Decoding multistage fluid-rock interactions in the Takab Iranian iron-ore deposit.

The Takab (NW Iran) BIF consists of alternating iron- and silica-rich layers. It formed ca 560 Ma in a back-arc basin from a mixing of seawater and hydrothermal fluids with incorporation of ca. 20% of terrigenous material [1]. The ore bodies are composed of magnetite with various textures (disseminated, banded, lenticular, nodular, and massive) mainly hosted in folded micaschists. The magnetite and/or the matrix may contain accessory minerals, monazite, barite, scheelite, and Fe-Mn-carbonates in nodular magnetite. In this study we show that the different types of magnetite layers recorded a variety of fluid-rocks interactions that occurred at moderate temperature (200-300°C) under variable but mostly reduced fO₂.

All magnetite types have low Ni and Cr (10-30 ppm) and V (< 100 ppm), and high Mn (1800-2600 ppm; up to 1% in nodular magnetite), characteristics of hydrothermal magnetite. Ti concentration is also low (15-200 ppm) except in disseminated magnetite, in which Ti (up to 1940 ppm) is correlated with Al and Mg. Moreover, all magnetite types show positive Eu and Y anomalies plus a negative Ce anomaly in nodular magnetite, typical for mixed seawater/hydrothermal fluid precipitation. The negative Ce anomaly of nodular magnetite is similar to that of the calcschist laminates.

The disseminated magnetite shows highly positive ∂⁵⁶Fe (+1.4 ‰) and ∂¹⁸ values (+2.2 ‰) testifying for a magmatic/high-T hydrothermal origin, also suggested by the trace element behavior.

The banded magnetite also shows mostly positive ∂⁵⁶Fe (up to +1.1 ‰) values, but a lighter oxygen isotopic composition (∂¹⁸ values=-2.5 to +1 ‰). This suggests that banded magnetite did not preserve the magmatic/high-T hydrothermal signature unlike disseminated magnetite, and was further affected by a hydrothermal alteration or re-equilibration with low-T fluids.

The nodular magnetite shows important differences from the other two types: mostly light ∂⁵⁶Fe values, indicative of a low-T hydrothermal fluid signature, and heavy ∂¹⁸O (+4 to ‰) values consistent with a magmatic/high-T hydrothermal origin. Decoupling of the Fe and O isotope signature suggests a more complex hydrothermal history. The presence of Cl-bearing apatite inclusion in the nodular magnetite supports the precipitation of low ∂⁵⁶Fe magnetite from a Cl-bearing hydrothermal fluid. Furthermore, the high ∂¹⁸O values possibly suggest a re-equilibration of a magmatic-hydrothermal fluid with carbonate rocks or mixing with fluid in equilibrium with the carbonate in the host rock. A likely scenario is the involvement of CO₂-bearing hydrothermal fluids produced during the decarbonatization of the close-by calcschist.

In conclusion, the most characteristic feature of the Takab BIF is the large predominance of the hydrothermal overprint on the volcano-sedimentary sequence throughout the formation and the evolutionary history of the iron ore deposit. The varied chemical and isotopic composition of the different magnetite types and the presence of accessory minerals point out both the variety of the fluids involved and the degree of the Sfluid-rock interactions [2, 3].

 [1] Honarmand et al., Precambrian Research, 2024; [2, 3] Wagner et al., Minerals, 2023, 2025.