Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the Polymetallic Madneuli Deposit, Lesser Caucasus, Georgia

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 architecture models created for old VMS provinces such ore deposit of the Bolnisi mining district, located about as the Cambrian Mount Read Volcanics, Tasmania, 50 km south of Tbilisi, close to the Georgian-Armenian Australia [15, 16]; the Cambro-Ordovician Mount Windsor border (Figure1(a)). Subprovince, Queensland, Australia [17]; the Proterozoic According to the majority of previous studies, the Skellefte district, Sweden [18]; the Archean Noranda formation of the Madneuli deposit is tightly linked to district, Quebec, Canada [19]; the Ordovician, Bathurst the evolution of Upper Cretaceous magmatism in the Mining Camp, New Brunswick, Canada [20]; and the Bolnisi district [2–5]. However, questions remain about the Upper Devonian to Lower Carboniferous Neves Corvo specific relationships with the local geological evolution. district (Iberian Pyrite Belt) in southern Portugal and Indeed, both volcanogenic massive sulfide (VMS) [6, Spain [21] have proven to be important in providing the 7] and porphyry-epithermal deposit models have been framework for ore deposit studies and exploration, and proposed [8]. Furthermore a genetic model combining both helpful in reconstructing the massive sulfide ore-forming environments and favoring a transitional volcanogenic environment and processes [21]. Our study based on massive sulfide-epithermal scenario with a transitional physical volcanology, volcanic and volcano-sedimentary submarine to subaerial environment was also proposed [9]. facies architecture and sedimentary basin analysis is the The most recent investigation interpreted the Madneuli first detailed approach of the Georgian Madneuli deposit. deposit as a transitional hydrothermal system with a In particular, this paper describes two types of rhyodacitic magmatic input formed in a submarine environment [10]. lobe-hyaloclastite, which are exposed in the open pit of the Madneuli deposit. They include (1) hyaloclastite In this contribution, we report detailed field and with pillow-like forms and (2) hyaloclastite with glass-like petrographic observations of the main volcanogenic selvages. Hyaloclastite with glass-like selvages refers to sedimentary lithofacies, which comprise the Upper a breccia facies, morphologically associated with carapace Cretaceous part of the stratigraphic record of the Bolnisi breccias occurring along the upper surface of the distal mining district, and we particularly focus on lobe- part of flows. By contrast, hyaloclastite with pillow- hyaloclastite exposures in the Madneuli open pit, which like forms is a pumiceous hyaloclastite, which consists of are singled out for first time in this area of the Lesser pumice fragments and volcanic glass. 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 2. Regional Geological Setting the key observations that need to be made in the case of lobe-hyaloclastites and their immediate host volcano- The Madneuli ore deposit is located in the Artvin-Bolnisi sedimentary environment to constrain their submarine zone, southern Georgia, which belongs to the Lesser depositional setting. Our study underlines the careful Caucasus belt (Figure 1(a)). The Lesser Caucasus records and detailed field and petrographic studies, which still a complex pre- to post-collisional history, documenting the need to be carried out in future investigations in similar convergence between the African/Arabian plates and the environments along the Lesser Caucasus, where the European margin during the closure of the Neotethys [8, submarine or subaerial depositional environment of rock 22, 23]. It consists of three main geological tectonic units is still very much debated and poorly constrained. zones, which are from SW to NE: (1) the South Armenian This investigation is also an important contribution to the Block of Gondwana affinity; (2) the ophiolitic Sevan-Akera understanding of the geological setting and the genesis suture zone; and (3) the Eurasian margin, which includes of the Madneuli polymetallic deposit, which is one of the the Kapan zone, the Somkheto-Karabakh island arc, the major ore deposits of the Lesser Caucasus (Figure 1(b)). Artvin-Bolnisi zone and the Adjara-Trialeti zone [1, 22, 23]. Thus, the identification and the interpretation of major The Artvin-Bolnisi zone represents the active Cretaceous lithofacial units is a powerful tool for determining the magmatic arc along the Lesser Caucasus and is the paleogeographic and the geotectonic environment of northeastern extremity of the Somkheto-Karabakh island volcanic successions spatially and genetically associated arc (Figure 1(b)). The Adjara-Trialeti zone to the north of with ore deposit formation. Previous descriptions the Artvin-Bolnisi zone (AT in Figure 1(a)) represents an and interpretations of the volcano-sedimentary complex associated Santonian-Campanian back-arc [1]. of the Madneuli deposit and other ore prospects of The Artvin-Bolnisi zone is characterized by a the Bolnisi ore district were succinct and did not Hercynian basement, which consists mainly of: (1) address physical volcanology and facies architecture a Late Proterozoic-Early Paleozoic basement, (2) a aspects of the host rocks. The volcanic facies Neoproterozoic-Cambrian granite basement complex, (3)

309 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 Upper Turonian Didgverdi suite and overlain by the complex, and (4) a Late Proterozoic-Early Paleozoic Lower Santonian Tandzia, Gasandami and Shorsholeti Tectonic Melange Zone [1, 22, 24]. In the Bolnisi region, suites (Figure 3). However, a more recent interpretation two basement complexes are exposed, and are called advocates an Upper Turonian-Coniacian stratigraphic age the Khrami and Loki salient. They are overlain by for the ore-hosting Mashavera suite, and an Upper Carboniferous volcanogenic sedimentary rocks, followed Turonian to the underlying Didgverdi suite (Vashakidze, by Jurassic sedimentary and volcanic rocks. The Jurassic pers. comm.1998). rocks consist of terrigenous, volcaniclastic and calc- A recent nanofossil study of the host rocks interprets alkaline magmatic arc rocks, including andesite, dacite, the Mashavera suite as Campanian [29]. Radiolaria rhyolite, basalt and volcaniclastic rocks intruded by identification from the host rocks is presently in progress granitoids [1]. to solve the host rock age inconsistencies. Recent The Bolnisi volcanic-tectonic depression consists of TIMS U-Pb dating of zircons from mafic dikes located Cretaceous, Paleogene, Pliocene and Quaternary rocks. in the southeastern part of the Madneuli open pit and Within the Artvin-Bolnisi zone, the Upper Cretaceous crosscutting the rhyodacitic extrusion yielded ages of 86- section is dominated by volcanic rocks consisting of calc- 87 Ma, therefore supporting a Coniacian-Santonian age alkaline basalt, andesite, dacite and rhyolite. Their of the host rocks of the Madneuli deposit [30]. 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 4. Overview of the major volcanic the Albian-Upper Cretaceous volcanogenic sedimentary and volcano-sedimentary lithofacies unit: 1) Albian-Cenomanian terrigenous-carbonate, 2) Turonian-Santonian volcanogenic and 3) Campanian- in the Madneuli open pit Maastrichtian carbonate units. This sequence is unconformably overlain by a Maastrichtian-Paleocene Stratigraphic relationships and textural characteristics of turbidite sequence (Figure 2). the host rocks of the Madneuli deposit are best exposed A Lower Eocene formation consists of terrigenous clastic in some key areas of the open pit (Figure 4)[12]. rocks. Middle Eocene volcanic rocks unconformably Identification and characteristics of facial units are based overlie older rocks and are conformably overlain by Upper on detailed studies of each existing mining level of the Eocene shallow-marine clastic rocks. The youngest rocks open pit. Our field-oriented observations throughout the in the region are Quarternary volcanic rocks and alluvial entire open pit and adjacent areas enabled us to collect sedimentary rocks [1, 22]. Besides the major Madneuli and interpret the different volcanic and sedimentary ore deposit, numerous ore occurrences are described in structures, outline their distribution and their relationship the Bolnisi region, and include Sakdrisi, David-Gareji, in the open pit and classify them into facies assemblages. Qvemo-Bolnisi, TsiteliSopeli, Darbazi and Beqtakari The different units were characterized based on variations (Figure 2). All of them are hosted by Cretaceous volcanic in composition and texture. Twelve lithofacies were and volcanogenic sedimentary rocks. 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 3. Stratigraphy of the Bolnisi ore units, described within the host-rock succession of the Madneuli deposit, are grouped in two facies assemblages: district a stratigraphically lower volcano-sedimentary facies assemblage and an upper volcanic facies assemblage The Bolnisi district is a Cretaceous magmatic region, (Figure 5). with complex, laterally and vertically variable regional In addition to these two facies assemblages, a stratigraphic relationships. The Upper Cretaceous rock granodioritic- to quartz dioritic porphyry has been formations are subdivided into five separate suites encountered during drilling beneath the Madneuli deposit, (Figure 3)[26, 27]. The host rock succession of the at a depth of 800-900 meters below the present day Madneuli deposit belongs to the Mashavera suite, and surface [29]. consists predominantly of lava, pyroclastic, volcanogenic The lower, bedded volcano-sedimentary facies assemblage sedimentary and other sedimentary rocks of rhyodacitic has an apparent thickness in the open pit of about 200 m composition (Figure 3). An Upper Turonian to Lower and predominates in the open pit (Figure 5). It hosts the Santonian age is currently attributed to the ore-bearing different ore types, including a stockwork vein zone in the Mashavera suite [26, 27], which is underlain by the western and northern parts, and a pyrite-telluride-gold

311 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 turbidites with well exposed Bouma Ta, Tb, and Tc silicified bedded sedimentary rocks alternate with tuff. divisions and these sedimentary rocks are volcanogenic Very fine-grained tuff, associated with vesiculated tuff [35, in origin, but transported and deposited by sedimentary 36] horizons, containing bioturbations and accretionary processes [37]. lapilli are present on all flanks of the open pit and may The pumice-rich volcaniclastic horizons of variable serve as marker horizons (see red horizons in Figure 5). thickness can be singled out in this volcano-sedimentary Vesiculated tuff is more indurated than the surrounding facies assemblage. In the lower part of the volcano- beds. Most vesicles in the tuff have a diameter of 0.5 sedimentary facies assemblage, the volcaniclastic facies is to 3 mm and a few reach 1 cm. The upper surfaces of strongly silicified, altered and mineralized. Hydrothermal vesiculated tuff are characterized by ripple marks, which alteration affects pervasively the pumice-rich rock and are interpreted as gravity flowage ripples [35]. In the can totally obliterate its original texture. In the upper upper levels of the open pit bedded sedimentary rocks of part of the open pit the volcaniclastic facies are less this complex consist of alternations of strongly silicified silicified, altered and mineralized, with the exception of marl, sandstone, turbiditic rock, volcanogenic mudstone, local abundant pyrite mineralization. The size of pumice and rare radiolarian-bearing horizons (see red and white range between 1 mm and 3 cm and more in some places. stars in Figure 5). Cross-bedding, slumps, load casts, Most of them have an elongated form and are flattened groove marks, wave and current ripples, and different and planar-stratified. bioturbations are also present in the volcano-sedimentary The stratigraphically upper volcanic facies assemblage bedded rocks, which are dominated by volcanogenic 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 aggregates of anhedral quartz, with a locally preserved pyroclastic flow with flow foliation, columnar jointed perlitic texture. The shapes of the 8 to 10 m-thick ignimbrite, rhyodacitic extrusion, non-stratified rhyolitic columnar jointed ignimbrite are rectangular. It contains to dacitic breccia facies, ignimbrite and lithic to pumice- crystals and rock fragments. The matrix is glassy and rich facies (Figure 5). The northern part of the Madneuli brown-colored. There is a typical spherulitic texture open pit is dominated by a 55 m thick rhyodacitic lava- of the volcanic glass with perlitic fractures, which is flow with flow foliation and columnar jointed ignimbrite. evidence of high-temperature devitrification of initially Flow-foliated rhyodacitic lava is strongly silicified. The glassy, welded ignimbrite [39]. A massive, rhyodacitic flow displays a 3 mm- to 5 cm- thick layering, defined by extrusion is present in the south-eastern part of the an alternation of pale siliceous bands and bands including open pit and is characterized by a granular, false clastic darker more phyllosilicate-rich material. The layering is and pyroclastic texture in some outcrops [38]. Locally mainly planar, with some local flow folding and finely this rhyodacite displays a pumiceous texture. Some bulbous cauliflower-like margins. A few cognate lava pumice clasts are replaced by chlorite and sericite. An clasts occur within the siliceous layers, revealing that interlayer of fine-grained tuff was described within this lithic inclusions are not always diagnostic of a pyroclastic body [29]. A 45 m thick rhyodacitic ignimbrite with a origin [38]. They consist of phenocrysts and rounded welding texture overlies the rhyodacitic lava flow with flow

313 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 the open pit within the dome structure (85 vol%), about contains no mineralization, but contains scarce silica-rich 1500 m wide and up to 100 m high [44–48]. It consists of xenoliths [4]. The volcano-sedimentary complex contains massive, non-vesicular rhyodacite, characterised by well- the lobe-hyaloclastite, described in more detail below, developed columnar joints (Figure 6(a)). The columns with distal parts consisting of Hyaloclastite with glass- have pentagonal and, in some cases rectangular outlines like selvages and pillow-like forms (see Hg and Hp in in cross section, the width of which is between 3 and 6 Figure 5). 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 cm- 5. Lobe-hyaloclastite flow in the sized fragments of fine-grained tuff. The periphery of the pumiceous hyaloclastiteis characteristic by flow banding Madneuli deposit (Figure 6(c)). The structure of the pumiceous hyaloclastite differs from the one of hyaloclastite with glass-like In the Madneuli open pit, it is possible to observe selvages, as they belong to different lobes, attributed fragments of lobe-hyaloclastite flow: massive (coherent) to different pulses of lava emplaced at the periphery of lava, flow-banded border zone of lava, carapace the dome.Fragments of poorly sorted and crudely layered breccia, individual lobes and two types of hyaloclastite: carapace breccia are present in the eastern and uppermost hyaloclastite with glass-like selvages and one with pillow- part of the open pit (Figure 6(d)). It consists of lobe like forms [40–43]. Detailed descriptions of each existing fragments, massive and flow banded, set in a hyaloclastite outcrop in the open pit allow us to interpret them matrix [33]. 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 5.1. Hyaloclastite with glass-like selvages open pit. In some parts, it is possible to recognize a gradational transition from the coherent part of the The best-exposed section of the hyaloclastite rock flow to quenched rocks forming the hyaloclastite. The formation is in the eastern part of the Madneuli open pit coherent rhyodacite facies is volumetrically dominant in (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 distinct border of a mass of igneous rock. It is usually ﬁne- joints is present in the south-eastern part of the open grained or glassy due to rapid cooling. Glass-like selvage pit, which lucks like the glass-like selvages of its distal is one of the main characteristic structures of hyaloclastite. part, and reveals a similar structure. The diameter of this It was formed by cooling, quenching and fracturing of its lobe is 13-15 m (Figure 7(a)). The term selvage means a external parts of rhyadacite lava during emplacement in

315 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 Silicified bedded Altered pelitic sedimentary rock, sandstone and siltstone with Flow transformation into turbidity volcano-sedimentary slide-slump unit; sedimentary rock with turbiditic nature [4]; currents; sandstone is the product of facies contains horizons with Radiolaria channelized mass flow deposition; subaqueous setting Fine-grained Massive or normally graded; recrystallized volcanic glass in the Shallow water sedimentation; in part accretionary lapilli groundmass; lapilli of various sizes, oval-shaped, filled with quartz; water-settled volcanic ash tuff and tuff with lapilli-rim type, with a core of coarse-grained ash, surrounded by bioturbation a rim of finer-grained ash Water-settled Inner flow stratification within a single layer shows fine-grained Resedimentation of shallow submarine pyroclastic fall lamination, normal grading, thick units with clasts and pyroclastic flow; down-slope transport by deposit reverse-graded at the top and fine-grained perlitic in the upper high concentration turbidity current [31] part Pumice- Predominantly matrix-supported pumice concentration zone; low Deposition from pulsatory pyroclastic richvolcaniclastic abundance of lithic clasts and crystals; stratified zone; current [32] (pyroclastic) facies fine-grained lithic clasts; sub-angular lapilli, locally vesicular Peperite In-situ mingling at the margins of intrusion or lava with Contact: wetsediment-hot lava, unconsolidated radiolarian-bearing sediment within a submarine subaqueous environment. Fluidal volcanic succession character Hyaloclastite Carapace rhyodacitic breccia flow; Hyaloclastite, with pillow like Lobe hyaloclastitefacies, reflects a shapes and glass-like selvages [5]; Groundmass with a perlitic continuous evolution of textures and structure; fractures defined by chlorite, and glass replaced by structures, formed during extrusion in quartz, feldspar, sericite and epidote 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 Columnar jointed ignimbrite; typical perlitic groundmass, with a Depositional setting below a storm-wave ignimbrite spherulitic texture, and oval-shaped quartz crystals. environment. High-temperature devitrification of volcanic glass Rhyodacitic Massive; evenly porphyritic groundmass micropoikilitic; locally Coherent facies of lava or volcanic dome extrusion pumiceous Non stratified Massive, poorly sorted, clast- to matrix supported; slabby rock Autoclastic breccia from the margins of rhyolitic-dacitic fragments, irregular, blocky and oval-shaped; local alteration, subaqueous lava or cryptodome breccia facies including silicification 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 Lithic, pumice and crystal fragments of different sizes; local Product of pyroclastic surges, which pumice-rich mudstone fragments with no sedimentary strata; no gradation. preceded or accompanied the main non-welded pyroclastic density current [34] ignimbrite

water. Cooling is typically more rapid than at its margins to be pervasively altered and acted as preferential fluid in contrasts to the internal parts of massive lava. During migration pathways [38]. Water has a great capacity cooling and devitrification, the glassy part of the lava to penetrate into fractures. The penetration of water developed a fine network permeability that enabled them 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 settings, in submarine lava flow-dome complex, such places this alteration is developed around the fracture as in Ponza in Italy [48], in the Early Devonian Ural zone and results in a rim texture (Figure 7(b)), which is volcanic rocks [52], pumiceous rhyolitic peperite, which defined by a glass-like selvage and in other places it is is associated with a rhyolitic sill which intruded a more pervasive and develops patch-like forms. wet, unconsolidated, submarine pumice breccia in the Cambrian Mount Read volcanic rocks in Australia [53] and Subaqueous lobe-haloclastite flows are identical to an example of silicic intrusion-dominated volcanic center subglacial dacite and rhyolite flows observed in Iceland at Highway-Reward, Australia [17]. and described by [50]. The subglacial, Quaternary dacite flow Blahnukur of the Torfejokull central volcanic complex Homogeneously devitrified cores remain relatively in South central Iseland is a prime example. Like impervious to hydrothermal alteration. During the lobe-Hyaloclastite flow at Noranda [51], rhyolitic lobes formation of hyaloclastite in the Madneuli open pit, at Blahnukur are characterized by a massive, typically the quenched selvage was broken and spalled. It is columnar-jointed glassy interior, flow-banded border zone, characterized by intense silicification, devitrification and and in situ brecciated glassy selvage, which are similar chloritization. At the outcrop scale, the hyaloclastite to glass-like selvages described in the Madneuli open gives the apparent impression of anautobreccia with pit. There are other analogue examples from submarine pale rims surrounding grey to green rock fragments

317 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 Figure5): (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.

(Figure6(b)). This false clastic, breccia structure was with glass-like selvages contains less than 30% of produced by the combined effects of devitrification, perlitic phenocrysts, including elongated sanidine crystals fracturing and pervasive hydrothermal alteration [38]. (Figure 9(a)). The groundmass consists of devitrified The pale colored rims within the hyaloclastite are 0.5 volcanic glass with a mosaic texture, radial-shaped to 3 cm-thick, and in thin section they have a similar crystals of K-feldspar and spherules of quartz. texture to the gray to green rock fragments, with the only Plagioclase microlites are surrounded by spherulites difference being that the pale-colored rims contain less (Figure 9(b)). Phenocrysts include quartz, plagioclase phenocrysts than the grey to green cores (Figure 8(a)). and K-feldspar of different sizes. In some places, they This type of hyaloclastite rock is characterized by a are associated with glomeroporphyric textures. Sericite perlitic texture, as recognized with a hand lens and in alteration affects K-feldspar and plagioclase crystals. thin section. In some exceptional cases, macro-perlitic Spherulites with fine-grained quartz and feldspar are textures can be recognized at the outcrop scale within products of high-temperature devitrification of silicic the Madneuli open pit (Figure 8(b)). This hyaloclastite volcanic glass. Subsequent recrystallisation of mosaic type contains round and oval-shaped amygdales filled quartz - feldspar destroyed or modified such original with quartz-chlorite or a fine-grained carbonate-clay devitrification textures [39]. association (Figure 8(c)-(d)). The groundmass contains perlitic cracks. Perlitic According to our petrographic descriptions, hyaloclastite 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 pillow-like forms is about 15-18 cm in length and 6-8 cm perlitic texture, in which the cracks are distinctly arcuate wide. The pillow-like forms has a local distribution and and concentrically arranged around spherical cores. associated with the bedded volcano sedimentary rocks. Hydration occurred after emplacement and during the The coherent lava is quite thick and compositionally later cooling history of the glass, or after complete cooling similar to hyaloclastite facies. They do not have rounded to surface temperature [39]. In thin sections, perlitic pillow forms, they are ﬂat and have elongated sigmoidal cracks, instead of crosscutting elongated K-feldspar shapes, most likelydue to the pressure of the overlying phenocrysts crystals, follow their edges (Figure 9(c)-(d)). rocks, or the water pressure. Intense hydrothermal alteration developed along these fractures. Such kind of hyaloclastiteis present elsewhere in the same Mashavera 5.2. Hyaloclastite with pillow-like forms suite, in the vicinity of the open pit of the Sakdrisi deposit (Figure 2), which reveals their regional development Hyaloclastite with pillow-like forms is exposed on three associated to diﬀerent lobes. It is the external part bench levels in the eastern part of the open pit (see of isolated lobes (pumiceous lava lobe), which were Hp in Figure 5), where typical small-elongated pillow- also described by [33]. The similar rhyolitic lobes and like shapes occur (Figure 7(c)). Along the same section, associated pumiceous hyaloclastiteis interpreted by [51] there is also a gradational transition from massive lava as a product of Subplinian to Plinian eruptions. to a pillow-like shaped part (Figure 7(d)). The size of

319 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 of glassy components. The center of Figure 10(a) shows glass shards also. It resembles other pumice-rich facies a relict pumice clast with a destroyed internal vesicular that are common in submarine volcanic successions. One microstructure. The brown rims of matrix shards are example of coherent pumiceous rhyolite and pymiceous affected by axiolitic devitrification [39]. Pumice clastsare hyaloclastite is found in the Cambrian Mount Read characterized by chilled margins and curviplanar surfaces. Volcanic rocks in Australia [53]. The matrix surrounding Shards of volcanic glass have preserved their platy and the pillows is a blue-colored altered rock of the same cuspate shapes (Figure 10(b)). Crystals of biotite are rhyodacitic composition as the pillow. The local thickness present and rare muscovite as well. The margins of K- of outcrops varies between 5 and 8 m. feldspars are partly resorbed. In some places, crystal In thin section, pillow-like hyaloclastite has a rhyodacitic relicts are totally replaced by chlorite. Sericite alteration composition with a porphyritic texture, whereas the overprints plagioclase crystals (Figure 10(c)). A pumice groundmass consists of relicts of volcanic glass replaced clast is replaced by sericite (Figure 10(d)). 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

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Figure 10. 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.

6. Whole-rock chemical aspects 7. Alteration

Based on our field and petrographic studies, the Upper Chemical analyses in Table 2 reveal a silica-rich nature Cretaceous rhyodacitic hyaloclastite from the Madneuli of the hyaloclastite rocks (high SiO2 contents of 69.94 open pit was affected by both low temperature and high to 77.77 wt%), which would classify them as rhyolite. temperature alteration. The low temperature alteration However, based on immobile trace and minor elements, includes: (1) hydration of volcanic glass resulting in the Zr/TiO2 vs. Nb/Y diagram (Figure 11) reveals partial replacement by clay minerals and chlorite, and a predominantly rhyodacitic/dacitic composition of the (2) open pore space (vesicles, amygdales) filling by Upper Cretaceous volcanic rocks from the Madneuli chlorite, and finely disseminated clay minerals and calcite. open pit (see red diamonds in Figure 8) with no Evidence for high temperature alteration is devitrification rhyolitic samples, including the four hyaloclastite samples of volcanic glass in glassy-like selvage type hyaloclastite, analyzed in this study (see green dots in Figure 8). in which the mosaic texture of volcanic glass is outlined by Therefore, we attribute the very high silica content to quartz and K-feldspar replacing spherulites, surrounded intense silicification during alteration of the hyaloclastite by a matrix of plagioclase microlites. Hyaloclastite rocks at the Madneuli ore deposit. locally contains columnar joints, which proves that the

321 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 8. Paleoenvironmental columnar joints can only form in a coherent material interpretation and emplacement as concluded in other studies on Miocene and Archean rhyolite hyaloclastite [55]. mechanism of lobe-Hyaloclastite More detailed and recent investigations about at Madneuli hydrothermal alteration mostly associated with the mineralization zones in the Madneuli deposit by [10] allowed us to establish the first alteration map for Figure 12(a) displays the relationship of the glassy- the Madneuli open pit. The following alteration zones like selvage lobe-hyaloclastite flow of our study with weredefined in the open pit: a silicified core, followed the coherent volcanic and adjacent rock units. The by a quartz-sericite-pyrite zone, a quartz-chlorite- hyaloclastite is located at the periphery of a rhyodacite sericiteand quartz-chlorite zone and weak regional lava lobe, therefore representing a gradual transition from chlorite-sericite. Also diagenetic/low temperature albite the massive, coherent part of the volcanic rock towards and chlorit [10]. Albite and chlorite are typical products its periphery at the contact with the volcano-sedimentary of seawater interaction with volcanic rocks at low rocks. The hyaloclastite rock likely consisted at the temperature [56, 57]. time of emplacement of unconsolidated bedded volcano-

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flow is massive in general, though locally ribbed, flow Table 2. Whole rock analyses of hyaloclastite rocks from the Madneuli open pit (concentrations in wt%): 1-2 hyaloclastite laminated and columnar jointed. The chaotic character with pillow like forms, 3-4 hyaloclastite with glassy like of the carapace breccia, their local distribution at the selvage. flow top, the absence of bedding and grading and lack 1 2 3 4 of broken crystals suggest an origin dominantly due to autobrecciation [33]. SiO2 74.74 77.77 75.50 69.94

TiO2 0.22 0.35 0.30 0.58 Al2O3 11.29 10.31 12.58 13.09 9. Model for the emplacement of Fe2O3 4.11 2.72 3.45 6.24 the lobe-hyaloclastite in the Madneuli MnO 0.08 0.07 0.04 0.17 MgO 2.91 1.83 1.14 2.49 deposit CaO 0.21 0.19 0.40 0.33 The hyaloclastite described in the Madneuli open pit Na2O 1.33 3.06 3.87 3.23 is associated with a submarine dome-like structure of K2O 1.43 0.58 0.90 1.08 felsic rhyodacite magmas and they were emplaced during

P2O5 0.05 0.06 0.05 0.15 several eruptive pulses [61–63]. It was accompanied LOI 2.95 2.85 1.85 2.56 by emplacement of isolated lobes. During the earliest Total 99.33 99.79 100.08 99.86 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 sedimentary units, with alternations of ash tuff, pumice silicified, altered and ore-bearing. The upper part consists tuff and sedimentary rocks. mostly of turbiditic rocks and bedded volcano-sedimentary Figure 12(b) shows that the central part of the pumiceous rocks. rhyodacitic lava lobe is surrounded by hyaloclastite In addition, numerous folds and fractures are present, formed in situ. The lobe is dissected in its external some of them being associated with uplift during the part and locally has a wavy-shaped outline (similar to formation of the dome structure. The newly rising the ones described by [39]), marked by the presence of magma could intrude along these faults or fracture small, pillow-like (or sigmoidal) forms at its periphery, systems and invade previously emplaced but still water- and forms a gradual transition from the massive, coherent saturated glass-like selvage hyaloclastite. The formation magmatic rock into the pillow-like shaped part. The lava of pumiceous hyaloclastite is related to second pulses of lobe is associated with a rhyodacitic lava flow with well- magmas. There are no constrains on the exact water depth exposed fluidal zonality in its external part. Peperite during formation of the hyaloclastite. The pumiceous is located at its contact with the volcano-sedimentary rhyodacitic hyaloclastite implies that volatile exsolution rock unit [58–60]. Identification of peperite during this was not inhibited by pressure [55], which indicates a study was of critical importance in clarifying the facies shallow water depth. The products of phreatomagmatic architecture and stratigraphy providing constraints on the eruption, represented by vesiculated fine-grained tuff age relationship and timing of intrusive episodes and associated with accretionary lapilli horizons and very sedimentation processes, as discussed previously by [53] fine-grained tuff, suggest that the associated eruption in the case of pumiceous peperite. was distal, at a distance of several km. Like in this Subaqueous felsic lavas can be divided into lobe- study, accretionary lapilli can also form in a submarine hyaloclastite flows, blocky subaqueous lava, domes, environment. According to [64] accretionary lapilli cryptodomes, and regionally extensive felsic lava [33]. can be found in subaqueous and redeposited deposits. Figure 13 is an idealized cross-section through a There are many examples, such as in the Devonian rhyolitic lobe-hyaloclastite flow, which illustrates the flow Lenneporphyr of Germany [65, 66], in the Haimaraka morphology and structures typical for proximal and distal Formation of Guyana [67], in the Tokiwa Formation of facies in such rock units. Japan [68] and in reworked deposits intercalated with The lobe hyaloclastite flow is inflated by successive pulses Paleogene volcanic rocks on the Voring Plateau in the of new magma, which feeds its large lobes. They generally North Sea [69]. Furthermore, accretionary lapilli were follow a very irregular path to the flow front, where describedin the deposits of the Ries impact crater in they form smaller lobes and locally they have small-sized southern Germany [70]. Deposits of hydromagmatic pillow-like shapes [33]. The Madneuli lobe-hyaloclastite eruption and hyaloclastite rocks are not contemporaneous.

323 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.

324 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; Hg- Hyaloclastite with glassy-like selvages. Not to scale.

The deposition of the lower part of the volcano- deep). The lack of pumiceous hyaloclastite in many sedimentary facies predate the formation of the upper part subaqueous lobe-hyaloclastite flows may simply reflect of this bedded sequence, which in turn predates formation their emplacement within deeper water [33]. of hyaloclastites and lobes, but it was still unconsolidated. 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 10. Conclusions of new magma. The lobe hyaloclastite described in this paper resembles hyaloclastite from other well known Two types of rhyodacitic lobes of lobe hyaloclastite flows deposits [49, 50, 55], which are common in submarine felsic are described for the first time in the Madneuli deposit successions and are one of the important characteristic of the Bolnisi mining district, Georgia: hyaloclastite facies for rocks hosting volcanogenic massive sulfide with glass-like selvages and hyaloclastite with pillow- deposits related to subaqueous felsic lavas/domes [33, 57, like forms, which represent the external part of individual 71–73]. lobes. The lobe structure with columnar jointing of the The association with a volcano-sedimentary complex, in internal part, described in the eastern part of the open which bedding textures are consistent with deposition pit is devoid of hyaloclastite, which indicates that the from turbidity currents, along with the presence of lobe was emplaced within the interior of the flow or slumps, cross-bedding, load casts, groove marks, wave dome during endogeneous growth [33]. This internal lobe and current ripples, different bioturbations and radiolaria- represents individual pulses of magma. bearing horizons, support a below wave-base submarine The absence of different resedimented rock fragments, depositional environment of the sedimentary rocks the gradational contact with coherent lava, their laterally associated with hyaloclastite at the Madneuli deposit. discontinuous character, and the absence of bedding Turbiditic volcano-sedimentrary rocks and hyaloclastite support their in situ hyaloclastitic nature. are present in the same stratigraphic section in the open Spherulites in the hyaloclastite are strong evidence pit, but were not coexisting during their formation. for high temperature devitrification of volcanic glass, The pumice-rich volcaniclastic rocks and also the fine- which was replaced by quartz and K-feldspar in the grained tuff with accretionary lapilli within the bedded groundmass. Classical perlitic fractures follow K-feldspar sedimentary and volcano-sedimentary complex in the open phenocrysts. This indicates that devitrification of volcanic pit are attributed to ashfall deposits of phreatomagmatic glass occurred after crystallization of phenocrysts and origin. 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. Acknowledgements The presence of pumiceous hyaloclastite in the subaqueous lobe hyaloclastite flow is a reliable evidence The research was supported by the Georgian National for shallow water depositional environment (<200 m Science Grant 204 and Swiss National Science

325 Architecture of Upper Cretaceous Rhyodacitic Hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia

Foundation through the research grant SNF 200020- deposits of the Caucasus and East PonticMetallotect. 113510 and SCOPES Joint Research Projects IB7320- Bulletin of the Mineral Research and Exploration, 111046 and IZ73Z0-128324. The authors would like 129, 2004, 1-16 to thank the other participants of the project: Tamara [8] Gugushvili V., Kutelia Z., Porphyry gold-copper Beridze, Stefano Gialli, Sophio Khutsishvili, Onise system of the Bolnisi mining district and analysis Enukidze and Ramaz Minigineishvili, and the staﬀ of the of two types of gold mineralization. Proceedings "Madneuli Mine" and Malkhaz Natsvlishvili for assistance, of the International Workshop: gold and base sharing geological information, and arranging access to metal deposits of the Mediterranean and the south the mine. Thanks to Jorge Relvas (Portugal) and Fernando Caucasus-challenges and opportunities, Tbilisi, 13- Tornos (Spain) for helpful discussions about facial units in 14 the Madneuli deposit. The research would not have been [9] Migineishvili R., Hybrid nature of the Madneuli Cu- possible without the support provided by SGA, SEG and Au deposit, Georgia. Bulgarian Academy of Sciences, IAVCEI organizations for attending many international proceedings of the 2005 Field Workshop, 127-132, conferences where the work was presented. 2012 [10] Gialli S., The controversial polymetallicMadneuli deposit, Bolnisi district, Georgia: hydrothermal alteration and ore mineralogy. Unpublished M.Sc. References thesis, University of Geneva, 2013, 1-143 [11] Popkhadze N., Moritz R., Gialli S., Beridze T., [1] Yilmaz A., Adamia Sh., Chabukiani A., Chkhotua T., Gugushvili V., Khutsishvili S., Major volcano- Erdogan K., Tuzcu S., Karabiyikoglu M., Structural sedimentary facies types of the Madneulipolymetallic correlation of the southern Transcaucasus (Georgia) deposit, Bolnisi district, Georgia: Implications for the - eastern Pontides (Turkey), Geological Society, host rock depositional environment. In: Erik Jonsson London, Special Publications, 173, 2000, 171-182 et al. (eds), Mineral deposit research for a high- [2] Bachaldin V.,Tvalchrelidze G., Some regulations tech world, 12th SGA Meeting, 12-15 August 2013, of formation and distribution of ore deposits in Sweden, Uppsala, 2, 576-579 volcanogenic rocks (Southern Georgia). Proceedings [12] Popkhadze N., Beridze T., Moritz R., Gugushvili of the Institution of Higher Education, Geological V., Khutsishvili S., Facies analysis of the volcano- Prospecting, 1, 1963, 61-72 (in Russian) sedimentary host rocks of the Cretaceous Madneuli [3] Malinovsky E., Sokolov A., Lezhepiokov L., massive sulphide deposit, Bolnisi district, Georgia. Structural-geological conditions and stages of Bulletin of the Georgia National Academy of formation of Madneuli copper-barite polymetallic Sciences, 3, 2009, 103-108 deposit (Lesser Caucasus), Geology of Ore Deposits [13] Popkhadze N., First evidence of hyaloclastites at 4, 1987, 44-57 Madneuli deposit, Bolnisi district, Georgia, Bulletin [4] Gugushvili V., Omiadze G., Ignimbrite volcanism and of the Georgia National Academy of Sciences, 6, ore mineralization (Bolnisi Ore District, the Lesser 2012, 83-90 Caucasus), Geology of Ore Deposits, 2, 1988, 105- [14] Mederer J., Moritz R., Ulianov A., Chiaradia 109 (in Russian) M., Middle Jurassic to Cenozoic evolution of arc [5] Kekelia S., Ambokadze A., Ratman I., Volcanogenic magmatsm during Neotethyssubduction and arc- deposits of base metals of paleoisland arc structures continent collision in the Kapan zone, southern and method of their prediction, Metsniereba, Tbilisi, Armenia, Lithos, 2013, 177, 61-78 1993, 0-96 [15] McPhie J., Allen R., Facies architecture of [6] Moon C., Gugushvili V., Kekelia M., Kekelia mineralized submarine volcanic sequences: Cambrian S., Migineishvili R., Otkhmezuri Z., Ozgur N., Mount Read Volcanics, western Tasmania, Economic Comparison of mineral deposits between Georgian Geology, 87, 1992, 587-596 and Turkish sectors of the Tethyanmetallogenic belt. [16] McPhie J., Allen R., Submarine, silicic, syn-eruptive In: Piestrzynski et al (eds), Mineral deposits at the pyroclastic units in the Mount Read Volcanics, Beginning of the 21st Century. 6th Biennial SGA Western Tasmania: inﬂuences of vent setting and Meeting Krakow, Poland, 26-29 August, 309-312, proximity on lithofacies characteristics. In: White J., 2001 Smellie J., Clague D., (Eds.), Explosive Subaqueous [7] Kekelia S., Kekelia M., Otkhmezuri Z., Ozgur N., Volcanism: Geophysical Monograph Series, 140, Moon C., Ore-forming systems in volcanogenic- 2003, 245-258 sedimentary sequences by the example of base metal

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