Shoshonitic pluton and an associated base-metal deposit in the Tarom–Hashtjin metallogenic zone, NW Iran: implication for tectono-magmatic evolution and metallogenic considerations

Abstract

Petrological, geochemical and zircon U–Pb and Hf isotope studies were carried out on the Hajseyran pluton, along the Tarom–Hashtjin metallogenic zone, as part of the Alborz–Azerbaijan magmatic belt, which is located in the central part of the Alpine–Himalayan orogenic belt. The Hajseyran pluton, which is made up of syenite, monzonite, quartz-monzonite and granite, yields a zircon U–Pb age of 39.20 ± 0.56 Ma (for a quartz-monzonite sample) with shoshonitic features. Enrichment in large ion lithophile elements, depletion in high-field-strength elements and negative anomalies of Nb, Ta and Ti in Chomalou samples suggest a subduction-related arc magmatic setting. The Chomalou epithermal Pb–Zn–Cu deposit occurs close to the Hajseyran pluton with the quartz–base metal sulfide veins predominantly hosted by Eocene volcanic rocks. The ore-forming processes include chalcopyrite and sphalerite–galena formation, with the latter the major mineralisation in this region. Microthermometric results from the liquid–vapour fluid-inclusion assemblages show that the homogenisation temperatures in the first-stage quartz range from 230 to 310 °C, in the sphalerite and second-stage quartz from 161 to 293 °C and in the late-stage quartz from 175 to 262 °C. The salinities of fluids decrease from early-stage quartz to late-stage quartz. The calculated stable isotope values of ore–fluid components (δ¹⁸O_water = +3.28‰ to −3.68‰; δ³⁴S_H2S = +5.5‰ to +7.8‰) show that magmatic fluids have played a significant role during mineralisation. The Chomalou deposit is interpreted as a volcanic-hosted intermediate-sulfidation type of epithermal mineralisation formed through shallow-level emplacement of the Hajseyran pluton, which formed via partial melting of phlogopite–amphibole-bearing sub-continental lithospheric mantle. The Neotethyan slab roll-back and concomitant asthenospheric upwelling led to the decompression melting of metasomatised sub-continental lithospheric mantle and the formation of several K-rich plutons and associated epithermal deposits in the Alborz–Azerbaijan magmatic belt.

KEY POINTS

1. The late Eocene Hajseyran pluton shows a shoshonitic, arc-related signature.

2. The Chomalou Pb–Zn–Cu deposit is an intermediate-sulfidation style of epithermal mineralisation.

3. Fluid inclusion and stable isotope data suggest the evolution of ore-fluid system from magmatic to meteoric.

4. Decompression melting of metasomatised sub-continental lithospheric mantle via slab roll-back led to the formation of Hajseyran pluton.

Keywords:

• shoshonitic pluton
• Alborz Mountains
• intermediate-sulfidation epithermal
• fluid inclusion
• isotope geochemistry
• Tarom–Hashtjin
• Iran

Acknowledgements

All logistical supports during fieldwork came from the University of Zanjan, Zanjan, Iran. The zircon data and oxygen isotope analyses on quartz were supported by the State Key Laboratory of Lithospheric Evolution at the Institute of Geology and Geophysics (grant no. 201503). We thank two anonymous reviewers.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Financial support of this research was provided by the offices of Vice Chancellor for Research and Technology from University of Zanjan and IASBS (Institute for Advanced Studies in Basic Sciences), Zanjan, Iran, as well as the IGGCAS (Institute of Geology and Geophysics, Chinese Academy of Sciences), Beijing, China.