de terras raras é oficialmente
considerada como crítica na Europa. Informação actualizada continua
a ser fundamental para aperfeiçoar sistemas de classificação de tipos
de depósitos e ocorrências que não se enquadram nesses regimes
classificativos pelo que devem ser adicionados aos existentes. Dois
exemplos em Portugal são Vale de Cavalos (Portalegre) e
Monfortinho (C. Branco).
Palavras-chave: Terras raras, Depósitos, Ocorrências, Tipos,
Classificação.
1
LNEG – Laboratório Nacional de Energia e Geologia, I.P., Unidade de
Recursos Minerais e Geofísica, Estrada da Portela - Zambujal, Apartado 7586,
2610-999 Amadora, Portugal.
*
Corresponding author / Autor correspondente: [email protected]
1. Introduction
Mineral deposits result from physical–chemical changes in
the atmosphere, hydrosphere and lithosphere; the latter
being made up of the crust and the upper mantle.
There are physical barriers hampering our access to
mineral deposits, such as mining depth controlled among
others by the geothermal gradients and the vast and deep
oceans, which cover more than 2/3 of the Earth’s surface.
Economic geology is not a discipline of its own but
rather a mixtum compositum of various disciplines of earth
sciences dedicated to finding new inorganic raw materials
and enhance the exploitation of those already known.
Geology and mineralogy are the key players in achieving
these goals (Dill, 2010).
The wealth of minerals and rocks which form the
building blocks of mineral deposits has very early sparked
attempts among geoscientists to classify mineral deposits
and refine their terminology. Lindberg (1922) and
Lindgren (1934) were among the pioneers who addressed
this problem in a modern way; numerous others followed
suit and joined in the attempt to put in order the newly
discovered mineral deposits worldwide as well as those
which have already been mined for ages.
Traditionally, the rare earth deposits and occurrences
have been placed under broad umbrellas but this trend is
changing as new information comes to light fueled largely
in Europe by the recognition that rare earth supply is
officially regarded as critical (UE, 2010).
Here we explore the rare earth elements, re-examine
the classification schemes in place and add to them using
new or little known information.
2. The rare earths
The term Rare Earth Elements, normally abbreviated to
REE, is misleading. Many of the REE are more widespread
in the earth's crust than elements widely known in our daily
life. For example, copper has an average concentration
lower than cerium, and
mercury averages lower than
thulium which is the rarest among the REE. About 200 rare
earth minerals are distributed in a wide variety of mineral
classes (Kanazawa & Kamitani, 2006).
Light rare earth elements (LREE) tend to occupy the
larger sites of 8–10 coordination numbers and concentrate
in carbonates and phosphates. On the other hand, heavy
rare earth elements (HREE) and Y occupy 6–8
coordination numbers sites and are abundant in oxides and
predominate as a part of phosphates. Additionally, the
middle rare earth elements (MREE), that include Sm, Eu,
Gd and Tb, also predominate as constituents of phosphate
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D. Oliveira, C. Inverno / Comunicações Geológicas (2014) 101, Especial II, 821-823
minerals. Actually, the most common ore minerals of REE
are phosphates: monazite, xenotime, ningyoite, florencite
and rhabdophane. They occur in metamorphic and
magmatic rocks and also result from supergene or other
alteration. Second most in abundance are REE carbonates
such as bastnaesite, synchysite, lanthanite and parasite
which are of widespread occurrence in carbonatites and
often accompanied by REE oxides, e.g., cerianite and
knopite and REE niobates–titanates, e.g., fergusonite and
aeschynite (Dill, 2010).
REE silicates (e.g. cerite) and REE arsenate (e.g.
kemmlitzite) are of minor economic or only academic
interest (Hak et al., 1969).
3. Classification of REE deposits
Classification, as in all classification schemes, is
invariably dependent on the amount of information
available. Classification schemes are often simplifications
and even oversimplifications of the natural complexity of
REE mineral deposits and occurrences. For example, in
many instances, a given REE deposit at the mining camp
level contains two or more REE deposit types.
A few known classification schemes are in use and
most are often compilations, e.g. (Dill, 2010; Walters &
Lusty, 2011; Lapido-Loureiro, 2013) of work carried out
by a variety of authors. According to Dill (2010), in terms
of the classification of REE deposits, traditionally there
has always been the separation into three broad classes (A
to C, below); taking into account the host rocks and
mineralogy, namely:
processes that concentrate and form REE minerals and
enable the formation of REE mineral deposits using key
examples. They believe that this creates a more robust
classification scheme than those late ones, though noting
that this classification as all classifications relies on the
amount of information available; further information may
change the classification type, as the case of the Bayan
Obo REE deposit in China testifies (Weng et al., 2013).
As stated above, all classification schemes are
invariably dependent on the amount of information
available. It is therefore important that all available
information be gathered to extend and complete this or any
other classification scheme.
The occurrence in Portugal of little-known types of
REE deposits and occurrences that fall outside the
presented classification schemes is noteworthy; specific
examples are of those occurring in the consolidated
sediments that outcrop in the northern Ossa Morena Zone
and in the southern Central Iberian Zone; namely, Vale de
Cavalos (Portalegre) and Monfortinho (C. Branco) (s.l.),
respectively, must be included in this classification scheme
(Table 1).
Table 1. The REE deposit type classification scheme proposed by Weng et
al. (2013) adapted to include the new Consolidated Sediment(*) class.
Tabela 1. Esquema classificativo dos depósitos de terras raras segundo
Weng et al. (2013) e a sua adaptação para incluir a nova classe
Consolidated Sediment(*).
3.1. Deposit types
A) Magmatic rare earth deposits
• REE-P–Nb–Ta–Y–F–(Be–Zr–Th) deposits related to
carbonatites
• REE-P–Ti deposits related to alkaline igneous
complexes
• REE-U–Nb-bearing
pegmatites
(in
places,
transitional into intragranitic deposits with Mo–W–
U–Be)
• REE-Nb–P–F– bearing hydrothermal iron deposits
• Be- and Y-bearing alkaline intrusive rocks (nepheline
syenite)
B) Structure-related rare earth deposits
• REE-F–Ba–Th- bearing vein-type deposits
C) Sedimentary rare earth deposits
• REE-(Ti–P–Nb) residual deposits / placers on
alkaline igneous and carbonatites complexes
• REE in bauxite
• Alluvial- to coastal REE placers
• REE-bearing phosphorites
• Ion adsorption clays
• REE-bearing coals
More recent work by Weng et al. (2013) proposes
slight modifications to this classification method (Table 1).
Their classification approach rather than focusing on the
mineralogy and host rocks focuses on the geological
1. REE occurrence – quartzite- and arkose-hosted.
a. Description – Vale de Cavalos and Monfortinho
(Portugal); deposit and occurrence of nodular
monazite-rich horizons prominently enriched in Eu,
within dark, fine-grained Arenigian quartzites
(Oliveira, 1998; Inverno et al., 1998, 2007).
b. Description – Beira Baixa and Monfortinho
(Portugal); Occurrence of concentrations of
Rare Earth deposit Classification
appreciable monazite and lesser xenotime in
Tertiary arkoses (Inverno et al., 2007; Rosa et al.,
2010).
There is no equivalent to the arkose-hosted REE
mineralization either in the Dill’s (2010) or Weng et al.’s
classifications. The closest slot in this last classification to
the quartzite-hosted REE deposit/occurrences of Vale de
Cavalos and Monfortinho is, at first approach, that of the
“Shale-hosted” REE deposits and occurrences known in
NW Spain (Sierras de Ancares y Laurel; Vaquero
Nazabal, 1979), France (Brittany; Donnot et al., 1973),
Belgium (Stavelot and Rocroi massifs; Burnotte et al.,
1989), Great Britain (Wales and SW G. Britain; Read et
al., 1987), United States (Alaska; Rosenblum & Mosier,
1983), Canada (Buckton; Weng et al., 2013)
and
elsewhere. Other than the host lithology, the genesis of
these REE deposits is quite different from the quartzitehosted REE deposits in Portugal (Inverno et al., 1998). In
fact the last would in a certain way be, though in distinct
rock types, more similar to the “Heavy mineral sands”
(e.g., WIM 150, Australia; Whitehouse, 2009) of Weng et
al.’s classification, but where consolidation of rock would
occur.
4. Final considerations
Classification schemes are information dependent.
Information remains the keystone in updating or refining
existing classification schemes. The less information
available, the weaker and vague the classification scheme
and vice versa.
In the case of the rare earth deposits and occurrences
types there are surely many data that are yet to surface
regarding specific aspects of the occurrence of these
critical elements. As their criticality rises on the supply
and demand chains, the more information will become
available to better constrain the mode of occurrence, host
rock lithology and their contents in LREE, MREE or
HREE.
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