Universit´e de Gen`eve
D´epartement de Min´eralogie Facult´e des Sciences
Prof. Robert Moritz
Regional setting, geological context and genetic aspects of polymetallic hydrothermal ore deposits
from the Kapan ore district, southern Armenia:
a contribution to the Mesozoic island arc metallogeny of the Lesser Caucasus
TH ` ESE
pr´esent´ee `a la Facult´e des Sciences de l’Universit´e de Gen`eve
pour obtenir le grade de Docteur `es sciences, mention Sciences de la Terre
par
Johannes Mederer
de Neumarkt i.d.OPf. (Allemagne)
Th`ese No 4587
Gen`eve
Atelier de reprographie ReproMail
Contents
Acknowledgements v
Abstract vii
Résumé en Français ix
Introduction and Overview of Thesis xi
Bibliography . . . xii
1 Base and precious metal mineralization in Middle Jurassic rocks of the Lesser Caucasus - A review of geology and metallogeny and new data from the Kapan, Alaverdi and Mehmana districts 1 1.1 Introduction . . . 3
1.2 Tectonic Setting and Regional Geology . . . 5
1.2.1 Main tectonic units in the central part of the Tethyan belt . . . 5
1.2.2 Geology of the Lesser Caucasus . . . 6
1.3 Ore deposits in Middle Jurassic rocks of the Eurasian margin . . . 10
1.3.1 Ore deposits in the Kapan mining district . . . 10
1.3.2 Ore deposits in the Alaverdi district . . . 16
1.3.3 Mineralization in the Drmbon deposit of the Mehmana district . . . . 19
1.4 Sulfur isotopes . . . 23
1.5 Geochemistry . . . 24
1.5.1 Sample material and analytical techniques . . . 24
1.5.2 Results . . . 24
1.6 Discussion . . . 27
1.6.1 Sulfur isotopes . . . 27
1.6.2 Geochemistry and geodynamic setting . . . 28
1.6.3 Ore-forming environments . . . 29
1.6.4 Implications for exploration . . . 31
1.7 Conclusions . . . 31
1.8 Bibliography . . . 32
2 Middle Jurassic to Cenozoic evolution of arc magmatism during Neotethys subduction and arc-continent collision in the Kapan Zone, southern Armenia 43 2.1 Introduction . . . 45
2.2 Geological Framework . . . 45
2.2.1 Geodynamic Setting . . . 45
2.2.2 Geology of the Lesser Caucasus . . . 46
2.3 Analytical techniques and sample material . . . 53
2.3.1 Whole rock geochemistry by XRF and LA-ICP-MS . . . 53
2.3.2 U-Pb Zircon dating by LA-ICP-MS . . . 53
2.3.3 Radiogenic isotopes . . . 56
2.4 Results . . . 57
2.4.1 Geochemistry . . . 57
2.4.2 U-Pb zircon geochronology . . . 60
2.4.3 Whole-rock Sr, Nd and Pb isotopes . . . 62
2.5 Discussion . . . 62
2.5.1 Age of intrusive rocks in the Kapan Zone . . . 62
2.5.2 Depositional environment of volcanic and volcaniclastic rocks . . . 62
2.5.3 Origin of magmatism in the Kapan Zone . . . 64
2.5.4 Setting of the Lesser Caucasus magmatic arc within the Tethyan belt 66 2.6 Conclusions . . . 67
2.7 Bibliography . . . 68
2.A Zircon data . . . 75
3 Geology, mineralization, isotopic constraints and absolute dating of base and precious metal ore deposits in the Kapan district, southern Armenia 79 3.1 Introduction . . . 81
3.2 Geological setting . . . 81
3.3 Mineralization in the Kapan district . . . 85
3.3.1 The Centralni West Cu deposit . . . 88
3.3.2 The Centralni East Cu-Au deposit . . . 88
3.3.3 The Shahumyan polymetallic Cu-Au-Ag-Zn±Pb deposit . . . 88
3.4 Analytical techniques and sample material . . . 91
3.4.1 Radiogenic isotope analyses . . . 91
3.4.2 Stable isotopes . . . 93
3.4.3 Geochronology . . . 97
3.4.4 Geochemistry of whole-rock ore samples . . . 101
3.5 Results . . . 102
3.5.1 Radiogenic isotopes . . . 102
3.5.2 Stable isotopes . . . 103
3.5.3 Geochronology . . . 104
3.5.4 Whole-rock geochemistry of ore samples . . . 107
3.6 Discussion . . . 107
3.6.1 Crosscutting relationships between dykes and mineralization . . . 107
3.6.2 The source of metals . . . 110
3.6.3 The source of hydrothermal fluids - evidence from Sr, O, C and H isotopes110 3.6.4 The source of sulfur and implications from sulfur isotopes . . . 112
3.6.5 Timing of mineralization . . . 116
3.6.6 Multiple ore-forming events in the Kapan district . . . 118
3.6.7 A magmatic-hydrothermal model of ore deposit formation . . . 119
3.7 Conclusions . . . 121
3.8 Bibliography . . . 121
Contents
3.A Mineral geochemistry (Microprobe and LA-ICP-MS), 40Ar/39Ar step-heating
isotope data and geochemical whole rock data of ore samples . . . 130
A Geological setting of the Lesser Caucasus 139 A.1 Geological units in the central part of the Tethyan belt between eastern Turkey and northern Iran . . . 139
A.1.1 Platforms . . . 139
A.1.2 Orogens . . . 139
A.1.3 Magmatic or volcanic belts and their associated basins . . . 143
A.2 Plate tectonic history . . . 144
A.2.1 Pre-Mesozoic history of the Scythian region . . . 144
A.2.2 Late Triassic to Late Jurassic Cimmerian orogeny and associated rifting 144 A.2.3 Early Cretaceous rift systems . . . 146
A.2.4 Upper Cretaceous - Paleocene inversion tectonics . . . 146
A.2.5 Caucasus orogeny . . . 147
A.3 Geology of the Lesser Caucasus . . . 147
A.3.1 Eurasian margin . . . 148
A.3.2 The Sevan-Akera suture zone . . . 148
A.3.3 South Armenian Block . . . 149
A.4 Bibliography . . . 149
B List of samples 157
Acknowledgements
Many people contributed to the success of this project during the last couple of years, and this thesis would not have been finished without their help.
First of all, big thanks to Robert Moritz who initiated this project and who offered me the opportunity to work in the Lesser Caucasus and to come to Geneva. Robert accompanied this research over the years with personal interest and always had an open door for discussion.
Thanks for encouraging me since the beginning to present my work at international confer- ences and workshops, where I could meet and discuss with colleagues and visit ore deposits in the field. Lluís Fontboté, Massimo Chiaradia, Richard Herrington and Marc Sosson are thanked for accepting to form part of the PhD committee.
I think it is a privilege to study and work at our Department at the University of Geneva and within the framework of ELSTE and other Swiss universities. I want to thank in general all the people who spend their time and efforts to provide the optimal conditions for us PhD students, and in particular Lluís Fontboté and Urs Schaltegger. Lluís always encouraged me to participate in courses and excursions which significantly improved my knowledge about ore forming processes.
In Geneva I want to thank Honza Catchpole, who reviewed large parts of this manuscript and Aldo Bendezú for many discussions and his magic eye with the hand lens and the micro- scope. Thanks to Kalin Kouzmanov who introduced me to work with the electron microprobe in Lausanne, to Richard Spikings for his support in the Argon lab and to Susanne Schmidt for the time we spent in the preparation of microscopy classes. Michèle Senn-Gerber, Jean-Marie Boccard, Fabio Capponi, Fred Arlaud and Sofia Saldana provided help with many technical and administrative issues. Mariel Reitsma, Roelant van der Lelij, Melissa Ortelli, Marie- Caroline Pinget, Stefano Gialli, Ryan Cochrane, Cyril Chelle-Michou, Cedric Schneider, Jörn Wotzlaw and many others at the University of Geneva helped with advice on sample prepa- ration and analytical topics. Hervé Rezeau is thanked for his help with the French abstract.
In Lausanne, Alexey Ulianov provided excellent support during the LA-ICP-MS sessions and Jorge Spangenberg, Kerstin Bauer and Benita Putlitz helped with stable isotope analyses.
I want to especially thank my Armenian and Georgian colleagues, whose knowledge and lo- gistical support was highly appreciated. These include Arman Vardanyan, Sergej Zohrabyan, Samvel Hovakimyan, Rafael Melkonyan, Rodrig Tayan, Vazgen Vardanyan, Tamara Beridze and Nino Popkhadze. Ross Overall, Istvan Márton and Alan Turner are thanked for their thoughts on the geology of the Kapan deposits and their support during my stay in the field.
I owe all my thanks to my family for their unconditional support and for always accepting my decisions. Finally I thank Cristina who was always with me during the last years and whose energy and positive attitude helped me tremendously. Gracias por todo, Cris!
Abstract
The Lesser Caucasus is part of the Tethyan metallogenic belt and formed by collision of the Gondwana-derived South-Armenian Block with the Eurasian margin in the Late Cretaceous to Early Paleocene. Long-lasting Mesozoic northeastwards subduction of the Neotethys below the Eurasian margin was re- sponsible for the evolution of the Jurassic- Cretaceous island-arc domain of the Lesser Caucasus, that can be divided into the Ka- pan Zone and the Somkheto-Karabakh Island Arc.
In the Kapan Zone, the stratigraphic col- umn is composed of volcanic and volcanosed- imentary rocks of the Middle Jurassic, Up- per Jurassic-Lower Cretaceous and Paleogene magmatic complexes and Quaternary basan- ite flows. Intrusive rocks within the Middle Jurassic magmatic complex do not outcrop, but tonalite clasts from steeply dipping peb- ble dykes were dated at a U-Pb zircon age of 165.6±1.4 Ma. Voluminous plutons in- truded in the Kapan Zone between 137.7±1.6 and 131.5±2.1 Ma and Paleogene gabbro was dated at 50.82±0.51 Ma. Subduction-related signature has been observed in igneous rocks from the Middle Jurassic and Upper Jurassic- Lower Cretaceous magmatic complexes, but also in Paleogene volcanic and intrusive rocks that formed after the subduction below the Lesser Caucasus magmatic arc had already ceased. The conserved subduction-related sig- nature in post-collisional igneous rocks from the Kapan Zone indicates partial melting from mantle that was either widely metasom- atized during preceding subduction, or meta- somatized during ongoing subduction further in the west. The primitive Pb, Sr and
Nd isotopic signatures of igneous rocks from the three magmatic complexes and Quater- nary basanite are in agreement with melt- ing from a primitive mantle source and lim- ited contribution of evolved crustal material.
Middle Jurassic to Cretaceous subduction- related igneous activity also took place in the Sanandaj-Sirjan Zone in Iran, but is absent in the Alborz mountains. This suggests a con- tinuation of the magmatic arc of the Lesser Caucasus to the southwest of the Kapan Zone, and indicates one segmented magmatic arc that was active along the Eastern Pontides, the Lesser Caucasus and the Sanandaj-Sirjan Zone.
The Kapan ore district in southern Ar- menia has multiple base and precious metal ore deposits that are hosted in Middle Juras- sic volcanic and volcaniclastic rocks with dominantly andesitic to dacitic composi- tion. Focus of this study are the Cen- tralni West Cu deposit, the Centralni East Cu±Au deposit and the Shahumyan Cu-Au- Ag-Zn±Pb deposit that occur in close prox- imity to each other in an area of 4x3 km2. The main mineralization styles in the dis- trict are steeply dipping east-west striking chalcopyrite-pyrite and polymetallic veins, but stockwork-type mineralization and prefer- ential replacement of host rock matrix in clas- tic rocks also occur. Mineralization is of dom- inantly intermediate sulfidation state (chal- copyrite, pyrite and tennantite-tetrahedrite), but smaller occurrences of high-sulfidation state mineral assemblages (luzonite, enar- gite, bornite, digenite and chalcocite) occur in places. Alteration varies from chlorite- carbonate-epidote±sericite alteration in the
Centralni West deposit to phyllic, argillic and poorly developed residual quartz alteration in the Centralni East and Shahumyan de- posits. Strontium and stable isotopes (O, C, H, S) in gangue and ore minerals from the three deposits indicate a dominantly mag- matic fluid source that mixed with variable amounts of seawater during the late carbon- ate stage. The Pb isotopic composition of sulfides and alunite from the Centralni West, Centralni East and Shahumyan deposits plot in a narrow range and overlap with the age- corrected Pb isotopic composition of Mid- dle Jurassic igneous rocks. The Pb isotopic data suggests that metals in the ore de- posits were derived from magmatic fluids that were exsolved upon crystallization of Mid- dle Jurassic intrusive rocks or leached from Middle Jurassic country rocks. Mineraliza- tion in the Kapan ore district occurred in pulses and 40Ar/39Ar mineralization ages of 161.78±0.79 Ma and 156.14±0.79 Ma have been obtained from hydrothermal muscovite in the Centralni West deposit and magmatic- hydrothermal alunite in the Shahumyan de- posit, respectively. Weighted average Re-Os model ages of 215.1±4.3 Ma and 146.0±3.1 Ma have been obtained from pyrite from the Centralni West and Centralni East deposits, respectively. However, the unreasonably old age for the Centralni West deposit question the reliability of the obtained ages and in- dicate disturbance of the Re-Os isotopic sys- tem in pyrite from the Kapan district. Based on field observations, ore and gangue min- eral assemblages, alteration styles and stable and radiogenic isotope analyses, we propose an intrusion-related scenario for the base and precious metal ore deposits with intermediate to high sulfidation state in the Kapan district.
Ore deposits in Middle Jurassic host rocks also occur in the Mehmana and Alaverdi districts of the Somkheto-Karabakh Island Arc. Polymetallic veins, massive strati- form replacement bodies at lithological con- tacts and stockwork type mineralization are hosted by sericitic and argillic altered vol-
canic and volcanosedimentary rocks. Middle Jurassic host rocks from the Mehmana and Alaverdi districts have subduction-related sig- nature and similar REE and trace element patterns as coeval rocks from the Kapan Zone.
The similarities between ore deposits and ig- neous host rocks from the Kapan Zone and the Somkheto-Karabakh Island Arc suggest that similar petrogenetic and ore-forming pro- cesses operated along the entire axis of the Lesser Caucasus magmatic arc. This opens the possibility for conceptional base metal ex- ploration models in other parts of the Lesser Caucasus magmatic arc, where similar de- posits might be hidden under barren cover rocks.
Résumé en Français
Le Petit Caucase se localise dans la ceinture métallogénique théthysienne entre les Pon- tides au nord de la Turquie et l’Iran. Il ré- sulte de la collision du bloc sud-Arménien avec l’Eurasie entre le Crétacé supérieur et le Paléogène inférieur. Durant le Mésozoïque, la subduction prolongée de la Néotéthys sous la marge eurasienne est responsable de l’évolution de l’arc volcanique du Pe- tite Caucase qui peut être divisé en deux zones: la «Kapan Zone» et l’arc insulaire du «Somkheto-Karabakh». Dans la «Ka- pan Zone», la colonne stratigraphique est composée de roches volcaniques et volcano- sédimentaires, et se divise entre les trois com- plexes magmatiques du Jurassique moyen, Jurassique supérieur à Crétacé inférieur et Paléogène. Les coulées de basanite Qua- ternaire couvrent les roches des trois com- plexes magmatiques. Les roches intrusives du complexe magmatique du Jurassique moyen n’affleurent pas dans la «Kapan Zone», mais elles se retrouvent sous forme de clastes dans des brèches qui suivent la forme de dykes.
Les datations U-Pb sur zircon donnent un âge de 165.6±1.4 pour les clastes, des âges en- tre 137.7±1.6 et 131.5±2.1 Ma pour les corps plutoniques de la «Kapan Zone» et un âge de 50.8±0.5 pour le gabbro Paléogène. Les roches magmatiques de la «Kapan Zone» sont caractérisées par des signatures géochimiques liées à un contexte de subduction. Cette par- ticularité se retrouve aussi dans les roches d’âge Paléogène qui se sont formées à la fin de l’épisode de subduction de la Néotéthys sous l’arc magmatique du Petit Caucase. Ces caractéristiques s’expliquent par la fusion par- tielle du manteau qui a été métasomatisé soit
pendant la précédente subduction, soit du- rant une subduction active localisée plus à l’ouest du Petit Caucase. Les signatures iso- topiques (Pb, Sr, Nd) des roches magmatiques de la «Kapan Zone» sont en accord avec la fusion du manteau sans influence significative d’assimilation de matériel d’origine crustale.
L’activité magmatique, d’âge Jurassique à Crétacé, liée à la subduction de la Néotéthys s’est aussi développé dans la «Sanandaj-Sirjan Zone» en Iran, cependant elle est absente dans les montagnes de l’Elbourz. Ces observations suggèrent une segmentation de l’arc magma- tique actif le long des Pontides en Turquie, du Petit Caucase en Arménie et de la «Sanandaj- Sirjan Zone» en Iran.
Dans le camp minier de Kapan au sud de l’Arménie, des gisements de métaux de bases et de métaux précieux sont encaissés dans des roches volcaniques et volcano-sédimentaires, de composition andésitique à dacitique, et d’âge Jurassique moyen. Cette étude porte sur les gisements de Cu (Centralni Ouest), de Cu±Au (Centralni Est) et de Cu-Au-Ag- Zn±Pb (Shahumyan), qui sont à proximité les uns des autres dans un espace de 4x3 km2. Dans le camp minier, les principales minéral- isations sont des veines à chalcopyrite-pyrite et des veines polymétalliques caractérisées par de fort pendage et une orientation est-ouest.
Dans une moindre mesure, des minéralisa- tions de type «stockwork» et des corps de remplacement sont aussi observés. Les as- semblages minéralogiques se caractérisent ma- joritairement par un état de sulfuration inter- médiaire (chalcopyrite, pyrite et tennantite- tétraédrite) mais la présence de minéraux in- dicateurs d’un état de haute sulfuration (lu-
zonite, énargite, bornite, digénite et chal- cocite) sont aussi observés à certains endroits.
Les types d’altération varient en fonction des gisements et de la proximité par rapport à la minéralisation. L’altération de type chlorite- carbonate-épidote et séricite est observée à Centralni Ouest alors que les altérations de type phyllique et argillique ainsi que des zones de silice résiduelle sont observées sur les gise- ments de Centralni Est et Shahumyan.
Les isotopes stables (O, C, H, S) et les isotopes du Sr dans les minéraux liés à la minéralisation et les minéraux de gangue in- diquent une signature magmatique prédom- inante pour la source des fluides hydrother- maux, cependant le mélange avec des quan- tités variables d’eau de mer lors de la dé- position tardive des carbonates est suggéré.
La composition isotopique du Pb dans les sulfures et dans les alunites des trois gise- ments est homogène et ressemble à la compo- sition isotopique des roches encaissantes d’âge Jurassique moyen dans le camp minier de Ka- pan. Les données suggèrent que les métaux ont été lessivés depuis les roches encaissantes ou bien exsolvés à partir d’un magma d’âge Jurassique moyen lors de la cristallisation en profondeur.
Plusieurs événements minéralisateurs sont reconnus dans le camp minier de Kapan. La muscovite d’origine hydrothermale du gise- ment de Centralni Ouest est datée à 161.8 Ma par 40Ar/39Ar et l’alunite du gisement Shahumyan à 156.1 Ma. Les résultats des datations de type Re-Os sur pyrite indiquent une perturbation du système isotopique à l’échelle du camp minier. Sur la base des observations de terrain, des assemblages minéralogiques de la gangue et des minérali- sations, des types d’altération et des résultats des isotopes stables et radiogéniques, nous proposons une mise en place des minérali- sation sliée à des intrusions formées en pro- fondeur.
Des gisements similaires à ceux décrits dans le camp minier de Kapan se retrou- vent aussi encaissés dans des roches d’âge
Jurassique moyen dans les camps miniers de Mehmana et Alaverdi dans l’arc insulaire du
«Somkheto-Karabakh». Des veines polymé- talliques et des corps de remplacement mas- sifs sont observés aux contacts lithologiques alors que la minéralisation de type «stock- work» se localise dans les halos d’altération séricitique et argilique des roches volcaniques et volcanosédimentaires encaissantes. Ces dernières partagent de nombreuses similarités géochimiques avec les roches encaissantes du même âge de la «Kapan Zone». Les simil- itudes entre les gisements et les roches en- caissantes dans la «Kapan Zone» et l’arc in- sulaire du «Somkheto-Karabakh» suggèrent une pétrogenèse et des processus de formation des minéralisations similaires tout le long de l’arc magmatique au Jurassique moyen dans le Petit Caucase. Cela ouvre la voie pour l’exploration des métaux dans les autres par- ties de l’arc magmatique du même âge, où des gisements similaires pourraient être cachés en dessous des roches stériles.
Introduction and Overview of Thesis
The Lesser Caucasus forms part of the Tethyan metallogenic belt and hosts a vari- ety of base and precious metal ore deposits that have so far received only little attention in Western literature. The region represents the opportunity to study the relationship be- tween changing geodynamic settings and re- lated magmatism and ore deposit formation.
Excluding mineralization in basement rocks, the oldest known deposits in the Lesser Caucasus formed during northeastwards sub- duction of the Neotethys below the Eurasian margin and their age varies from Middle Jurassic to Lower Cretaceous. The ore bodies are polymetallic veins, massive replacement bodies at lithological contacts and porphyry- and epithermal deposits (Hemon et al., 2012;
Mederer et al., in revision; Moritz et al., 2012). During regional extension in the Up- per Cretaceous, volcanogenic-massive sulfide deposits with transitional features towards epithermal gold deposits formed (Gialli et al., 2012; Moritz et al., 2012). Large Cenozoic porphyry Mo-Cu and epithermal precious metal deposits are associated with composite plutons that intruded after the collision of the South-Armenian Block with the Eurasian margin (Hovakimyan, 2011; Moritz et al., 2011,2012).
The aim of this thesis is a better un- derstanding of the geological setting and depositional environment of base and pre- cious metal ore deposits hosted by Middle Jurassic volcanic and volcanosedimentary rocks in the island-arc domain of the Lesser Caucasus. Several genetic scenarios had been proposed, including a volcanogenic massive sulfide (Schmidt et al.,1985) and a porphyry-
related scenario (Wolfe and Gossage, 2009).
On the basis of detailed descriptive and an- alytical studies of the ore deposits and their subduction-related host rocks, we propose an intrusion-related magmatic-hydrothermal scenario for their genesis.
Chapter 1
A review of the tectonic setting and the re- gional geology of the Lesser Caucasus are presented together with a compiled transna- tional geological map of the region. Ore de- posits hosted by Middle Jurassic rocks from the Kapan, Alaverdi and Drmbon districts are presented and their mineralization and alter- ation styles compared. Trace- and REE el- ement geochemical data from Middle Juras- sic country rocks add arguments to evaluate the ore-forming environment during the Mid- dle to Upper Jurassic along the Lesser Cau- casus island arc domain. This chapter is an invited review with new data for publication in Ore Geology Reviews and is accepted pend- ing some revisions.
Chapter 2
Robust geochemical and geochronological data for Middle Jurassic to Quaternary ig- neous rocks from the Lesser Caucasus mag- matic arc have not been obtained so far from the Kapan Zone in southern Armenia. We present first U-Pb zircon ages (LA-ICP-MS) of Middle Jurassic, Lower Cretaceous and Eocene intrusive rocks from the Kapan Zone.
The stratigraphic column of this tectonic unit
allows a detailed investigation and discussion of the evolution and origin of Middle Jurassic to Quaternary magmatism in the Lesser Cau- casus. Trace and REE geochemical data and the isotopic composition of volcanic and intru- sive rocks are used to constrain the magmatic evolution of the Kapan Zone, that hosts sev- eral hydrothermal ore deposits. This chapter is a manuscript currently in press in Lithos.
Chapter 3
The Chapter provides detailed descriptions of the hydrothermal mineralization styles in the Kapan ore district, where base and pre- cious metal ore deposits are hosted by Mid- dle Jurassic volcanic and volcanosedimentary rocks. 40Ar/39Ar and Re-Os mineralization ages are presented for the Centralni West, Centralni East and Shahumyan deposits that occur within an area of 3x4 km2. Stable (O, S, C, H) and radiogenic (Pb, Sr) isotope analy- ses are presented and used to trace the source for metals and fluids in the hydrothermal ore deposits. The data is integrated into a genetic model for ore deposit formation in the Kapan ore district. This chapter is a manuscript in- tended to be submitted to Economic Geology.
Appendix A
This appendix chapter contains a detailed re- view of the geological units in the central part of the Tethyan belt between eastern Turkey and northern Iran and expands the geologic information provided in Chapter 1. It also summarizes the plate tectonic history of the Lesser Caucasus and adjacent regions from Paleozoic to Cenozoic time.
Bibliography
Gialli, S., Moritz, R., Popkhadze, N., Gugushvili, V., Migineishvili, R., Spangenberg, J., 2012. The Mad- neuli Polymetallic Deposit, Lesser Caucasus, Geor- gia: A Transitional System with Magmatic Input in a Submarine Environment, in: Hedenquist, J.W., Fontboté, L. (Eds.), Integrated Exploration and
Ore Deposits, Proceedings SEG 2012 Conference, Lima, Peru. p. Poster 38.
Hemon, P., Moritz, R., Ramazanov, V., 2012. The Gedabek Epithermal Cu-Au Deposit, Lesser Cau- casus, Western Azerbaijan: Geology, Alterations, Petrography and Evolution of the Sulfidation Fluid States, in: Hedenquist, J.W., Fontboté, L. (Eds.), Integrated Exploration and Ore Deposits, Proceed- ings SEG 2012 Conference, Lima, Peru. p. Poster 50.
Hovakimyan, S.E., 2011. Origin and Paleotectonic Conditions of Formation of Ore-Containing Frac- tures of the Lichk-Aygedzor Ore Field of South- ern Armenia, Lesser Caucasus, in: Abstract Vol- ume, 9th Swiss Geoscience Meeting, 11-13 Novem- ber 2011, Zurich, p. 114.
Mederer, J., Moritz, R., Zohrabyan, S.A., Vardanyan, A.V., Melkonyan, R.L., Ulianov, A., in revision.
Base and precious metal mineralization in Middle Jurassic rocks of the Lesser Caucasus - A review of geology and metallogeny and new data from the Kapan, Alaverdi and Mehmana districts. Ore Ge- ology Reviews .
Moritz, R., Mederer, J., Ovtcharova, M., Selby, D., Chiaradia, M., Popkhadze, N., Gugushvili, V., Migineshvili, R., Melkonyan, R., Tayan, R., Vardanyan, A., Havokimyan, S., Ramazanov, V., Mansurov, M., 2011. Major Cu, Au and Mo de- posits of the Lesser Caucasus: Products of diverse geodynamic settings, in: Abstract Volume, 9th Swiss Geoscience Meeting, 11-13 November 2011, Zurich, pp. 100–101.
Moritz, R., Selby, D., Ovtcharowa, M., Mederer, J., Melkonyan, R., Hovakimyan, S.E., Tayan, R., Pop- khadze, N., Gugushvili, V., Ramazanov, V., 2012.
Diversity of geodynamic settings during Cu, Au and Mo ore formation in the Lesser Caucasus: new age constraints, in: Proceedings 1st triennial EMC meeting, Frankfurt, Germany, p. 745.
Schmidt, A.I., Sher, L.S., Portnoy, A.L., Minyn, D.A., 1985. Perfection of the methods of large scale prog- nosis of pyrite, copper-molybdenum and gold min- eralization on the basis of structural-formational analysis of long-term areas of Kapan ore bearing zone of Armenian SSR. Central Research Institute of Geological Prospecting for Base and Precious Metals (TsNIGRI), Ministry of Natural Resources, Russia, Moscow (in Russian).
Wolfe, B., Gossage, B., 2009. Technical Report for the Kapan Project, Kapan, Armenia. Coffey Mining Pty Ltd on behalf of Deno Gold Mining Company CJSC.
Chapter 1
Base and precious metal mineralization in Middle Jurassic rocks of the Lesser Caucasus - A review of geology and metallogeny and new data from the Kapan, Alaverdi and Mehmana districts
Johannes Mederera, Robert Moritza, Sergej Zohrabyanb, Arman Vardanyanb, Rafael Melkonyanb, Alexey Ulianovc
aEarth and Environmental Sciences Section, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland
bInstitute of Geological Sciences, National Academy of Sciences of Armenia, 24A Marshall Baghramian Avenue, Yerevan 0019, Armenia
cInstitute of Earth Sciences, University of Lausanne, Géopolis, 1015 Lausanne, Switzerland
Ore Geology Reviews, accepted with some revisions
Highlights
• Different mineralization styles caused various genetic models in the past.
• We present a compiled transnational geological map with major ore deposits.
• The Lesser Caucasus magmatic arc is a discontinuous segmented arc.
• Middle Jurassic rocks are favorite hosts for copper-gold and polymetallic mineralization.
• Mineralization hosted by Middle Jurassic rocks formed from magmatic-hydrothermal systems.
The polymetallic Cu-Au-Ag-Zn±Pb, Cu-Au and Cu deposits in the Kapan, Alaverdi and Mehmana mining districts of Armenia and the Nagorno-Karabakh region form part of the Tethyan belt. They are hosted by Middle Jurassic rocks of the Lesser Caucasus paleo-island arc, which can be divided into the Kapan Zone and the Somkheto-Karabakh Island Arc.
Mineralization in Middle Jurassic rocks of this paleo-island arc domain formed during the first of three recognized Mesozoic to Cenozoic metallogenic epochs. The Middle Jurassic to Lower Cretaceous metallogenic epoch comprises the described deposits of this contribution and porphyry Cu, skarn and epithermal deposits related to Upper Jurassic and Lower Cre- taceous intrusions. The second and third metallogenic epochs of the Lesser Caucasus are represented by Upper Cretaceous volcanogenic massive sulfide (VMS) deposits with tran- sitional features towards epithermal mineralization and by Eocene to Miocene world-class porphyry Mo-Cu and epithermal precious metal deposits, respectively.
The ore deposits in the Kapan, Alaverdi and Mehmana mining districts are poorly un- derstood and former authors named them copper-pyrite, Cu-Au or polymetallic deposits.
Different genetic origins were proposed for their formation, including VMS and porphyry- related scenarios. The ore deposits in the Kapan, Alaverdi and Mehmana mining districts are characterized by different mineralization styles, which include polymetallic veins, massive stratiform replacement ore bodies at lithological contacts, and stockwork style mineraliza- tion. Sericitic, argillic and advanced argillic alteration assemblages are widespread in the deposits which have intermediate to high-sulfidation state mineral parageneses that consist of tennantite-tetrahedrite plus chalcopyrite and enargite-luzonite-colusite, respectively. The ore deposits are spatially associated with differentiated calc-alkaline intrusions, character- ized by a dominantly Cu-Au metal budget and pebble dykes are widespread. Publishedδ34S values for sulfides and sulfates are in agreement with a magmatic source for the bulk sulfur whereas publishedδ34S values of sulfate minerals partly overlap with the isotopic composi- tion of contemporaneous seawater. Published mineralization ages demonstrate discrete ore forming pulses from Middle Jurassic to the Upper Jurassic-Lower Cretaceous boundary, in- dicating time gaps of 5 to 20 Ma in between the partly subaqueous deposition of the host rocks and the epigenetic mineralization.
Most of the described characteristics indicate an intrusion-related origin for the ore deposits in Middle Jurassic rocks of the Lesser Caucasus, whereas a hybrid VMS-epithermal-porphyry scenario might apply for deposits with both VMS- and intrusion-related features.
The volcanic Middle Jurassic host rocks for mineralization and Middle to Upper Jurassic intrusive rocks from the Somkheto-Karabakh Island Arc and the Kapan Zone show typical subduction-related calc-alkaline signature. They are enriched in LILE such as K, Rb and Ba and show negative anomalies in HFSE such as Nb and Ta. The ubiquitous presence of amphibole in Middle Jurassic volcanic rocks reflects magmas with high water contents.
Flat REE patterns ([La/Yb]N = 0.89-1.23) indicate a depleted mantle source and concave- upward (listric-shaped) MREE-HREE patterns ([Dy/Yb]N = 0.75-1.21) suggest melting from a shallow mantle reservoir. Similar trace element patterns of Middle Jurassic rocks from the Somkheto-Karabakh Island Arc and the Kapan Zone indicate that these two tectonic units form part of one discontinuous segmented arc. Similar petrogenetic and ore-forming processes operated along its axis and Middle Jurassic volcanic and volcanosedimentary rocks constitute the preferential host for polymetallic Cu-Au-Ag-Zn±Pb, Cu-Au and Cu mineral- ization, both in the Somkheto-Karabakh Island Arc and the Kapan Zone.
Keywords
Tethyan belt, Lesser Caucasus, Armenia, Nagorno-Karabakh, VMS-epithermal-porphyry transition
1.1. Introduction
1.1 Introduction
The present day Lesser Caucasus moun- tain range is situated in the central part of the Tethyan belt and formed by collision of the Gondwana-derived South-Armenian Block with the Eurasian margin (Adamia et al., 1981; Golonka, 2004; Sosson et al., 2010;Stampfli and Borel,2002). The Tethyan belt extends from western Europe, through Turkey, the Lesser Caucasus, Iran, Pak- istan, Southeast Asia, to the southwest Pa- cific. The belt is well-known for its metal endowment and examples of prominent de- posits are the Murgul VMS deposit in eastern Turkey (Schneider et al.,1988), the Kisladag porphyry Au deposit in west-central Turkey (Yigit,2009), the Çöpler porphyry-epithermal Au deposit in east-central Anatolia (İmer et al.,2013) or the Sungun porphyry Cu de- posit in northern Iran (Hezarkhani, 2006).
This study is focused on the Lesser Cauca- sus orogen which extends in a NW-SE direc- tion from southern Georgia to Armenia and Azerbaijan. We present a review of the main tectonic units and the geodynamic evolution of the orogen, an area that was given little attention in western literature to date. The complex geodynamic evolution of the Lesser Caucasus is reflected by a variety of ore de- posits that formed during successive geody- namic settings. Excluding basement rocks, three metallogenic epochs have been distin- guished in the Lesser Caucasus which range in age from Middle Jurassic to Neogene (Moritz et al.,2012).
The oldest and first metallogenic epoch in the Lesser Caucasus is of Middle Juras- sic to Lower Cretaceous age and is recog- nized in the paleo-island arc environment of the Eurasian margin. The ore deposits that formed during this time can be divided into two categories, based on host-rock charac- teristics. The main focus of this contribu- tion is on mineralization hosted by Middle Jurassic rocks, which includes the polymetal- lic base and precious metal vein deposits, the
stockwork-style mineralization and the mas- sive ore bodies from the Kapan, Alaverdi and Mehmana districts. The second class of deposits which also formed during the old- est metallogenic epoch includes porphyry Cu, precious metal epithermal and skarn deposits which are related to and hosted by Upper Jurassic to Lower Cretaceous intrusions. Ex- amples include the Teghout porphyry Cu de- posit in the Alaverdi district (Amiryan et al., 1987; Melkonyan and Akopyan, 2006), the Shikahogh porphyry Cu-Au-Mo deposit 20 km south of Kapan (Achikgiozyan et al.,1987) or the Gedabek epithermal Cu-Au deposit in Western Azerbaijan (Hemon et al., 2012).
Recent Re-Os dating of molybdenite yielded an age of 145.85±0.59 Ma for the Teghout porphyry Cu deposit (Moritz et al., 2011).
The second metallogenic epoch comprises Up- per Cretaceous VMS deposits with transi- tional features towards epithermal mineral- ization, also recognized in the Eastern Pon- tides (Kekelia et al.,2004;Moon et al.,2001).
An example is the polymetallic Madneuli de- posit that is hosted by volcanic and subvol- canic rocks which formed in a shallow subma- rine environment (Gialli et al., 2012; Moritz et al.,2012). The youngest and third metal- logenic epoch includes large porphyry Mo-Cu deposits associated with Eocene to Miocene intrusions, such as the world-class Kadjaran deposit (Karamyan,1978; Mkrtchyan,1969), and low- and high-sulfidation deposits with poorly defined Eocene to Miocene ages such as Zod (Konstantinov et al.,2010;Kozerenko, 2004) or the recently discovered Amulsar de- posit.
Whereas Middle Jurassic volcanic and vol- canosedimentary rocks also occur elsewhere in the Tethyan metallogenic belt, such as in the Eastern Pontides of northern Turkey or in the Sanandaj-Sirjan Zone of Iran, base and pre- cious metal ore deposits in Middle Jurassic rocks have not been reported so far outside the Lesser Caucasus. As in Turkey (Yigit, 2006, 2009) or in the Panagyurishte region of Bulgaria (von Quadt et al., 2005), known
5642
0 m Elevation
Black Sea
Caspian Sea
Fig.2 Scythian Platform
GC GC
Fig.2
Alborz LC
SKIA KZ ZSZ TM
IAES EAP
KH
EP SAS
ZFTB
SSZ UD
CentralIran Arabian Platform
0 150 300km
35° N40° N
40° E 45° E 50° E
?
Figure 1.1: Digital elevation model of the central part of the Tethyan belt illustrating the location of the main tectonic units mentioned in the text. The dashed white line indicates the approximate location of the Zagros suture zone, while the dashed black line indicates the Izmir-Ankara-Erzinkan and the Sevan-Akera suture zones. Its continuation towards the South is not well defined and marked by the question mark. EAP-East Anatolian Platform; EP-Eastern Pontides; GC-Greater Caucasus; IAES-Izmir-Ankara-Erzinkan suture; KH-Khoy ophiolitic complex; KZ-Kapan Zone;
LC-Lesser Caucasus; SAS-Sevan-Akera suture; SKIA-Somkheto Karabakh Island Arc; SSZ-Sanandaj-Sirjan zone; TM- Talesh mountains; UD-Urumieh Dokhtar magmatic arc; ZFTB-Zagros fold and thrust belt; ZSZ-Zagros suture zone.
VMS, porphyry and epithermal ore deposits in the Tethyan metallogenic belt did generally not form before the Upper Cretaceous. The Middle Jurassic rock hosted ore deposits of the Lesser Caucasus that formed during Mid- dle Jurassic and Lower Cretaceous time rep- resent a particular metallogenic epoch in the Tethyan realm and are therefore presented in this contribution.
The Centralni East, Centralni West and Shahumyan deposits of the Kapan district, the Alaverdi, Shamlugh and Akhtala deposits of the Alaverdi district and the Drmbon de- posit of the Mehmana district will be de- scribed. These deposits were called copper- pyrite, copper-gold and polymetallic deposits by different authors (Bagdasaryan et al.,1969;
Khachaturyan,1977;Maghakyan,1954), they are not well understood and their genesis has been debated, with genetic models including a volcanogenic massive sulfide (Schmidt et al., 1985) or a porphyry-related origin (Wolfe and
Gossage,2009). Similarities and differences in mineralization style and host rocks are pre- sented for these deposits, which we classi- fied based on their metal endowment to avoid terms that imply a specific genetic scenario.
We present new whole-rock geochemical and compiled sulfur isotopic data from the Ka- pan, Alaverdi and Mehmana districts to char- acterize Middle Jurassic host rocks and ore forming sulfides, respectively. The origin of the deposits is discussed within the context of the regional geologic history, with special reference to the northeastwards subduction of the Neotethys below the Eurasian margin and related magmatic activity. The results contribute to a better understanding of the geodynamic evolution of the Lesser Cauca- sus and its diverse metallogenic inheritance, which are the consequences of complex pro- cesses of ocean opening, ocean subduction and obduction, micro-plate accretion and ex- humation.
1.2. Tectonic Setting and Regional Geology
1.2 Tectonic Setting and Regional Geology
For a better understanding of the geodynamic and tectonic evolution of the Lesser Cauca- sus, the geology of the adjacent main tectonic units (Fig. 1.1) are briefly reviewed. Subse- quently, we present the geology of the three main constituting units of the Lesser Cauca- sus: the Eurasian margin, the South Arme- nian Block and the Sevan-Akera suture zone.
1.2.1 Main tectonic units in the central part of the Tethyan belt
The important surrounding orogens of the Lesser Caucasus include the Greater Cau- casus, the Alborz and the Zagros. The doubly verging fold- and thrust belt of the Greater Caucasus resulted from the closure of a deep Mesozoic - Cenozoic back-arc basin north of the subduction-related arc of the Lesser Caucasus (Gamkrelidze,1986;Hafken- scheid et al., 2006; Mosar et al., 2010; Zo- nenshain and Le Pichon, 1986). The basin shortened and closed as a consequence of the north-directed movement of the Arabian plate that resulted in a dynamic and still active continent-continent collision (Reilinger et al., 2006;Vernant et al.,2004).
TheAlborzmountain chain of northern Iran separates the Caspian Lowlands to its north from the Central Iran Plateau to its south.
It extends from the southern Lesser Caucasus of Armenia and Azerbaijan in the west to- wards the Paropamisus mountains of north- ern Afghanistan in the east. The late Pre- cambrian basement of the orogen is covered by Paleozoic to Middle Triassic platform sedi- mentary rocks. These rocks were only slightly affected by the Middle-Late Triassic Eocim- merian orogeny (Davoudzadeh and Schmidt, 1984; Saïdi et al., 1997) which resulted from northwards Paleotethys subduction and the subsequent collision of the Gondwana derived Cimmerian terranes (including the Iran Plate,
Sanandaj-Sirjan terrane and Afghan Block) with Eurasia (Barrier and Vrielynck, 2008;
Sengör et al.,1984;Stampfli and Borel,2002).
The Upper Triassic-Lower Cretaceous silici- clastic rocks of the Shemshak Formation were derived from the erosion of the rising Cim- merian mountain chain further north and un- conformably cover Middle Triassic shallow- water platform carbonates (Fürsich et al., 2005;Wilmsen et al.,2009).
The Zagros Mountains are regarded as the result of Neotethys subduction between Arabia and Eurasia and can be divided in a broader sense into three NW-SE striking subparallel belts which are from southwest to northeast the external Zagros Fold and Thrust belt (Mohajjel et al., 2003; Stöcklin, 1968), the internal Sanandaj-Sirjan (SSZ) tec- tonic zone (Agard et al.,2005;Mohajjel et al., 2003; Shahbazi et al.,2010) and the Tertiary Urumieh-Dokhtar magmatic belt (Berberian and Berberian,1981;Förster,1978). The Za- gros suture zone is regarded as the boundary between Arabia to the south and Iran to the north (Berberian and King,1981).
The Sanandaj-Sirjan zone (SSZ) formed as an Andean-like margin during the north- eastwards subduction of the Neotethys un- der its southern margin (Agard et al., 2005;
Ghasemi and Talbot,2006; Mahmoudi et al., 2011; Shahbazi et al., 2010). The onset of subduction is a matter of debate and ages proposed range from Late Triassic or Early Jurassic (Arvin et al., 2007; Berbe- rian and King, 1981) to Late Jurassic (Mo- hajjel et al., 2003). The calc-alkaline mag- matism in the SSZ shifted towards the north- east and the Urumieh-Dokhtar magmatic belt formed during most of the second half of the Mesozoic (Agard et al., 2005; Sengör, 1990) with calc-alkaline magmatism as young as Miocene in the NW of the SSZ (Omrani et al., 2008). Most of the metamorphic rocks of the Sanandaj-Sirjan zone are metamorphosed equivalents of Paleozoic and Mesozoic units of greenschist facies metamorphic grade but lo- cal conditions reached amphibolite facies near
some plutons (Agard et al., 2005; Baharifar et al.,2004;Mohajjel et al.,2003). Metamor- phism in the Sanandaj-Sirjan belt is the result of a complex protracted Late Precambrian to Tertiary evolution with various deformation events and local greenschist to amphibolite fa- cies metamorphism (Moritz et al.,2006).
The ophiolites found at the northern mar- gin of Arabia were emplaced during the col- lision of the Arabian plate with an island arc in the southwest of the SSZ (Agard et al., 2005; Mohajjel et al., 2003). Final Neotethys closure concluded in the collision of the central Iranian plateau with the NE margin of the Arabian plate between the late Mesozoic (Sheikholeslami et al., 2008) and Miocene (Mohajjel et al., 2003). This re- sulted in the formation of the Zagros Fold and Thrust Belt as part of the Alpine-Himalayan mountain chain, and marks the position of the Neotethys suture zone (Agard et al., 2005;Berberian and Berberian,1981;Mohaj- jel et al.,2003).
The Eastern Pontides in the northeast of Turkey (Fig. 1.1) are bounded by the Izmir-Ankara-Erzincan ophiolitic suture to the south and the Black Sea to the north.
Okay and Sahintürk(1997) describe the het- erogeneous pre-Jurassic metamorphic base- ment in the area which is overlain by a Mesozoic sedimentary sequence starting with a Liassic marine transgression. Lower to Middle Jurassic volcanic rocks and volcano- sedimentary units in the Eastern Pontides (Yilmaz et al., 2003; Yilmaz and Kandemir, 2006) are interpreted to be related to a vol- canic arc (Şen, 2007). Platform carbonates of Middle Jurassic (Callovian) to lower Cre- taceous age cover the volcanogenic sequence over the entire Pontides (Tüysüz,1999). Cre- taceous to Miocene arc volcanism continued in the Eastern Pontides and a number of ore deposits hosted by Upper Cretaceous tuffa- ceous horizons show similarities with Kuroko type VMS deposits (Akinci,1984).
1.2.2 Geology of the Lesser Caucasus
The Lesser Caucasus is regarded as a Tethyan suture zone and from northeast to south- west the three main tectoni units are the Eurasian margin, the Sevan-Akera suture zone, and the Gondwana-derived South- Armenian Block (Milanovsky,1968;Nalivkin, 1976;Sosson et al.,2010).
Eurasian margin
The continued Devonian to Triassic subduc- tion of the Paleotethys below the southern margin of Laurasia (i.e., the East-European craton) caused the northwards migration of the Cimmerian and Transcaucasus terranes which collided with the Eurasian margin dur- ing the Cimmerian orogeny at Triassic to Lower Jurassic time (Dercourt et al., 2000;
Golonka, 2004; Stampfli et al., 2001). At the same time, the northwards subduction of the Paleotethys caused rifting at the northern margin of Gondwana and the opening of the Neotethys south of the Cimmerian and Tran- scaucasus terranes (Golonka, 2004; Sengör, 1984; Stampfli and Borel, 2004). The sub- sequent Middle Jurassic to Upper Cretaceous northeastwards subduction of the Neotethys below the Eurasian margin and the formerly accreted terranes is evidenced by the re- lated magmatic arc in the Lesser Caucasus (Barrier and Vrielynck,2008; Golonka, 2004;
Stampfli and Borel, 2004). The Jurassic- Cretaceous island arc of the Lesser Cauca- sus evolved as a consequence, and is di- vided into the Somkheto-Karabakh Island Arc (SKIA) and the Kapan Zone (KZ) further south (Gevorkyan and Aslanyan,1997). The SKIA extends over about 350 km from south- ern Georgia over northern Armenia into the Nagorno-Karabakh region. The KZ extends about 70 km from southern Armenia into northern Iran (Fig. 1.2). Kazmin et al.(1986) interpret the KZ as the original direct con- tinuation of the SKIA which was displaced
Taurides-Anatolides and South- Armenia accreted terranes Arabian platform
European margin Iran accreted terrane
Tatvan
Agri
Erzurum
Kars
Artvin
Batumi Gyumri Yerevan
Tbilisi Ganja Nakhchivan Khoy Tabriz Urmia
Ahar
Stepanakert (Khankendi)
Baku Rasht Zanjan Tehran Sanandaj
Caspian Sea
Black Sea Iraq Syria
Azerbaijan
Georgia Armenia Turkey
Georgia
SKIA
SASZ
KZ
Eastern Pontides IAES SAB
?
Alborz
SSZ UD
ZSZ
Greater Caucasus ~100 km further North Iran
44°46°48°50°52° Eocene rocks
Sea and lakes Plio- and Pleistocene rocks Oligocene and Miocene rocks Paleocene rocks Cenozoic intrusive rocks Mesozoic intrusive rocks Mesozoic ophiolite complexes
Jurassic rocks
Cretaceous rocks Paleozoic to Triassic platform sedimentary and metamorphic basement rocks Proterozoic basement rocks
International border
City
km0100 Mine or major prospect Sutures
Geological map of the Tethyan belt from eastern Turkey to western Iran, highlighting the Lesser Caucasus SARI GUNAY
ZARSHURAN
SUNGUN
AGARAK
AMULSAR KAPAN KADJARAN
DRMBONZOD
GEDABEK
ALAVERDI
MADNEULI MURGUL Supposed major faults
42° Fig. 3
40° 38° 36°
Figure 1.2: Geological map of the Tethyan belt from eastern Turkey to western Iran, highlighting the Lesser Caucasus, Mesozoic and Cenozoic intrusive rocks and ophiolites. Based on maps fromBairamov et al. (2008), Bingöl(1989), Emami et al.(1993),Gudjabidze(2003),Kharzyan(2005) andLotfi et al.(1993). IAES-Izmir-Ankara-Erzinkan suture;
45° 46° 47°
41°
40°
39°
Stepanakert (Khankendi) Alaverdi
district Fig. 7
Kapan district Fig. 4
Mehmana district Fig. 8
Nakhchivan Yerevan
Ganja
Khoy
Ahar Azerbaijan Armenia
Turkey
Georgia
Iran
SAB
Eocene rocks Sea and lakes Plio- and Pleistocene rocks Oligocene and Miocene rocks Paleocene rocks
Cenozoic intrusive rocks Mesozoic intrusive rocks Mesozoic ophiolite complexes Jurassic rocks
Cretaceous rocks Paleozoic to Triassic platform sedimentary and metamorphic basement rocks
Proterozoic basement rocks Eocene rocks
Sea and lakes Plio- and Pleistocene rocks Oligocene and Miocene rocks Paleocene rocks
Cenozoic intrusive rocks Mesozoic intrusive rocks Mesozoic ophiolite complexes Jurassic rocks
Cretaceous rocks Paleozoic to Triassic platform sedimentary and metamorphic basement rocks
Proterozoic basement rocks International border
City km
0 50
Sutures
Supposed major faults
SASZ
European margin SKIA
Figure 1.3: Amplification of the geological map in Fig. 1.2 that shows the compiled geology of the Lesser Caucasus.
The Kapan, Alaverdi and Mehmana districts are also shown. Abbreviations as in Fig. 1.2
1.2. Tectonic Setting and Regional Geology
by sublatitudinal strike-slip movement. How- ever, as W-NE trending strike-slip faults cut- ting both continental and Jurassic-Cretaceous oceanic crust were described in the Neotety- han region (Jackson, 1992; Kopp, 1997), the apparent offset between the SKIA and the KZ to its southwest could also be the result of an original transform-fault type offset between the two tectonic provinces. Khain(1975) and Melkonyan et al. (2000) propose a NW-SE striking double-sided subduction in between the KZ and the SKIA, thus refusing the sce- narios that the KZ and the SKIA might form parts of one and the same discontinuous arc.
Poorly exposed Permian quartz-mica schists from the Azerbaijan part of the SKIA (Rb- Sr isochron age of 293±7 Ma, Bagdasaryan et al.,1978) constitute the oldest rocks in the area and represent the Hercynian basement of the Eurasian margin. A pile of more than 7000 m of volcanic and volcaniclastic rocks of mainly Jurassic-Cretaceous age dominates most of the island arc (Nikishin et al.,2001).
It is characterized by Lower Jurassic shales and clastic rocks followed by Middle Juras- sic volcanic and volcaniclastic rocks interca- lated with marine sedimentary rocks, indicat- ing deposition in a semipelagic environment.
Magmatism is related to the northeastwards Neotethys subduction below the Eurasian margin, with maximum volcanic activity dur- ing Bajocian-Bathonian times (Kazmin et al., 1986). Middle Jurassic volcanic rocks con- sist of a differentiated sequence of basalt, an- desite, dacite and rhyolite, and typically reach 3 to 3.5 km thickness. The overlying upper Jurassic to Lower Cretaceous sequence is 1 to 1.5 km thick with an increasing dominance of sedimentary and coarse volcaniclastic rocks (Kazmin et al.,1986).
The Sevan-Akera suture zone
The Sevan-Akera suture zone (Fig. 1.2) ex- tends over almost 400 km in a NW-SE direc- tion from north of the Sevan Lake towards the Nagorno-Karabakh region (Adamia et al.,
1977;Maghakyan et al.,1985). Various ophi- olitic fragments are remnants of the North- ern Neotethys ocean and mark this suture between the Eurasian margin to the north- northeast and the South Armenian block to the south-southwest (Adamia et al., 1981;
Knipper et al., 1986; Sengör and Yilmaz, 1981; Stampfli et al., 2001). Remnants of oceanic crust can also be found in the Vedi (Adamia et al., 1981; Knipper and Sokolov, 1977) and Zangezur (Knipper and Khain, 1980) areas. Based on similar geological, petrological, geochemical and age features, they are all interpreted to be remnants of a single obducted sequence which was obducted on the South Armenian Block (SAB) between 88-83 Ma (Galoyan et al.,2007;Galoyan et al., 2009; Rolland et al., 2009b). The ophio- lite occurrences of Vedi and Zangezur are un- roofed nappes and slivers of ophiolites that were thrusted onto the SAB towards the south during continental convergence (Knipper and Khain, 1980; Sosson et al., 2010). Radiolar- ites of Middle Jurassic (Bajocian) age directly cover ophiolitic lavas along the Vedi River (Danelian et al.,2008;Danelian et al.,2010).
The timing of collision between the SAB and the Eurasian margin is poorly defined and Late Cretaceous to Early Paleocene collision ages were proposed byRolland et al.(2009a) and Sosson et al. (2010), respectively. To- wards the east, the Sevan-Akera suture zone of Armenia is interpreted as the eastwards prolongation of the Ankara-Erzincan suture zone in northern Turkey (Okay and Tüysüz, 1999;Yilmaz et al.,2000).
South Armenian Block
The South Armenian Block (SAB) is com- monly regarded as a continental terrane of Gondwanian origin (Knipper and Khain, 1980; Monin and Zonenshain, 1987). It is characterized by a metamorphic basement, an incomplete Paleozoic sedimentary succes- sion, minor Triassic, Jurassic and Cretaceous formations and Paleogene and Neogene de-
trital and volcanogenic rocks (Sosson et al., 2010). Alkaline basalt of Middle Jurassic age in the Nakhichevan area as well as Aalenian to Bajocian sedimentary rocks indicate a paleo- latitude of 21.5◦N ± 3.7◦ (2000 km south of its present position) based on paleomag- netic data (Bazhenov et al.,1996). Abundant Eocene to Miocene magmatic activity gener- ated large-volume composite intrusions such as the Meghri Pluton in southern Armenia af- ter the collision of the SAB with the Eurasian margin (Melkonyan et al.,2008).
1.3 Ore deposits in Middle Jurassic rocks of the Eurasian margin
Ore deposits hosted by Middle Jurassic rocks (Table 1.1) can be found in the SKIA of Arme- nia and Azerbaijan and in the subparallel KZ in southern Armenia. Together both regions represent the Jurassic-Cretaceous island-arc domain of the Lesser Caucasus, which devel- oped as a consequence of long-lasting north- eastwards subduction of the Neotethys below the Eurasian margin (Adamia et al.,1977;Ri- cou et al., 1986). Calc-alkaline volcanogenic and volcaniclastic series dated as Bajocian to Santonian (i.e., Middle Jurassic to Upper Cre- taceous) dominate in the island-arc domain of the Lesser Caucasus (Adamia et al.,1981;
Khain and Koronousky, 1997) whereas eco- nomic mineralization of the below described deposits is concentrated in Middle Jurassic rocks of Bajocian and Bathonian age.
1.3.1 Ore deposits in the Kapan mining district
The Kapan mining district is located in south Armenia at 39◦13'N and 46◦24'E in the center of the KZ (Fig. 1.3). Three main magmatic complexes can be distinguished in the almost 6500 meter thick pile of volcanic and volcani- clastic rocks of the Kapan district: the Mid- dle Jurassic, the Upper Jurassic-Lower Cre-
taceous, and the Paleogene magmatic com- plex (Achikgiozyan et al., 1987). The three main deposits of the district, from west to east the Centralni West Cu-deposit, the Centralni East Cu±Au deposit and the Shahumyan polymetallic Cu-Au-Ag-Zn±Pb deposit (Ta- ble 1.1), are all hosted by rocks of the Middle Jurassic magmatic complex (Fig. 1.4). Metal zonation is observed on a district-scale with copper-rich mineralization in the west chang- ing progressively to higher Zn, Pb, Au and Ag concentrations in the east. The industrial ex- ploitation of the Kapan deposits dates back to the middle of the 19th century (Tadevossyan, 2005). Different state and private companies have operated the mines since then. Accurate production numbers are difficult to estimate, but taking into account all available data, at least 370.000 tons of copper were mined in the district since 1953 (Wolfe and Gossage, 2009). The Centralni East open pit opera- tion ceased production in 2005 with the for- mal take-over by the private company Deno Gold. The Centralni West underground oper- ation was closed in 2008 and nowadays only the polymetallic Shahumyan deposit is an ac- tive mine, producing annually about 600.000 tons of ore at about 1.53 ppm Au, 29 ppm Ag, 0.27 % Cu and 1.52 % Zn.
