Proceedings Chapter
Reference
Structural control of the Cenozoic porphyry Cu-Mo, epithermal and skarn deposits and prospects, Central Tethyan belt, Lesser Caucasus
HOVAKIMYAN, Samvel, et al.
Abstract
The Cenozoic evolution of the central segment of the Tethyan metallogenic belt is dominated by the oblique convergence and final collision of Gondwana-derived terranes and the Arabian plate with Eurasia, which created a favorable setting for the formation of the highly mineralized Meghri-Ordubad pluton inthe southernmost Lesser Caucasus, in the Zangezur-Ordubad mining district. Paleostress reconstructions indicate anti-clockwise rotation from NE-oriented compression during the early and middle Eocene to NNW-oriented compression during the Pliocene. During the Eocene the N-S oriented faults are consistent with dextral strike-slip tectonics, correspond to synthetic faults and control the main porphyry Cu-Mo and epithermal deposits and prospects. The sinistral E-W oriented en- échelon faults correspond to antithetic faults. This strike-slip kinematics is consistent with the regional NE-oriented compression in the Zangezur-Ordubad district and concomitant with final subduction of the Neotethys along the Eurasian margin. Sinistral strike-slip kinematics along the E-W oriented faults resulted in clockwise rotation of the [...]
HOVAKIMYAN, Samvel, et al . Structural control of the Cenozoic porphyry Cu-Mo, epithermal and skarn deposits and prospects, Central Tethyan belt, Lesser Caucasus. In: Proceedings of the 14th SGA Biennial Meeting . 2017.
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Structural control of the Cenozoic porphyry Cu-Mo, epithermal and skarn deposits and prospects, Central Tethyan belt, Lesser Caucasus
Samvel Hovakimyan, Robert Moritz University of Geneva, *HQHYDSwitzerland
Rodrik Tayan, Rafael Melkonyan, Marianna Harutyunyan
Institute of Geological Sciences of the National Academy of Sciences of the Republic of Armenia
Abstract. The Cenozoic evolution of the central segment of the Tethyan metallogenic belt is dominated by the oblique convergence and final collision of Gondwana-derived terranes and the Arabian plate with Eurasia, which created a favorable setting for the formation of the highly mineralized Meghri-Ordubad pluton inthe southernmost Lesser Caucasus, in the Zangezur-Ordubad mining district.
Paleostress reconstructions indicate anti-clockwise rotation from NE-oriented compression during the early and middle Eocene to NNW-oriented compression during the Pliocene.
During the Eocene the N-S oriented faults are consistent with dextral strike-slip tectonics, correspond to synthetic faults and control the main porphyry Cu-Mo and epithermal deposits and prospects. The sinistral E-W oriented en- échelon faults correspond to antithetic faults. This strike-slip kinematics is consistent with the regional NE-oriented compression in the Zangezur-Ordubad district and concomitant with final subduction of the Neotethys along the Eurasian margin. Sinistral strike-slip kinematics along the E-W oriented faults resulted in clockwise rotation of the individual blocks of the Zangezur-Ordubad district. During the Oligocene and Miocene anti-clockwise rotation of the main paleostress compressional orientation resulted in reactivation of existing ore-controlling structures in a sinistral strike-slip tectonic regime, which is consistent with the re-orientation of the tectonic plate kinematics and re- organization of the Arabia-Eurasia collision.
1 Introduction
The Zangezur-Ordubad mining district consists of the Meghri-Ordubad pluton in the southernmost Lesser Caucasus, Central Tethyan belt (Moritz et al. 2016a). It was formed by repeated intrusive activity from the mid- Eocene subduction to the Miocene post-collisional evolution of the southernmost Lesser Caucasus, with pulsed ore formation (Karamyan 1978; Melkonyan et al.
2008; Moritz et al. 2016a, b; Rezeau at al. 2016). It is a very fertile area and contains sixteen porphyry Cu-Mo, twenty epithermal Au and base metal and three skarn W, Cu-Mo, Fe deposits and prospects.
Regional strike-slip faults played an important role in
the control of the porphyry Cu-Mo, epithermal and skarn systems hosted by the Meghri-Ordubad pluton.
The aim of this study is to understand how ore deposit and prospect location, ore body geometry in the Zangezur- Ordubad mining district are linked to the long-lasting Eocene to Mio-Pliocene regional strike-slip tectonics, and far-field plate tectonic evolution from Eocene subduction to Miocene post-collision.
We discuss the paleostress and the kinematic environment of the major strike-slip and oblique-slip ore- controlling faults throughout the Cenozoic tectonic evolution of the Meghri-Ordubad pluton based on detailed structural field mapping of the ore districts, stereonet compilation of ore-bearing fractures and vein orientations in the major porphyry and epithermal deposits, and the paleostress reconstructions.
2 Major regional district-scale structures
The Zangezur-Ordubad region consists of the Nakhitchevan and Zangezur tectonic blocks (Fig. 1), which behaved as two separate entities since the middle Eocene (Tayan et al. 1976), when uplifting of the Zangezur block started with respect to the Nakhitchevan block. The two tectonic blocks are separated by the NNW- oriented dextral strike-slip Salvard-Ordubad fault zone (Fig. 1; Tayan et al. 1976). To the east, the major regional NNW-oriented, dextral strike-slip Khustup-Giratagh fault zone separates the Zangezur block from the Kapan block, in other words the Mesozoic Eurasian margin (Fig. 1).The Zangezur block is crosscut by numerous N-S, E-W, NE- and NW-oriented faults, which are the main ore- and magma-controlling structures of the Zangezur-Ordubad district (Fig. 1). The majority of the Cenozoic magmatism, ore deposits and prospects are confined to a central N-S- oriented array, located between the Khustup-Giratagh and the Salvard-Ordubad fault zones, and with a distinct discordant orientation with respect to the two regional fault zones (Fig.1). It has been defined as the central magma- and ore-controlling zone by previous authors, and it records a long lasting and multi-stage tectonic and metallogenic evolution, with structurally controlled magmatic intrusions and ore occurrences (Fig. 1;
Karamyan 1978; Tayan 1998).
Figure 1. Structural map of the Zangezur-Ordubad mining district (after Karamyan et al. 1974; Tayan et al. 1998;
Hovakimyan et al. 2016) with locations of the major porphyry Cu-Mo and epithermal Au-polymetallic and skarn Fe, Cu-Mo and W deposits and prospects.
3 Eocene tectonic and ore control regime Paleostress reconstructions reveal a NE-SW-oriented compressional setting during the early and middle Eocene, which was favorable for dextral displacements along the two major, regional NNW-oriented Khustup-Giratakh and Salvard-Ordubad strike- slip faults (Fig.1). This resulted in the formation of a N-S oriented transrotational basin, known as the Central magma and ore- controlling zone (Tayan 1998).
Since the Eocene the N-S oriented faults are consistent with subparallel dextral strike-slip tectonics and correspond to synthetic faults (Fig. 1; Tashtun, Meghriget, Spetry, Tey, Terterasar, Dastakert and Nshanakar faults) and the sinistral E-W oriented en-échelon faults correspond to antithetic faults (Fig. 1; Aramazd, Voghji, Meghrasar, Meghriget-Cav, Bughakyar and Agarak faults).
This strike-slip kinematics is consistent with the regional NE-oriented compression in the Zangezur-Ordubad mining district revealed by the paleostress indicators and corresponding to Eocene SW- to NE-oriented plate convergence and Neotethys subduction beneath Eurasia along the Lesser Caucasus-Zagros segment of the Tethyan belt (Barrier and Vrielynck 2008). Sinistral strike-slip kinematics along the E-W oriented faults resulted in clockwise rotation of the individual blocks of the Zangezur-Ordubad district (Figs. 1 and 2). Such block rotation is typical for strike-slip fault systems, with the blocks rotating synthetically with respect to the main master faults (Kim et al. 2014).
During the Eocene, dextral displacement along the major N-S oriented strike-slip faults were favorable for creating of NE-oriented en-échelon normal faults or extension fractures. These faults can be observed from the regional scale down to the ore body scale in the porphyry and epithermal systems. The N-S oriented faults, in particular at their intersection with E-W and NE-oriented faults, were important ore-controlling structures for the emplacement of major porphyry Cu-Mo (e.g., Dastakert, Hanqasar, Aygedzor and Agarak) and epithermal Au, base
metal (e.g., Tey-Lichkvaz and Terterasar) deposits and prospects.
The NE- to NNE-oriented structures were important controls of the skarn deposits and prospects. They occur mainly in the northern and southern Bargushat blocks (Fig.1). The Svaranc Fe skarn deposit related with the regional NNW-oriented Khustup-Giratagh dextral strike- slip fault and the Kefashen Cu-Mo and W prospects occur at the contact of the Geghi intrusion and Middle Devonian to Late Permian carbonate rock and shale along a NW- oriented thrust zone (Fig.1; Harutyunyan 1995).
Figure 2. Clockwise block rotation in the Zangezur-Ordubad mining district.
4 Oligocene tectonic regime: structural
control of the world class Kadjaran porphyry deposit
The paleostress data indicate a progressive anti-clockwise rotation of the main stress axes from regional NE-oriented compression (Fig. 3a) to NNE-oriented compression during the Oligocene (Fig. 3b) and to N-S compression during the Miocene. Therefore, since the Oligocene, magmatic intrusions and ore deposits were progressively emplaced under a different regional tectonic regime than their Eocene counterparts.
The Oligocene giant Kadjaran porphyry deposit formed in the immediate vicinity of the NNW oriented Tashtun and E-W oriented Voghji fault intersection (Fig. 1). The E- W-, NE- and N-S oriented ore and dike controls reveal that structures inherited from the Eocene dextral strike- slip tectonics were reactived during the Oligocene.
However, the reverse motion along the E-W and NE- oriented faults controlling the 26-27 Ma-old ore-bearing veins at the Kadjaran porphyry deposit is not compatible with dextral strike-slip tectonics under a NE-oriented compression. These reverse fault geometries indicate that ore formation at Kadjaran occurred under a different tectonic regime, more consistent with progressive NNE- to N-S oriented compression compatible with the paleostress rotation from the late Oligocene to Miocene (Fig. 3b).
In the Kadjaran deposit, the early porphyry veinlets are predominantly NE- to NNE-oriented and late chalcedony and carbonate veinlets crosscutting 22 Ma-old granodiorite porphyry dikes are mainly steeply-dipping E-
W oriented, which is consistent with N-S oriented compression during the Miocene.
Figure 3. Characteristics of the major N-S oriented ore- controlling faults. a from the early and middle Eocene to early Oligocene. b during the late Miocene with the formation of the Meghri – Tey graben.
5 Miocene to Pliocene tectonic regime:
structural control of the Lichk porphyry- epithermal system
Paleostress reconstructions in the Zangezur-Ordubad region indicate a progressive rotation of the major compression from NNE-oriented during the Oligocene to N-S-oriented during the Miocene and finally to NNW- oriented during the Pliocene, which is consistent with the re-orientation of the tectonic plate kinematics and re- organization of the Arabia-Eurasia collision since at least the late Miocene (Allen et al. 2004; Austermann and Iaffladano 2013).
The early Miocene E-W oriented extensional setting was favorable for intruding the early Miocene porphyritic granite-granodiorite of the Voghji massif along the footwall of the Tashtun fault (Fig. 1). This reveals that the Tashtun fault had an essentially oblique, normal fault behavior and controlled the western boundary of the Meghri-Tey graben since the early Miocene (Figs. 1 and 3b; Tayan et al. 1998). The eastern boundary of the graben was controlled by the N-S oriented Meghriget fault (Figs.
1 and 3b). The geometry and N-S orientation of the Meghri-Tey graben is consistent with E-W and NE- orientation of the main extensional stress axes during the Miocene and Pliocene, respectively (Fig. 1).
The Miocene Lichk prospect is hosted by a 22.2 Ma- old porphyritic granodiorite emplaced within a release bend between the western and eastern segments of the Tashtun fault (Fig. 1). The geometry of the release bend is inconsistent with dextral kinematics along the Tasthtun fault. However, it could have formed during sinistral kinematics. Dextral kinematics is recorded along the eastern segment of the Tashtun fault during the Oligocene,
with displacement of the early Oligocene diorite porphyry dike. During the Miocene mineralization event, the Tashtun fault had a sinistral oblique-slip kinematics, which is recorded by sinistral displacement of segments of the epithermal system controlled by the NE-oriented Lichk fault. Thus, we conclude that the Lichk deposit and its host granodiorite porphyry were emplaced during Miocene sinistral reactivation of the Tashtun fault, which was behaving as a dextral tectonic system until the early Oligocene. The switch of the Tashtun fault behavior resulted in the development of a pull-apart basin and the formation of the Lichk porphyry - epithermal system. It coincides with the progressive rotation of the main compressional orientation from the early Oligocene to the Miocene, and then the Pliocene (Fig. 3).
6 Discussion and conclusions
The proposed tectonic model explains the structural control of porphyry Cu-Mo, epithermal and skarn deposits and prospects of the Zangezur- Ordubad mining during the early Eocene to Mio-Pliocene tectonic evolution of the district in the context of regional strike-slip tectonics.
The progressive change of the main compressional paleostress orientation recorded in the Zangezur-Ordubad region, rotating from NE during the early and middle Eocene to NNE during the early Oligocene to N-S during the Miocene and finally to NNW- during the Pliocene, is consistent with the re-orientation of the tectonic plate kinematics from Eocene subduction to Mio-Pliocene post- collision.
During the Eocene NE-oriented compression created the essentially dextral strike-slip tectonic regime along the major N-S oriented strike-slip fault. The structures formed during Eocene dextral strike-slip faulting, concomitant with final subduction of the Neotethys, were repeatedly reactivated during the subsequent Neogene tectonic evolution of the Zangezur-Ordubad region, as one evolved from a subduction to a post-subduction geodynamic setting.
The Oligocene and Miocene deposits and prospects were formed essentially in a sinistral strike-slip tectonic regime, which created the favorable geometry and adequate conditions for the emplacement of vein and stockwork-type porphyry Cu-Mo deposits, including the giant Oligocene Kadjaran deposit and the Miocene Lichk prospect.
This contribution underscores the importance of regional strike-slip tectonics as a fundamental control on the formation of Cenozoic porphyry-epithermal systems and associated magmatism within the Lesser Caucasus, which is comparable to many other metallogenic belts.
Acknowledgements
This study was financially supported by the National Academy of Sciences of Republic of Armenia, the
SCOPES projects IB7620-118901 and IZ73Z0-128324 and the Swiss National Science Foundation projects 200020- 138130, 200020-155928 and 200020-168996. S.
Hovakimyan would like to thank the Swiss government for awarding the Swiss Government Excellence Postdoctoral Scholarship for the 2014-2015 academic year, Foundation Ernst et Lucie Schmidheiny from the University of Geneva for a scholarship for the 2015-2016 academic year, Foundation Azad and the Swiss Chapter of the Armenian General Benevolent Union (AGBU).
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