Available online at www.sciencedirect.com ScienceDirect

Procedia Earth and Planetary Science 17 ( 2017 ) 913 – 916

15th Water-Rock Interaction International Symposium, WRI-15H Hydrothermal systems in the salt domes of south Iran

M. Mortazavia,1, S. Heuss-Assbichlerb, M. Shahria

aDepartment of Geology, Hormozgan University, P.O.Box 3995, Bandar Abbass, Iran bDepartment for Earth and Environmental Sciences, LM-Universität München, Theresienstr. 41, 80333 Munich, German

Abstract

The intrusion of less dense rock salt into denser overburden sediments can create salt diapirs. In the north of the Persian Gulf about 200 salt-gypsum domes are exposed, partially forming impressive salt glaciers. With the emergence of the salt domes exotic blocks of igneous and metamorphic rocks are also brought out to the surface. They are often intensively altered. Minerals, such as albite, quartz, hematite, coarse grained dolomite and apatite are widespread. The growth of blue amphiboles (magnesio- riebeckite) in veins of altered rocks indicate an intensive interaction of hydrothermal fluids with the host rocks in the salt diapirs. © 20172017 Published The Authors. by Elsevier Published B.V. Thisby Elsevier is an open B.V. access article under the CC BY-NC-ND license (Peerhttp://creativecommons.org/licenses/by-nc-nd/4.0/-review under responsibility of the organizing). committee of WRI-15. Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Hormoz formation, salt diapir, igneous rock, hydrothermal interaction, magnesio-riebeckite, blue amphibole, hematite

1. Introduction

The Hormoz Formation in the south of Iran, in the northern part of the Persian Gulf, is composed of sequences of evaporates, shales, siltstones and carbonates. They are exposed in more than 200 salt-gypsum domes (Figure 1). More than 118 salt domes crop out while the others are still buried. Some of them, like those on Hormuz Island jut out of the waters of the Persian Gulf. Various blocks of igneous and metamorphic rocks are transported to the surface. The origin of these rocks within the salt plugs is still under debate. Of particular interest is the formation of partly coarse grained idiomorphic minerals within the salt domes implicating an interaction of hot fluids with the rocks. The origin of this hydrothermal process is still puzzling.

* Corresponding author. Tel.: 0098 (0) 917 361 6592 E-mail address: [email protected]

1878-5220 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 doi: 10.1016/j.proeps.2017.01.016 914 M. Mortazavi et al. / Procedia Earth and Planetary Science 17 ( 2017 ) 913 – 916

2. Geology

In the Neoproterozoic to Early Cambrian, Iran was located on the northeastern margin of the Gondwana supercontinent where great masses of evaporites and carbonates were deposited1. These deposits crop out on the Gondwana margin preserved in Oman, Pakistan (Salt Range), India, Turkey, Qatar, Morocco, Yemen, Saudi Arabia, Jordan and Western Africa. The Hormoz Formation which in total may reach a thickness of 1000 m, consists mainly of evaporites (salt and gypsum), carbonates (dark limestone and dolomite), and additionally shales and siltstones. In various salt domes igneous rocks comprising rhyolite, rhyodacite, trachyte, diabasic dikes as well as rhyolitic tuffs and ignimbrites are exposed. The rhyolites are inferred to be formed by subduction-related I-type magmas generated in an active continental margin setting. Analysis of zircon crystals from rhyolites exposed in the Hormoz Island yielded an age of 558 ± 7 Ma 2. Some of the igneous rocks are affected by metamorphism up to amphibolite-facies. The volcanic rocks are often intensively altered. In some areas the units contain iron or copper with the potential for bulk-mineable mineralization at economic grades.

Figure 1 Geological map of Iran modified after 3 showing the distribution of the exposed salt domes. The study area (Gahkom salt dome) in the south of the Main Zagros thrust is marked by a circle.

M. Mortazavi et al. / Procedia Earth and Planetary Science 17 ( 2017 ) 913 – 916 915

Gahkom salt dome which emerged 100 km north of Bandar Abbas City on the southeast of the Zagros Mountains (see Figure 1) contains a sequence of black and yellow dolomites and siltstone together with large sequences of tuff reflecting sedimentation of volcanic rocks in a shallow sea basin (Figure 2). In the central part these rocks are intensely altered. They are composed of dolomite, gypsum, feldspar and white clinochlor. In some parts they also contain illite and muscovite. It is remarkable that halite bearing salt formations are not exposed.

Hormoz Island

Figure 2 Geological map of the salt dome Gahkom, South Iran

3. Hydrothermal mineralizations in the salt domes

Coarse grained minerals with euhedral shapes are often observed in the salt domes. Crystals of calcite, black dolomite and apatite up to 2 cm in sizes are widespread. In one locality garnet, epidote and quartz cover the surface of magmatic rocks. Layers with banded iron and other Fe-bearing minerals are also common. In Gahkom, the outcrops are often red or pink in color indicating the migration of iron and manganese rich fluids (Figure 3a). Coarse grains of euhedral hematite forming aggregates were frequently observed within the sediments (Figure 3b). Twinned albite crystals with a size up to 1 cm occur in the siliceous dolomites. In the strongly altered rocks irregular veins with blue fibrous magnesio-riebeckite have been documented (Figure 3c). In addition, larger crystals grow along the fractures as shown in Figure 3d. They occur together with aegerine-augite, albite or chlorite and are occasionally associated with ferro-winchite. In thin sections the mafic minerals like aegerine-augite crystals show a bleached rim pointing to leaching processes in the magmatic rocks.

916 M. Mortazavi et al. / Procedia Earth and Planetary Science 17 ( 2017 ) 913 – 916

Fission track dating of an apatite crystal from Hormuz Island yields an age of 55.4 ± 2.6 Ma for a cooling temperature of approximately 100°C 4. Considering all these observations and indications suggests a young age of these hydrothermal activities.

Figure 3 A) Iron rich fluid penetrating the host rock, B) coarse grains of euhedral hematite, C) irregular veins with blue magnesio- riebeckite within strongly altered rocks, D) fibrous amphiboles growing perpendicular to the crack wall

4. Conclusion

Assemblages of newly formed and occasionally coarse grained minerals such as dolomite, apatite, garnet, epidote and hematite were observed in various salt domes which are obviously the result of hydrothermal activities. In Gahkom, typically blue amphiboles grow along irregular cracks. All these observations point to an intensive local interaction of hot hydrothermal fluids with various rocks within the salt dome.

Acknowledgements

Funding of this research by the Hormozgan University is gratefully acknowledged.

References

1. Stöcklin, J. Salt deposits of the Middle East. Geological Society of America Special Papers 88 (1968) 157-182. 2. Faramarzi, N. S., Amini, S., Schmitt, A. K., Hassanzadeh, J., Borg, G., McKeegan, K., ... & Mortazavi, S. M., Geochronology and geochemistry of rhyolites from Hormuz Island, southern Iran: A new record of Cadomian arc magmatism in the Hormuz Formation. Lithos 236 (2015): 203-211. 3.Nabavi & Agha Nabati, Geological Survey of Iran 2001 4. Hurford AJ, Grunau HR & Stöcklin J. Fission track dating of an apatite crystal from Hormuz Island, Iran. Journal of Petroleum Geology, 7(4), (1984) 365-380.