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

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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- 308 N. Popkhadze et al. 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. 310 N. Popkhadze et al. 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
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