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Cent. Eur. J. Geosci. • 6(3) • 2014 • 308-329
DOI: 10.2478/s13533-012-0182-z
Central European Journal of Geosciences
Architecture of Upper Cretaceous Rhyodacitic
Hyaloclastite at the polymetallic Madneuli deposit,
Lesser Caucasus, Georgia
Research Article
Nino Popkhadze1∗ , Robert Moritz2 , Vladimer Gugushvili1
1 Al. Janelidze institute of Geology of I. Javakhishvili Tbilisi State University, 0186 Tbilisi, Georgia
2 Earth and Environmental Sciences, University of Geneva, 1205 Geneva, Switzerland
Received 27 September 2013; accepted 30 May 2014
Abstract: This study focuses on a well-exposed section of the Artvin-Bolnisi zone located in the open pit of the
Madneuli ore deposit, Lesser Caucasus, Georgia. Detailed field and petrographic observations of the main
volcano-sedimentary lithofacies of its Upper Cretaceous stratigraphic succession were carried out. Whole rock
geochemistry studies support the interpretation of intense silicification of the rocks, and supports our petrographic
studies of samples from the Madneuli open pit, including lobe-hyaloclastite described in detail during this study.
A particular focus concerned lobe-hyaloclastite exposures in the Madneuli open pit, singled out for first time in this
area of the Lesser Caucasus. Two types of hyaloclastite are recognized at the Madneuli deposit: hyaloclastite with
pillow-like forms and hyaloclastite with glass-like selvages. The petrographic description shows a different nature
for both: hyaloclastite with glass-like selvages represented by devitrification of volcanic glass, which is replaced by
quartz and K-feldspar overgrowth of crystals in the groundmass and elongated K-feldspar porphyry phenocrysts.
Perlitic cracks were identified during thin section observation. The Hyaloclastite with pillow-like forms consists of
relicts of volcanic glass and large pumice clasts replaced by sericite. Key observations are presented in the case
of lobe-hyaloclastite and their immediate host volcano-sedimentary environment to constrain their depositional
setting. A paleoreconstruction of their environment is proposed, in which hyaloclastite record the interaction of
magma emplaced in unconsolidated volcano-sedimentary rocks associated with a submarine rhyodacite dome,
emplaced during several magmatic pulses. Our study shows that the predominant part of the host rock sequence
of the Madneuli polymetallic deposit was deposited under submarine conditions, which is in agreement with
volcanogenic massive sulfide models or transitional, shallow submarine magmatic to epithermal models that
were proposed by previous studies.
Keywords: Hyaloclastite • lobe-hyaloclastite • pillow-like forms • glass-like selvages • facies
© Versita sp. z o.o.
1.
∗
Introduction
E-mail: [email protected]
The Cretaceous Artvin-Bolnisi zone of Georgia belongs to
the Lesser Caucasus and was formed during northeastward
subduction of the Tethys below the Eurasian margin. This
study focuses on a well-exposed section of the Artvin-
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Bolnisi zone, in the open pit of the Madneuli polymetallic
ore deposit of the Bolnisi mining district, located about
50 km south of Tbilisi, close to the Georgian-Armenian
border (Figure 1(a)).
According to the majority of previous studies, the
formation of the Madneuli deposit is tightly linked to
the evolution of Upper Cretaceous magmatism in the
Bolnisi district [2–5]. However, questions remain about the
specific relationships with the local geological evolution.
Indeed, both volcanogenic massive sulfide (VMS) [6,
7] and porphyry-epithermal deposit models have been
proposed [8]. Furthermore a genetic model combining both
environments and favoring a transitional volcanogenic
massive sulfide-epithermal scenario with a transitional
submarine to subaerial environment was also proposed [9].
The most recent investigation interpreted the Madneuli
deposit as a transitional hydrothermal system with a
magmatic input formed in a submarine environment [10].
In this contribution, we report detailed field and
petrographic observations of the main volcanogenic
sedimentary lithofacies, which comprise the Upper
Cretaceous part of the stratigraphic record of the Bolnisi
mining district, and we particularly focus on lobehyaloclastite exposures in the Madneuli open pit, which
are singled out for first time in this area of the Lesser
Caucasus [11–13]. Our main aims are to describe the
emplacement, fragmentation and eruption processes that
operated in the area, to constrain the volcano-sedimentary
depositional environments. We particularly emphasize
the key observations that need to be made in the case
of lobe-hyaloclastites and their immediate host volcanosedimentary environment to constrain their submarine
depositional setting. Our study underlines the careful
and detailed field and petrographic studies, which still
need to be carried out in future investigations in similar
environments along the Lesser Caucasus, where the
submarine or subaerial depositional environment of rock
units is still very much debated and poorly constrained.
This investigation is also an important contribution to the
understanding of the geological setting and the genesis
of the Madneuli polymetallic deposit, which is one of the
major ore deposits of the Lesser Caucasus (Figure 1(b)).
Thus, the identification and the interpretation of major
lithofacial units is a powerful tool for determining the
paleogeographic and the geotectonic environment of
volcanic successions spatially and genetically associated
with ore deposit formation.
Previous descriptions
and interpretations of the volcano-sedimentary complex
of the Madneuli deposit and other ore prospects of
the Bolnisi ore district were succinct and did not
address physical volcanology and facies architecture
aspects of the host rocks.
The volcanic facies
architecture models created for old VMS provinces such
as the Cambrian Mount Read Volcanics, Tasmania,
Australia [15, 16]; the Cambro-Ordovician Mount Windsor
Subprovince, Queensland, Australia [17]; the Proterozoic
Skellefte district, Sweden [18]; the Archean Noranda
district, Quebec, Canada [19]; the Ordovician, Bathurst
Mining Camp, New Brunswick, Canada [20]; and the
Upper Devonian to Lower Carboniferous Neves Corvo
district (Iberian Pyrite Belt) in southern Portugal and
Spain [21] have proven to be important in providing the
framework for ore deposit studies and exploration, and
helpful in reconstructing the massive sulfide ore-forming
environment and processes [21]. Our study based on
physical volcanology, volcanic and volcano-sedimentary
facies architecture and sedimentary basin analysis is the
first detailed approach of the Georgian Madneuli deposit.
In particular, this paper describes two types of rhyodacitic
lobe-hyaloclastite, which are exposed in the open pit of
the Madneuli deposit. They include (1) hyaloclastite
with pillow-like forms and (2) hyaloclastite with glass-like
selvages. Hyaloclastite with glass-like selvages refers to
a breccia facies, morphologically associated with carapace
breccias occurring along the upper surface of the distal
part of flows. By contrast, hyaloclastite with pillowlike forms is a pumiceous hyaloclastite, which consists of
pumice fragments and volcanic glass.
2.
Regional Geological Setting
The Madneuli ore deposit is located in the Artvin-Bolnisi
zone, southern Georgia, which belongs to the Lesser
Caucasus belt (Figure 1(a)). The Lesser Caucasus records
a complex pre- to post-collisional history, documenting the
convergence between the African/Arabian plates and the
European margin during the closure of the Neotethys [8,
22, 23]. It consists of three main geological tectonic
zones, which are from SW to NE: (1) the South Armenian
Block of Gondwana affinity; (2) the ophiolitic Sevan-Akera
suture zone; and (3) the Eurasian margin, which includes
the Kapan zone, the Somkheto-Karabakh island arc, the
Artvin-Bolnisi zone and the Adjara-Trialeti zone [1, 22, 23].
The Artvin-Bolnisi zone represents the active Cretaceous
magmatic arc along the Lesser Caucasus and is the
northeastern extremity of the Somkheto-Karabakh island
arc (Figure 1(b)). The Adjara-Trialeti zone to the north of
the Artvin-Bolnisi zone (AT in Figure 1(a)) represents an
associated Santonian-Campanian back-arc [1].
The Artvin-Bolnisi zone is characterized by a
Hercynian basement, which consists mainly of: (1)
a Late Proterozoic-Early Paleozoic basement, (2) a
Neoproterozoic-Cambrian granite basement complex, (3)
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Figure 1.
(a) Location of the Madneuli deposit in the Bolnisi region [1]. Abbreviations: S - Scythian Platform; GCS - Greater Caucasus Suture;
T - Transcaucasus; AT - Southern Black Sea Coast-Achara-Trialeti Unit; AB - Artvin-Bolnisi Unit; P - Pontides; BK - Bayburt-Karabakh
Imbricated Unit; NALCS - North Anatolian-Lesser Caucasian Suture; AI - Anatolian-Iran Platform. (b) Geological map of the Lesser
Caucasus, highlighting Mesozoic and Cenozoic intrusive rocks, ophiolites, and major ore districts [14] SAB-South Armenian Block;
SASZ-Sevan Akera suture zone; SKIA-Somkheto Karabakh island arc.
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a Middle-Late Carboniferous microcline granite basement
complex, and (4) a Late Proterozoic-Early Paleozoic
Tectonic Melange Zone [1, 22, 24]. In the Bolnisi region,
two basement complexes are exposed, and are called
the Khrami and Loki salient. They are overlain by
Carboniferous volcanogenic sedimentary rocks, followed
by Jurassic sedimentary and volcanic rocks. The Jurassic
rocks consist of terrigenous, volcaniclastic and calcalkaline magmatic arc rocks, including andesite, dacite,
rhyolite, basalt and volcaniclastic rocks intruded by
granitoids [1].
The Bolnisi volcanic-tectonic depression consists of
Cretaceous, Paleogene, Pliocene and Quaternary rocks.
Within the Artvin-Bolnisi zone, the Upper Cretaceous
section is dominated by volcanic rocks consisting of calcalkaline basalt, andesite, dacite and rhyolite. Their
thickness reaches up to 3000-4000 m. Volcanic rocks were
deposited in a shallow marine to subaerial environment [1,
22]. Three main formations are distinguished within
the Albian-Upper Cretaceous volcanogenic sedimentary
unit: 1) Albian-Cenomanian terrigenous-carbonate, 2)
Turonian-Santonian volcanogenic and 3) CampanianMaastrichtian carbonate units.
This sequence is
unconformably overlain by a Maastrichtian-Paleocene
turbidite sequence (Figure 2).
A Lower Eocene formation consists of terrigenous clastic
rocks. Middle Eocene volcanic rocks unconformably
overlie older rocks and are conformably overlain by Upper
Eocene shallow-marine clastic rocks. The youngest rocks
in the region are Quarternary volcanic rocks and alluvial
sedimentary rocks [1, 22]. Besides the major Madneuli
ore deposit, numerous ore occurrences are described in
the Bolnisi region, and include Sakdrisi, David-Gareji,
Qvemo-Bolnisi, TsiteliSopeli, Darbazi and Beqtakari
(Figure 2). All of them are hosted by Cretaceous volcanic
and volcanogenic sedimentary rocks.
3. Stratigraphy of the Bolnisi ore
district
The Bolnisi district is a Cretaceous magmatic region,
with complex, laterally and vertically variable regional
stratigraphic relationships. The Upper Cretaceous rock
formations are subdivided into five separate suites
(Figure 3) [26, 27]. The host rock succession of the
Madneuli deposit belongs to the Mashavera suite, and
consists predominantly of lava, pyroclastic, volcanogenic
sedimentary and other sedimentary rocks of rhyodacitic
composition (Figure 3). An Upper Turonian to Lower
Santonian age is currently attributed to the ore-bearing
Mashavera suite [26, 27], which is underlain by the
Upper Turonian Didgverdi suite and overlain by the
Lower Santonian Tandzia, Gasandami and Shorsholeti
suites (Figure 3). However, a more recent interpretation
advocates an Upper Turonian-Coniacian stratigraphic age
for the ore-hosting Mashavera suite, and an Upper
Turonian to the underlying Didgverdi suite (Vashakidze,
pers. comm.1998).
A recent nanofossil study of the host rocks interprets
the Mashavera suite as Campanian [29]. Radiolaria
identification from the host rocks is presently in progress
to solve the host rock age inconsistencies. Recent
TIMS U-Pb dating of zircons from mafic dikes located
in the southeastern part of the Madneuli open pit and
crosscutting the rhyodacitic extrusion yielded ages of 8687 Ma, therefore supporting a Coniacian-Santonian age
of the host rocks of the Madneuli deposit [30].
4. Overview of the major volcanic
and volcano-sedimentary lithofacies
in the Madneuli open pit
Stratigraphic relationships and textural characteristics of
the host rocks of the Madneuli deposit are best exposed
in some key areas of the open pit (Figure 4) [12].
Identification and characteristics of facial units are based
on detailed studies of each existing mining level of the
open pit. Our field-oriented observations throughout the
entire open pit and adjacent areas enabled us to collect
and interpret the different volcanic and sedimentary
structures, outline their distribution and their relationship
in the open pit and classify them into facies assemblages.
The different units were characterized based on variations
in composition and texture. Twelve lithofacies were
singled out in our study for the first time at the Madneuli
deposit. Descriptions and interpretations of the twelve
principal facies are summarized in Table 1. Lithofacies
units, described within the host-rock succession of the
Madneuli deposit, are grouped in two facies assemblages:
a stratigraphically lower volcano-sedimentary facies
assemblage and an upper volcanic facies assemblage
(Figure 5).
In addition to these two facies assemblages, a
granodioritic- to quartz dioritic porphyry has been
encountered during drilling beneath the Madneuli deposit,
at a depth of 800-900 meters below the present day
surface [29].
The lower, bedded volcano-sedimentary facies assemblage
has an apparent thickness in the open pit of about 200 m
and predominates in the open pit (Figure 5). It hosts the
different ore types, including a stockwork vein zone in the
western and northern parts, and a pyrite-telluride-gold
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Figure 2.
Geological map of the Bolnisi ore district [25].
vein corridor in the eastern part (Figure 5). Strongly
silicified bedded sedimentary rocks alternate with tuff.
Very fine-grained tuff, associated with vesiculated tuff [35,
36] horizons, containing bioturbations and accretionary
lapilli are present on all flanks of the open pit and may
serve as marker horizons (see red horizons in Figure 5).
Vesiculated tuff is more indurated than the surrounding
beds. Most vesicles in the tuff have a diameter of 0.5
to 3 mm and a few reach 1 cm. The upper surfaces of
vesiculated tuff are characterized by ripple marks, which
are interpreted as gravity flowage ripples [35]. In the
upper levels of the open pit bedded sedimentary rocks of
this complex consist of alternations of strongly silicified
marl, sandstone, turbiditic rock, volcanogenic mudstone,
and rare radiolarian-bearing horizons (see red and white
stars in Figure 5). Cross-bedding, slumps, load casts,
groove marks, wave and current ripples, and different
bioturbations are also present in the volcano-sedimentary
bedded rocks, which are dominated by volcanogenic
turbidites with well exposed Bouma Ta, Tb, and Tc
divisions and these sedimentary rocks are volcanogenic
in origin, but transported and deposited by sedimentary
processes [37].
The pumice-rich volcaniclastic horizons of variable
thickness can be singled out in this volcano-sedimentary
facies assemblage. In the lower part of the volcanosedimentary facies assemblage, the volcaniclastic facies is
strongly silicified, altered and mineralized. Hydrothermal
alteration affects pervasively the pumice-rich rock and
can totally obliterate its original texture. In the upper
part of the open pit the volcaniclastic facies are less
silicified, altered and mineralized, with the exception of
local abundant pyrite mineralization. The size of pumice
range between 1 mm and 3 cm and more in some places.
Most of them have an elongated form and are flattened
and planar-stratified.
The stratigraphically upper volcanic facies assemblage
is mainly of rhyodacitic composition and consists of the
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Figure 3.
Lithostratigraphic column of the Bolnisi ore district (N. Popkhadze). Data from [27, 28].
following facial units (from bottom to top): rhyodacitic
pyroclastic flow with flow foliation, columnar jointed
ignimbrite, rhyodacitic extrusion, non-stratified rhyolitic
to dacitic breccia facies, ignimbrite and lithic to pumicerich facies (Figure 5). The northern part of the Madneuli
open pit is dominated by a 55 m thick rhyodacitic lavaflow with flow foliation and columnar jointed ignimbrite.
Flow-foliated rhyodacitic lava is strongly silicified. The
flow displays a 3 mm- to 5 cm- thick layering, defined by
an alternation of pale siliceous bands and bands including
darker more phyllosilicate-rich material. The layering is
mainly planar, with some local flow folding and finely
bulbous cauliflower-like margins. A few cognate lava
clasts occur within the siliceous layers, revealing that
lithic inclusions are not always diagnostic of a pyroclastic
origin [38]. They consist of phenocrysts and rounded
aggregates of anhedral quartz, with a locally preserved
perlitic texture. The shapes of the 8 to 10 m-thick
columnar jointed ignimbrite are rectangular. It contains
crystals and rock fragments. The matrix is glassy and
brown-colored. There is a typical spherulitic texture
of the volcanic glass with perlitic fractures, which is
evidence of high-temperature devitrification of initially
glassy, welded ignimbrite [39]. A massive, rhyodacitic
extrusion is present in the south-eastern part of the
open pit and is characterized by a granular, false clastic
and pyroclastic texture in some outcrops [38]. Locally
this rhyodacite displays a pumiceous texture. Some
pumice clasts are replaced by chlorite and sericite. An
interlayer of fine-grained tuff was described within this
body [29]. A 45 m thick rhyodacitic ignimbrite with a
welding texture overlies the rhyodacitic lava flow with flow
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Figure 4.
Panorama of the Madneuli open pit: (a) view from the eastern part towards the west, (b) view toward the south from the top of the hill
in the northern edge of the open pit.
foliation in the northern upper part of the open pit, which
contains no mineralization, but contains scarce silica-rich
xenoliths [4]. The volcano-sedimentary complex contains
the lobe-hyaloclastite, described in more detail below,
with distal parts consisting of Hyaloclastite with glasslike selvages and pillow-like forms (see Hg and Hp in
Figure 5).
5. Lobe-hyaloclastite flow in the
Madneuli deposit
In the Madneuli open pit, it is possible to observe
fragments of lobe-hyaloclastite flow: massive (coherent)
lava, flow-banded border zone of lava, carapace
breccia, individual lobes and two types of hyaloclastite:
hyaloclastite with glass-like selvages and one with pillowlike forms [40–43]. Detailed descriptions of each existing
outcrop in the open pit allow us to interpret them
and understand the emplacement mechanism and facies
of rhyodacite dome recognized in the open pit. The
lobe-hyaloclastite flow creates a dome structure in the
open pit. In some parts, it is possible to recognize
a gradational transition from the coherent part of the
flow to quenched rocks forming the hyaloclastite. The
coherent rhyodacite facies is volumetrically dominant in
the open pit within the dome structure (85 vol%), about
1500 m wide and up to 100 m high [44–48]. It consists of
massive, non-vesicular rhyodacite, characterised by welldeveloped columnar joints (Figure 6(a)). The columns
have pentagonal and, in some cases rectangular outlines
in cross section, the width of which is between 3 and 6
m. The coherent rhyodacite facies contains abundant 4
to 7 cm-sized green and subsidiary grey macrocrystalline
enclaves (Figure 6(b)), and occasional 10 to 30 cmsized fragments of fine-grained tuff. The periphery of the
pumiceous hyaloclastiteis characteristic by flow banding
(Figure 6(c)). The structure of the pumiceous hyaloclastite
differs from the one of hyaloclastite with glass-like
selvages, as they belong to different lobes, attributed
to different pulses of lava emplaced at the periphery of
the dome.Fragments of poorly sorted and crudely layered
carapace breccia are present in the eastern and uppermost
part of the open pit (Figure 6(d)). It consists of lobe
fragments, massive and flow banded, set in a hyaloclastite
matrix [33].
5.1.
Hyaloclastite with glass-like selvages
The best-exposed section of the hyaloclastite rock
formation is in the eastern part of the Madneuli open pit
(see Hg and Hp in Figure 5). A ring structure of isolated
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Figure 5.
Facies distribution map in the Madneuli open pit (this study), with ore zone locations from [10].
lobe within the massive facies [49] with internal columnar
joints is present in the south-eastern part of the open
pit, which lucks like the glass-like selvages of its distal
part, and reveals a similar structure. The diameter of this
lobe is 13-15 m (Figure 7(a)). The term selvage means a
distinct border of a mass of igneous rock. It is usually finegrained or glassy due to rapid cooling. Glass-like selvage
is one of the main characteristic structures of hyaloclastite.
It was formed by cooling, quenching and fracturing of its
external parts of rhyadacite lava during emplacement in
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Table 1.
Summary of the main volcano-sedimentary facies of the Madneuli deposit.
Lithofacies
Characteristics
Interpretation
Volcano-sedimentary facies assemblage
Altered pelitic sedimentary rock, sandstone and siltstone with
Silicified bedded
volcano-sedimentary slide-slump unit; sedimentary rock with turbiditic nature [4];
contains horizons with Radiolaria
facies
Flow transformation into turbidity
currents; sandstone is the product of
channelized mass flow deposition;
subaqueous setting
Fine-grained
accretionary lapilli
tuff and tuff with
bioturbation
Shallow water sedimentation; in part
Massive or normally graded; recrystallized volcanic glass in the
groundmass; lapilli of various sizes, oval-shaped, filled with quartz; water-settled volcanic ash
lapilli-rim type, with a core of coarse-grained ash, surrounded by
a rim of finer-grained ash
Water-settled
pyroclastic fall
deposit
Inner flow stratification within a single layer shows fine-grained
lamination, normal grading, thick units with clasts and
reverse-graded at the top and fine-grained perlitic in the upper
part
Resedimentation of shallow submarine
pyroclastic flow; down-slope transport by
high concentration turbidity current [31]
Pumicerichvolcaniclastic
(pyroclastic) facies
Predominantly matrix-supported pumice concentration zone; low
abundance of lithic clasts and crystals; stratified zone;
fine-grained lithic clasts; sub-angular lapilli, locally vesicular
Deposition from pulsatory pyroclastic
current [32]
Peperite
In-situ mingling at the margins of intrusion or lava with
unconsolidated radiolarian-bearing sediment within a submarine
volcanic succession
Contact: wetsediment-hot lava,
subaqueous environment. Fluidal
character
Hyaloclastite
Carapace rhyodacitic breccia flow; Hyaloclastite, with pillow like
shapes and glass-like selvages [5]; Groundmass with a perlitic
structure; fractures defined by chlorite, and glass replaced by
quartz, feldspar, sericite and epidote
Lobe hyaloclastitefacies, reflects a
continuous evolution of textures and
structures, formed during extrusion in
response to rapid chilling and quench
fragmentation of lava by water or by wet
hyaloclastite formed from previous
lobes [33]
Volcanic facies assemblage
Rhyodacite lava-flow Shards of felsic rocks along flow foliation. Porphyry structure with Coherent facies of volcanic dome
with flow foliation
plagioclase, K-feldspar and quartz phenocrysts; perlitic
(cryptodome) or volcanic sill
groundmass, amygdales filled with quartz; local strong silicification
Columnar-jointed
ignimbrite
Columnar jointed ignimbrite; typical perlitic groundmass, with a
spherulitic texture, and oval-shaped quartz crystals.
Depositional setting below a storm-wave
environment. High-temperature
devitrification of volcanic glass
Rhyodacitic
extrusion
Massive; evenly porphyritic groundmass micropoikilitic; locally
pumiceous
Coherent facies of lava or volcanic dome
Non stratified
rhyolitic-dacitic
breccia facies
Massive, poorly sorted, clast- to matrix supported; slabby rock
fragments, irregular, blocky and oval-shaped; local alteration,
including silicification
Autoclastic breccia from the margins of
subaqueous lava or cryptodome
Ignimbrite
Welded ignimbrite containing lapilli and crystal fragment or lapilli Deposition from pyroclastic flow
and matrix. Crystal fragments are plagioclase, orthoclase, and
quartz; glass shards with cuspate and platy shapes. Some local
strong silicification
Lithic- to
pumice-rich
non-welded
ignimbrite
Lithic, pumice and crystal fragments of different sizes; local
mudstone fragments with no sedimentary strata; no gradation.
water. Cooling is typically more rapid than at its margins
in contrasts to the internal parts of massive lava. During
cooling and devitrification, the glassy part of the lava
developed a fine network permeability that enabled them
Product of pyroclastic surges, which
preceded or accompanied the main
pyroclastic density current [34]
to be pervasively altered and acted as preferential fluid
migration pathways [38]. Water has a great capacity
to penetrate into fractures. The penetration of water
was accompanied by hydrothermal alteration, which can
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Figure 6.
Representative examples of hyaloclastite outcrops at Madneuli open pit: a - columnar joints in the coherent part of the lobe hyaloclastite,
b - The example of microcrystalline enclaves in the coherent rhyodacite facies, c - flow banding of the periphery part of Hyaloclastite
with pillow-like forms, d - Carapace breccias in the uppermost part of the open pit.
locally result in different colors of the rocks [38]. In some
places this alteration is developed around the fracture
zone and results in a rim texture (Figure 7(b)), which is
defined by a glass-like selvage and in other places it is
more pervasive and develops patch-like forms.
Subaqueous lobe-haloclastite flows are identical to
subglacial dacite and rhyolite flows observed in Iceland
and described by [50]. The subglacial, Quaternary dacite
flow Blahnukur of the Torfejokull central volcanic complex
in South central Iseland is a prime example. Like
lobe-Hyaloclastite flow at Noranda [51], rhyolitic lobes
at Blahnukur are characterized by a massive, typically
columnar-jointed glassy interior, flow-banded border zone,
and in situ brecciated glassy selvage, which are similar
to glass-like selvages described in the Madneuli open
pit. There are other analogue examples from submarine
settings, in submarine lava flow-dome complex, such
as in Ponza in Italy [48], in the Early Devonian Ural
volcanic rocks [52], pumiceous rhyolitic peperite, which
is associated with a rhyolitic sill which intruded a
wet, unconsolidated, submarine pumice breccia in the
Cambrian Mount Read volcanic rocks in Australia [53] and
an example of silicic intrusion-dominated volcanic center
at Highway-Reward, Australia [17].
Homogeneously devitrified cores remain relatively
impervious to hydrothermal alteration.
During the
formation of hyaloclastite in the Madneuli open pit,
the quenched selvage was broken and spalled. It is
characterized by intense silicification, devitrification and
chloritization. At the outcrop scale, the hyaloclastite
gives the apparent impression of anautobreccia with
pale rims surrounding grey to green rock fragments
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Figure 7.
Representative examples of hyaloclastite outcrops at Madneuli (see Hg and Hp locations in Figure 5): (a) - Margins of a lobe hyaloclastite
flow with internal columnar joints, (b) - Carapace breccias, (c) - Pillow-like shapes in hyaloclastite, (d) -Transitional zone from massive
to pillow-structured parts in pillow-like hyaloclastite.
(Figure 6(b)). This false clastic, breccia structure was
produced by the combined effects of devitrification, perlitic
fracturing and pervasive hydrothermal alteration [38].
The pale colored rims within the hyaloclastite are 0.5
to 3 cm-thick, and in thin section they have a similar
texture to the gray to green rock fragments, with the only
difference being that the pale-colored rims contain less
phenocrysts than the grey to green cores (Figure 8(a)).
This type of hyaloclastite rock is characterized by a
perlitic texture, as recognized with a hand lens and in
thin section. In some exceptional cases, macro-perlitic
textures can be recognized at the outcrop scale within
the Madneuli open pit (Figure 8(b)). This hyaloclastite
type contains round and oval-shaped amygdales filled
with quartz-chlorite or a fine-grained carbonate-clay
association (Figure 8(c)-(d)).
According to our petrographic descriptions, hyaloclastite
with glass-like selvages contains less than 30% of
phenocrysts, including elongated sanidine crystals
(Figure 9(a)). The groundmass consists of devitrified
volcanic glass with a mosaic texture, radial-shaped
crystals of K-feldspar and spherules of quartz.
Plagioclase microlites are surrounded by spherulites
(Figure 9(b)). Phenocrysts include quartz, plagioclase
and K-feldspar of different sizes. In some places, they
are associated with glomeroporphyric textures. Sericite
alteration affects K-feldspar and plagioclase crystals.
Spherulites with fine-grained quartz and feldspar are
products of high-temperature devitrification of silicic
volcanic glass. Subsequent recrystallisation of mosaic
quartz - feldspar destroyed or modified such original
devitrification textures [39].
The groundmass contains perlitic cracks.
Perlitic
cracks developed in response to hydration of the glass.
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Figure 8.
Representative examples of hyaloclastite outcrops at Madneuli open pit: (a) - pale-colored rim of glass-like selvages in the outcrop, (b)
- classical macro-perlitic texture in the outcrop, (c), (d) - ovel-shaled amygdales in the hyaloclastite rocks.
Hyaloclastite with glass-like selvages has a classic
perlitic texture, in which the cracks are distinctly arcuate
and concentrically arranged around spherical cores.
Hydration occurred after emplacement and during the
later cooling history of the glass, or after complete cooling
to surface temperature [39]. In thin sections, perlitic
cracks, instead of crosscutting elongated K-feldspar
phenocrysts crystals, follow their edges (Figure 9(c)-(d)).
5.2.
Hyaloclastite with pillow-like forms
Hyaloclastite with pillow-like forms is exposed on three
bench levels in the eastern part of the open pit (see
Hp in Figure 5), where typical small-elongated pillowlike shapes occur (Figure 7(c)). Along the same section,
there is also a gradational transition from massive lava
to a pillow-like shaped part (Figure 7(d)). The size of
pillow-like forms is about 15-18 cm in length and 6-8 cm
wide. The pillow-like forms has a local distribution and
associated with the bedded volcano sedimentary rocks.
The coherent lava is quite thick and compositionally
similar to hyaloclastite facies. They do not have rounded
pillow forms, they are flat and have elongated sigmoidal
shapes, most likelydue to the pressure of the overlying
rocks, or the water pressure. Intense hydrothermal
alteration developed along these fractures. Such kind of
hyaloclastiteis present elsewhere in the same Mashavera
suite, in the vicinity of the open pit of the Sakdrisi deposit
(Figure 2), which reveals their regional development
associated to different lobes. It is the external part
of isolated lobes (pumiceous lava lobe), which were
also described by [33]. The similar rhyolitic lobes and
associated pumiceous hyaloclastiteis interpreted by [51]
as a product of Subplinian to Plinian eruptions.
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Figure 9.
Petrographic observations of glassy-like selvage hyaloclastite. (a) - Elongated phenocryst of K-feldspar. (b) - Associated pale-colored
and gray-brown alteration (crossed nicols). Formation of perlitic cracks, the same field of view in crossed polarized light and plane
polarized light. (c) - Perlitic cracks in glassy groundmass, note that they do not crosscut K-feldspar phenocrysts (crossed nicols). (d) Perlitic cracks (plane polarized light).
Coherent rhyodacitic lava is pumiceous and consists of
glass shards also. It resembles other pumice-rich facies
that are common in submarine volcanic successions. One
example of coherent pumiceous rhyolite and pymiceous
hyaloclastite is found in the Cambrian Mount Read
Volcanic rocks in Australia [53]. The matrix surrounding
the pillows is a blue-colored altered rock of the same
rhyodacitic composition as the pillow. The local thickness
of outcrops varies between 5 and 8 m.
In thin section, pillow-like hyaloclastite has a rhyodacitic
composition with a porphyritic texture, whereas the
groundmass consists of relicts of volcanic glass replaced
by finely disseminated quartz and K-feldspar. Large
pumice clasts are also present. Locally, the groundmass
has a fluidal nature. In some places, the matrix displays a
vitriclastic texture accentuated by axiolitic devitrification
of glassy components. The center of Figure 10(a) shows
a relict pumice clast with a destroyed internal vesicular
microstructure. The brown rims of matrix shards are
affected by axiolitic devitrification [39]. Pumice clastsare
characterized by chilled margins and curviplanar surfaces.
Shards of volcanic glass have preserved their platy and
cuspate shapes (Figure 10(b)). Crystals of biotite are
present and rare muscovite as well. The margins of Kfeldspars are partly resorbed. In some places, crystal
relicts are totally replaced by chlorite. Sericite alteration
overprints plagioclase crystals (Figure 10(c)). A pumice
clast is replaced by sericite (Figure 10(d)).
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Figure 10.
6.
Petrographic observations of pumice hyaloclastite: (a) - Axiolitic devitrification of glass. (b) - Remnants of platy and cuspate shaped
volcanic glass and pumice. (c) - Sericite microcrystals replacing a plagioclase crystal (crossed polarized light). (d) - Pumice clast
replaced by sericite.
Whole-rock chemical aspects
Chemical analyses in Table 2 reveal a silica-rich nature
of the hyaloclastite rocks (high SiO2 contents of 69.94
to 77.77 wt%), which would classify them as rhyolite.
However, based on immobile trace and minor elements,
the Zr/TiO2 vs. Nb/Y diagram (Figure 11) reveals
a predominantly rhyodacitic/dacitic composition of the
Upper Cretaceous volcanic rocks from the Madneuli
open pit (see red diamonds in Figure 8) with no
rhyolitic samples, including the four hyaloclastite samples
analyzed in this study (see green dots in Figure 8).
Therefore, we attribute the very high silica content to
intense silicification during alteration of the hyaloclastite
rocks at the Madneuli ore deposit.
7.
Alteration
Based on our field and petrographic studies, the Upper
Cretaceous rhyodacitic hyaloclastite from the Madneuli
open pit was affected by both low temperature and high
temperature alteration. The low temperature alteration
includes: (1) hydration of volcanic glass resulting in
partial replacement by clay minerals and chlorite, and
(2) open pore space (vesicles, amygdales) filling by
chlorite, and finely disseminated clay minerals and calcite.
Evidence for high temperature alteration is devitrification
of volcanic glass in glassy-like selvage type hyaloclastite,
in which the mosaic texture of volcanic glass is outlined by
quartz and K-feldspar replacing spherulites, surrounded
by a matrix of plagioclase microlites. Hyaloclastite
locally contains columnar joints, which proves that the
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Figure 11.
Zr/TiO2 vs. Nb/Y diagram after [54] showing the compositional range of mafic and felsic volcanic rocks of the Bolnisi region (red
diamonds: samples from the Madneuli open pit, and blue triangles: samples out of the open pit) in contrast to the hyaloclastite
samples of this study (green dots).
glass granules were cemented at high temperature, since
columnar joints can only form in a coherent material
as concluded in other studies on Miocene and Archean
rhyolite hyaloclastite [55].
More detailed and recent investigations about
hydrothermal alteration mostly associated with the
mineralization zones in the Madneuli deposit by [10]
allowed us to establish the first alteration map for
the Madneuli open pit. The following alteration zones
weredefined in the open pit: a silicified core, followed
by a quartz-sericite-pyrite zone, a quartz-chloritesericiteand quartz-chlorite zone and weak regional
chlorite-sericite. Also diagenetic/low temperature albite
and chlorit [10]. Albite and chlorite are typical products
of seawater interaction with volcanic rocks at low
temperature [56, 57].
8. Paleoenvironmental
interpretation and emplacement
mechanism of lobe-Hyaloclastite
at Madneuli
Figure 12(a) displays the relationship of the glassylike selvage lobe-hyaloclastite flow of our study with
the coherent volcanic and adjacent rock units. The
hyaloclastite is located at the periphery of a rhyodacite
lava lobe, therefore representing a gradual transition from
the massive, coherent part of the volcanic rock towards
its periphery at the contact with the volcano-sedimentary
rocks. The hyaloclastite rock likely consisted at the
time of emplacement of unconsolidated bedded volcano-
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Table 2.
Whole rock analyses of hyaloclastite rocks from the
Madneuli open pit (concentrations in wt%): 1-2 hyaloclastite
with pillow like forms, 3-4 hyaloclastite with glassy like
selvage.
1
2
3
4
SiO2
74.74
77.77
75.50
69.94
TiO2
0.22
0.35
0.30
0.58
Al2 O3
11.29
10.31
12.58
13.09
Fe2 O3
4.11
2.72
3.45
6.24
MnO
0.08
0.07
0.04
0.17
MgO
2.91
1.83
1.14
2.49
CaO
0.21
0.19
0.40
0.33
Na2 O
1.33
3.06
3.87
3.23
K2 O
1.43
0.58
0.90
1.08
P 2 O5
0.05
0.06
0.05
0.15
LOI
2.95
2.85
1.85
2.56
Total
99.33
99.79
100.08
99.86
sedimentary units, with alternations of ash tuff, pumice
tuff and sedimentary rocks.
Figure 12(b) shows that the central part of the pumiceous
rhyodacitic lava lobe is surrounded by hyaloclastite
formed in situ. The lobe is dissected in its external
part and locally has a wavy-shaped outline (similar to
the ones described by [39]), marked by the presence of
small, pillow-like (or sigmoidal) forms at its periphery,
and forms a gradual transition from the massive, coherent
magmatic rock into the pillow-like shaped part. The lava
lobe is associated with a rhyodacitic lava flow with wellexposed fluidal zonality in its external part. Peperite
is located at its contact with the volcano-sedimentary
rock unit [58–60]. Identification of peperite during this
study was of critical importance in clarifying the facies
architecture and stratigraphy providing constraints on the
age relationship and timing of intrusive episodes and
sedimentation processes, as discussed previously by [53]
in the case of pumiceous peperite.
Subaqueous felsic lavas can be divided into lobehyaloclastite flows, blocky subaqueous lava, domes,
cryptodomes, and regionally extensive felsic lava [33].
Figure 13 is an idealized cross-section through a
rhyolitic lobe-hyaloclastite flow, which illustrates the flow
morphology and structures typical for proximal and distal
facies in such rock units.
The lobe hyaloclastite flow is inflated by successive pulses
of new magma, which feeds its large lobes. They generally
follow a very irregular path to the flow front, where
they form smaller lobes and locally they have small-sized
pillow-like shapes [33]. The Madneuli lobe-hyaloclastite
flow is massive in general, though locally ribbed, flow
laminated and columnar jointed. The chaotic character
of the carapace breccia, their local distribution at the
flow top, the absence of bedding and grading and lack
of broken crystals suggest an origin dominantly due to
autobrecciation [33].
9. Model for the emplacement of
the lobe-hyaloclastite in the Madneuli
deposit
The hyaloclastite described in the Madneuli open pit
is associated with a submarine dome-like structure of
felsic rhyodacite magmas and they were emplaced during
several eruptive pulses [61–63]. It was accompanied
by emplacement of isolated lobes. During the earliest
pulses, the upper part of the lava was directly extruded
in the volcano-sedimentary bedded unconsolidated rocks.
The lower part of these rocks is the product of
phreatomagmatic explosion. The latter one is strongly
silicified, altered and ore-bearing. The upper part consists
mostly of turbiditic rocks and bedded volcano-sedimentary
rocks.
In addition, numerous folds and fractures are present,
some of them being associated with uplift during the
formation of the dome structure.
The newly rising
magma could intrude along these faults or fracture
systems and invade previously emplaced but still watersaturated glass-like selvage hyaloclastite. The formation
of pumiceous hyaloclastite is related to second pulses of
magmas. There are no constrains on the exact water depth
during formation of the hyaloclastite. The pumiceous
rhyodacitic hyaloclastite implies that volatile exsolution
was not inhibited by pressure [55], which indicates a
shallow water depth. The products of phreatomagmatic
eruption, represented by vesiculated fine-grained tuff
associated with accretionary lapilli horizons and very
fine-grained tuff, suggest that the associated eruption
was distal, at a distance of several km. Like in this
study, accretionary lapilli can also form in a submarine
environment.
According to [64] accretionary lapilli
can be found in subaqueous and redeposited deposits.
There are many examples, such as in the Devonian
Lenneporphyr of Germany [65, 66], in the Haimaraka
Formation of Guyana [67], in the Tokiwa Formation of
Japan [68] and in reworked deposits intercalated with
Paleogene volcanic rocks on the Voring Plateau in the
North Sea [69]. Furthermore, accretionary lapilli were
describedin the deposits of the Ries impact crater in
southern Germany [70].
Deposits of hydromagmatic
eruption and hyaloclastite rocks are not contemporaneous.
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Figure 12.
Schematic paleoreconstruction of the relationships of a lobe hyaloclastite flow with adjacent rock types and schematic logs showing
the textural facies characteristics. (a) Hyaloclastite with glassy like selvages and adjacent volcano-sedimentary rock. (b) Contact of
pillow-like hyaloclastite with volcano-sedimentary bedded rocks. Not to scale.
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N. Popkhadze et al.
Figure 13.
Schematic sketch of the lobe hyaloclastite flow of this study (modified from [33]). Hp-Hyaloclastite with pillow-like forms; HgHyaloclastite with glassy-like selvages. Not to scale.
The deposition of the lower part of the volcanosedimentary facies predate the formation of the upper part
of this bedded sequence, which in turn predates formation
of hyaloclastites and lobes, but it was still unconsolidated.
10.
Conclusions
Two types of rhyodacitic lobes of lobe hyaloclastite flows
are described for the first time in the Madneuli deposit
of the Bolnisi mining district, Georgia: hyaloclastite
with glass-like selvages and hyaloclastite with pillowlike forms, which represent the external part of individual
lobes. The lobe structure with columnar jointing of the
internal part, described in the eastern part of the open
pit is devoid of hyaloclastite, which indicates that the
lobe was emplaced within the interior of the flow or
dome during endogeneous growth [33]. This internal lobe
represents individual pulses of magma.
The absence of different resedimented rock fragments,
the gradational contact with coherent lava, their laterally
discontinuous character, and the absence of bedding
support their in situ hyaloclastitic nature.
Spherulites in the hyaloclastite are strong evidence
for high temperature devitrification of volcanic glass,
which was replaced by quartz and K-feldspar in the
groundmass. Classical perlitic fractures follow K-feldspar
phenocrysts. This indicates that devitrification of volcanic
glass occurred after crystallization of phenocrysts and
perlitic cracks formed at the end. Columnar joints, which
occur inside the lobe flow in the Madneuli open pit, also
support a high temperature of formation.
The presence of pumiceous hyaloclastite in the
subaqueous lobe hyaloclastite flow is a reliable evidence
for shallow water depositional environment (<200 m
deep). The lack of pumiceous hyaloclastite in many
subaqueous lobe-hyaloclastite flows may simply reflect
their emplacement within deeper water [33].
Both types of hyaloclastite, which differ texturally, are
lobe hyaloclastite. The formation processes took place in
one lobe body, which was inflated by successive pulses
of new magma. The lobe hyaloclastite described in
this paper resembles hyaloclastite from other well known
deposits [49, 50, 55], which are common in submarine felsic
successions and are one of the important characteristic
facies for rocks hosting volcanogenic massive sulfide
deposits related to subaqueous felsic lavas/domes [33, 57,
71–73].
The association with a volcano-sedimentary complex, in
which bedding textures are consistent with deposition
from turbidity currents, along with the presence of
slumps, cross-bedding, load casts, groove marks, wave
and current ripples, different bioturbations and radiolariabearing horizons, support a below wave-base submarine
depositional environment of the sedimentary rocks
associated with hyaloclastite at the Madneuli deposit.
Turbiditic volcano-sedimentrary rocks and hyaloclastite
are present in the same stratigraphic section in the open
pit, but were not coexisting during their formation.
The pumice-rich volcaniclastic rocks and also the finegrained tuff with accretionary lapilli within the bedded
sedimentary and volcano-sedimentary complex in the open
pit are attributed to ashfall deposits of phreatomagmatic
origin.
Acknowledgements
The research was supported by the Georgian National
Science Grant 204 and Swiss National Science
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Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia
Foundation through the research grant SNF 200020113510 and SCOPES Joint Research Projects IB7320111046 and IZ73Z0-128324. The authors would like
to thank the other participants of the project: Tamara
Beridze, Stefano Gialli, Sophio Khutsishvili, Onise
Enukidze and Ramaz Minigineishvili, and the staff of the
"Madneuli Mine" and Malkhaz Natsvlishvili for assistance,
sharing geological information, and arranging access to
the mine. Thanks to Jorge Relvas (Portugal) and Fernando
Tornos (Spain) for helpful discussions about facial units in
the Madneuli deposit. The research would not have been
possible without the support provided by SGA, SEG and
IAVCEI organizations for attending many international
conferences where the work was presented.
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