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Looking for Noble and Rare Metals in
Black Shale Formations of Armenia:
The Primary Investigations
Arshavir Hovhannisyan1, Yelena Panova2, Shahen Khachatryan3, Varduhi
Siradeghyan4, Karen Hambaryan4 And Yeghis Keheyan5*
1Laboratory of Ore Deposits of the Institute of Geological Sciences of NAS RA, Republic of Armenia
2Department of Geochemistry, St Petersburg University, Russia
3Faculty of Geography and Geology, Yerevan State University, Armenia
4Laboratory of Ore Deposits of the Institute of Geological Sciences of NAS RA, Republic of Armenia
5Formerly Prime Researcher at CNR, Department of Chemistry, University of Rome ‘La Sapienza’, Italy
Introduction
Numerous studies on alpine geodynamics and patterns of spatial distribution of metallic
and non-metallic deposits of the Lesser Caucasus, depending on the geodynamic regime of
development of folded areas of Armenia [1], allowed to theoretically predict and later prove in
practice the metallogeny of terrigenous-carboniferous complexes [2-6]. These complexes are
widespread in Armenia. They are represented by terrigenous-coal-bearing and terrigenous-
carbonate-coal-bearing deposits with small thickness of brown coals, oil shales, bituminous
tuff-conglomerates and tuff breccias. The total thickness of the complexes is highly variable
and is between 60-120 and 200m on average. They spread hundreds of meters, and in some
cases from 2-3 to 5km [7-9]. The age of the complexes ranges from the Lower Triassic to
Pliocene (Triassic, Jurassic, Eocene, Oligocene, Miocene, Pliocene), which indicates the multi-
stage of mineralization all over Phanerozoic. In most cases, these deposits have gentle dips
and they outcrop, or have subsurface emplacement and are easy for open-pit mining. The
materials value of the Armenian solid caustobiolites in terms of metallogeny. It is necessary
Crimson Publishers
Wings to the Research Research Article
*Corresponding author: Yeghis
Keheyan, Formerly Prime Researcher at
CNR, Department of Chemistry, University
of Rome ‘La Sapienza’, Piazzale Aldo Moro
5, 00185 Rome, Italy
Submission: May 24, 2023
Published: June 22, 2023
Volume 11 - Issue 4
How to cite this article: Arshavir
Hovhannisyan, Yelena Panova, Shahen
Khachatryan, Varduhi Siradeghyan, Karen
Hambaryan And Yeghis Keheyan*. Looking
for Noble and Rare Metals in Black Shale
Formations of Armenia: The Primary
Investigations. Aspects Min Miner Sci.
11(4). AMMS. 000769. 2023.
DOI: 10.31031/AMMS.2023.11.000769
Copyright@ Yeghis Keheyan, This
article is distributed under the terms of
the Creative Commons Attribution 4.0
International License, which permits
unrestricted use and redistribution
provided that the original author and
source are credited.
ISSN: 2578-0255
Aspects in Mining & Mineral Science 1298
Abstract
The shortage of liquid and gaseous fossil fuels in the world growing day by day has led to intensive searches
for new energy sources. The focus on solid fuels has increased dramatically, particularly on oil shales and
combined. Moreover, solid caustobiolites, in addition to being source of energy, also have another “merit”
undiscovered until recent decades: they are natural accumulators of noble and rare metals. More than
40 deposits and occurrences of brown coal and oil shales are known within the borders of Armenia.
Prospecting work of coal and oil shales, as well as of bituminous formations, has been done for decades.
However, in general, the study of fuel and energy resources of the Republic was unplanned, unsystematic,
and it is currently in unsatisfactory condition. Moreover, the existing preconditions for expanding the raw
share in the energy balance of the Republic of Armenia. More than 16 deposits of brown coal and oil shales
were studied in detail in Armenia during the Soviet years. According to the data received, exploitation of
perspective in terms of fuel raw materials. However, solid fossil fuels have another “aspect” of usefulness,
Targeted combined studies of noble and rare metals in the black-shale complexes are a fundamentally new
Keywords: Armenia; Black shale; Noble and rare metals; Brown coals; Metal bearing
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AMMS.MS.ID.000769.11(4).2023
to implement large-scale complex projects, which will include both
research and geological exploration work. Since 2021, within the
scope of cooperation between the Institute of Geological Sciences of
NAS RA and the Institute of Earth Sciences of Saint Petersburg State
University, explorations have started related to the issue of noble
and rare metals in brown coals and oil shales. In fact, in the work,
an attempt is made to discuss the prospects of the metalbearing of
gray coals and black shale in Armenia. This is especially relevant
in the sense that the aforementioned geological formations have
very low quality as combustible minerals. And their complex use:
obtaining combustion energy and extracting metals can give a real
economic effect in Armenia, which is poor in natural fuels.
Fieldwork, Materials and Methods
In fact, we try to determine the possibilities of the detailed
and comprehensive analysis of black-shale formations of Armenia,
with the assessment of their current hypsometric level and the
perspectives of their application. To study the problem, our
geological-geochemical group has already sampled the previously
known 9 occurrences and deposits of brown coals, oil (black)
shales, and bituminous limestones (Figure 1). Among more than
40 geological sites we have chosen the ones which are physically
more accessible and are more representative. The total number
of samples is 30. The material for research was samples of coals,
oil shales, and bituminous limestones of deposits and occurrences
of Armenia, which are shown in the schematic map (Figure 1).
The samples were taken according to the stratum thickness, from
bottom to top. The distribution of rhenium and trace elements in
black shales has been analyzed through inductively coupled plasma
mass spectrometry (ICP MS).
Figure 1: The schematic map and areas of distribution of sampled deposits and occurrences.
In this case, we used the methodology and analysis algorithm
developed for brown coals [10]. To decompose the samples and
special purity, further cleaned through distillation, and deionized
water were used. The prepared solutions were analyzed with
Agilent-7700 and ELAN-6100 DRC devices. The spectrometry
of atomic absorption allows to implement quantitative analysis
for determining the contents of noble metals (Au, Pt, Pd, Os, Ir,
Ro, Ru), which was carried out according to the method of full
opening with the help of acid with additional melting of the
residue. The weighed portion of the powdered sample weighing
10g was decomposed according to the method developed in the
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AMMS.MS.ID.000769.11(4).2023
laboratory. The sample decomposition method is based on partial,
non-oxidative destruction of the sample by acid mixture, washing,
drying, and ashing the residue, fusing it with sodium peroxide and
dissolving the mixture in hydrochloric acid. The introduction of
oxalic acid into the mixture of destructive reagents prevents the
passage of noble metals into the solution and ensures separation
of metals in the solid phase, if they are present in the sample in the
and the mixture of sulfuric and phosphoric acids contributes to
metals. The destruction of carbon substances is of particular
importance: it eliminates the need for pre-ashing process of carbon
samples. The analyses of the prepared solutions were carried out by
the method of atomic absorption spectrometry with electrothermal
atomization on Analyst-800 devices.
Geochemical features of studied rocks
Comparisons of the contents of Armenian black-shale rocks
(coals, oil shales and bituminous sedimentary rocks, in general, 30
samples) sampled by us with the Clark numbers of other similar
occurrences in the world are presented in Table 1. Compared to
Clarke numbers, the highest values in the black-shale formation
rocks of Armenia (Table 1) are set for gold and silver. Platinum
and palladium are traditionally present in small amounts but can
be considered as accompanying elements. The high content of
-elements - vanadium and uranium should not be ignored. Among
the chalcophile elements, the following are distinguished according
to their high contents (in descending order of Clark numbers of
concentration): Co, Cu, Ni, Mo, Zn, Pb. Due to the fact that there
are deposits of different genesis (primary and secondary) and
ages in the territory of Armenia, an individual approach to the
study of each of them is required. It is known that the formation
of deposits in carbonaceous terrigenous-volcanogenic strata is of
a complex nature and is caused by the processes of sedimentation,
metamorphism, and superimposed metasomatism. It is established
that elevated contents of noble metals in black shales can be
controlled by faults and are due to the appearance of hydrothermal-
metasomatic processes, with redistribution and introduction of
noble metals by deep solutions [11-17]. In recent decades, close
attention has been paid to rhenium in carbon shales, in which it is
accumulated in high contents. Rhenium extremely rarely occurs as
an independent mineral therefore, it is present as an isomorphic
admixture in the composition of more than 50 minerals, and in an
organic and argillaceous matter, it is in a dispersed condition. The
Clarke of rhenium in black shales is 0.3-0.8g/t [18], which is six
times more than its Clarke in other sedimentary rocks. Possessing
organophilic properties, it is a characteristic element of carbon
shales [19,20].
Table 1: The average geochemical contents of analyzed elements in clayey and black shales, in ppm.
Chemical Elements Black-Shale Rocks of Armenia Clay Shales by Sklyarov et al. [31] Black Shales By Ketris, Yudovich [18,19]
U 1-75 3,7 8,5
V 3-134 130 205
Mo 1-40 2,6 20
Cu 3-170 48 70
Co 5-435 19 19
Ni 3-104 74 70
Zn 2-133 93 130
Pb 1-120 20 21
Rb 5-150 150 74
Sr 25-504 330 190
Au 0,002-0,83 0,0033 0,007
Ag 0,06-10,5 0,072 1
Pt 0,002-0,0061 0,001 0,0009
Pd 0,002-0,027 0,001 -
Re 0,005-0,042 0,0001 0,3-0,8
In the last decades, thanks to the development of modern
analytical methods of determination of rhenium in a carbon matrix,
new data have been obtained on the content of rhenium in the
geological bodies of the sedimentary cover [21]. Improvement
in analytical equipment and methods for the determination of
rhenium has made it possible to carry out mass analyzes with a low
detection threshold. The geochemical “specialization” of oil shales
is given below (Table 2) within the sedimentary cover of the East
European Platform [21,22]. It has been established that Dictyonema
shales, oil shales-kuckersites and carbon shales of different basins
have “rhenium specialization”. The connection of metals with
black shales may be of genetic nature or be spatially linked [23].
presence of an organic component, in the second - by the common
sedimentary basins, which are equally favorable for the formation
of carbon rocks and accumulation of metals [23]. Currently,
black-shale formations, along with ores of magmatic origin, are
considered to be a potential source of stratiform mineralization
of precious metals. Black shales provide a new source of raw
materials for the metals e.g., gold, rhenium, and platinum group.
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In this regard, it is important to understand their distribution and
type of formation in black-shales, to create a basis for forecasting,
developing prospecting methods for sites of this type, and creating
technological enrichment schemes for the metals present in them.
Table 2: Geochemical specialization formulas of oil shales in Eastern Europe and Russia and its vicinity [19]. Note: the
number before the element sign indicates the concentration Clarke of the element.
Basin (Area) Geochemical Specialization Formula
Eastern (Russian) part of the Baltic basin. Dictyonema shales 350Mo 160Sc 143Re 111Bi 75As 52U 24Cd 13Cu 12V 11Tl 10Ag 10Au
7Pb 6Sb 5Hg 4B 4Co 3Zn 2F 2Sn 2Ni 2Nb
Eastern (Russian) part of the Baltic basin. Kuckersites 1000Re 160Sc 14Ag 11Mo 5Hg 4Pb 4B 3Zn 2Sn
Yarenga basin 112Re 101Cd 66Mo 27As 12Zn 7V 5Ag 5Ni 4Cu 3P 2Ba 2Cr
Timan-Pechora basin 165Re 150Bi 50Mo 12U 7B 3Ag 3Pb 2Sn 2Tl 2V 2Zn 2Hg
The forms of occurrence of chemical elements in nature
According to Safronov [24], chemical elements are found in
nature in mineral and non-mineral forms. The latter include simple
and complex ions, colloidal, adsorbed, and suspended particles.
In addition, they are divided into mobile and strongly bonded
ones according to extraction from rocks (through simple leaching
without destroying the mineral lattice). At the same time, it is a fact
that the non-mineral mass of an element in nature is greater than
its proportion in the mineral form [24]. Literature data show that
the form of occurrence of noble metals in black shale complexes is
that. It is due to dispersed precipitation of noble metals (about
[25,26]. If platinoid minerals are quite distinguishable in Poland,
as well as most regions of Russia, the presence of platinoids is
determined only by chemical-analytical methods [23]. At the same
time, they can occur in soluble and insoluble organic matter, i.e., in
nickel, copper, silver, molybdenum, zinc, lead and mercury minerals,
in form of platinoid admixtures.
According to Gurskaya [12], The minerals of noble metals,
revealed in black shales, form a complicated complex, which is
divided into several classes: 1) native metals, 2) metallic solid
4) arsenides and sulfo-arsenides, 5) selenides, 6) tellurides, 7)
bismuthides, 8) antimonides. Lots of new minerals were revealed
in black-shale rocks, including, their compounds with biophilic
elements - selenium, phosphorus, and arsenic. Since the forms
of occurrence of noble metals and the range of satellite elements
determine their technological properties, the issue of determining
the mineral composition in black-shale rocks becomes a primary
one. Traditionally, the forms of occurrence of chemical elements in
nature are divided into the following types: easily extractable; ores
and arsenides; ores in which noble metals are part of organic matter
[27]. Traditional technological processes of extraction of refractory
ores do not take into account the behavior characteristics of noble
metals in shales, as a result, it is impossible to completely extract
them. The following forms of noble metals are known in black
shales: mineral and micromineral, dispersed, i.e., in associations
with clay and organic matters, and water-soluble substances.
Studies in this direction will be carried out for Armenian samples
in the near future. Obviously, each type requires certain processing
conditions. Mineralogical research of black shales of Cambrian-
Ordovician of the Baltic paleo-basin has allowed detecting, native
metals and intermetallic compounds in the siltstone-sandstone,
admixture: Aunat (native), Pd, Os-Ir, Au-Ag, Au-Cu, Au-(Cu)-Hg, Au-
Hg, Ag-Cu, Pt-Fe. Along with them, the samples contain native Fe, Al,
Cu, Fe-Ni, Ni-Fe, Cu-Zn, as well as tellurides and bismuth oxides [23].
Further, the following mineralization phases of noble metals were
chalcopyrite with an admixture of gold. In association with pyrite,
the native elements occur with the admixtures of Ni, Co, Zn, Cu, As,
with sphalerite - with the admixtures of Cd, Fe and pentlandite.
Noble metals and organic matter: The most important forms
of existence of these elements in carbon rocks can be absorbed (in
organic matter and clay minerals) and colloidal-saline. In recent
years, the method of bio-oxidation based on the action of iron- and
sulfur-oxidizing bacteria has been applied to the complex types of
black-shale ores. In this case, it is important to have the presence
of mobile, weakly bonded, colloidal-water-soluble forms of metals
in the rock. The method is successfully used on an industrial scale
in many countries around the world, e.g., in Poland, Canada, South
Africa, and Finland [13,27-29]. The black-shale formations contain
up to 50 rel.% of Corg (organic). A close correlation between rheium
and organic matter has been established. The increase in rhenium
concentration up to 0.1-0.2g/t is accompanied by an increase in the
amount of Corg.
Noble metals and clay components of shales: In order
to detect the connection between the clay components and
noble metals of black shales we have analyzed the whole-rock
composition of black shales and the clay fraction separately by the
ICP MS method (Table 3). As seen in Table 3, the clay component is
mostly several times enriched by noble metals and other elements
compared to the BSh. Enrichment of the clay part of shale is typical
for rhenium, uranium, molybdenum and vanadium. This is due to
the higher absorption capacity of clay minerals. The clay fraction
is enriched by platinum and palladium by about 7-10 times, and
rhenium content in the clay part is 3-7 times more than in the black
shale sample.
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Table 3: Concentration of chemical elements in the studied samples of black shale (BSh) and in its clay component (CC),
in g/t.
Sample Pd Pt Au Re U V Mo
BSh CC BSh CC BSh CC BSh CC BSh CC BSh CC BSh CC
9 0,06 0,45 0,012 0,11 0,067 0,093 0,15 1,23 14 119 279 1760 43 782
35 0,05 0,36 0,011 0,089 0,034 0,089 0,22 1,14 172 310 1483 2360 176 217
114 0,07 0,77 0,078 0,55 0,078 0,13 0,29 1,56 152 204 860 653 214 118
Colloidal-water-soluble forms of noble metals in black
shales: Chemical elements in the black shale rocks can be in a
dispersed form and be represented by particles of ultra-small size
- less than 1000nm. The content of this fraction varies in different
analysis and use of traditional technology schemes of extraction. At
the same time, there is evidence that rare and dispersed elements
are naturally concentrated in porous sections of rocks with content
several times higher than in the sample as a whole, and higher than
the limit of detection by analytical methods [22,30]. The important
results that shed light on the issue of the forms of occurrence of
noble metals and rhenium in black shales were obtained during the
segregation of the so-called “submicron fractions” from the rock
[10]. Submicron fraction is a part of the sample in which chemical
elements are in ionic, molecular and colloidal forms, 1-1000nm in
size, which is extracted by water solution under specially selected
conditions.
Experiments on a sample with the content of rhenium of
1.75g/t and the output of submicron fraction from the sample
- 1.94 weight % allowed to determine the content of rhenium in
submicron fraction of 22.6g/t. The extraction of this element in the
colloidal-saline fraction was 25.3 rel.% [10]. The analysis of water
solutions makes it possible to realize the possibilities of the ICP MS
method to the maximum extent, since there is no negative effect
of additionally introduced chemical reagents on the results, which
leads to decrease in the limits of detection of elements. The content
of petrogenic oxides in the solution of Submicron Fraction (SMF) is
obtain real values of microelements without introducing additional
corrections. The result of the work with real concentrations, and
not near detection limits is the high reproducibility of the analysis
and decrease in determination error. The accumulation factor for
the noble metals and rhenium is quite high, except for silver (Table
4). We assume that this may indicate the accumulation of silver
in mineral form, unlike the noble metals and Re. Thus, analysis of
matters according to the mineralogical and geochemical features
of carbon rocks allows expressing an opinion about the content of
gold, silver, platinoids, and rhenium in four possible forms. Each
mineralization (pyrite-marcasite, chalcopyrite, galenite and other
is determined by its high absorption capacity, c) in organic matter
of rocks, d) in the composition of mobile (ionic, molecular and
colloidal) forms [31-33].
Table 4: Noble metals in black shales and their submicron fractions, g/t [17].
Element Average for the Baltic Paleobasin Accumulation Ratio (SMF/BSh)
SMF 70 Samples BSh for 70 Samples)
Pt 0,20 0,018 11,2
Pd 4,86 0,654 7,4
Au 0,29 0,034 8,4
Ag 0,91 1,309 0,7
Re 2,1 0,214 9,8
Conclusion
a. Black-shale formations are widely spread and are in
from the Lower Triassic to Pliocene and they are represented
by terrigenous-carbonate formations, brown coals (lignite), oil
shales, bituminous mudstones, sandstones, and limestones.
b. Black-shale formations in Armenia are enriched by gold,
silver, rhenium, and platinoids. Among the satellite elements,
there are Co, Cu, Ni, Mo, Zn, and Pb.
c. It is necessary to clarify the macro- and micro-mineral
forms of noble metals, rhenium and satellite-elements in the
black-shale formations of Armenia.
forms of the above-mentioned chemical elements, however,
their extraction will require an integrated approach.
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