# The role of S 3 •- in molybdenum transport by hydrothermal fluids revealed by in situ X-ray absorption spectroscopy

Maria A Kokh, Stephan Klemme, Christian Schmidt, Jean-Louis F

Hazemann, Denis Testemale, Clement Laskar, Manuela Borchert, Elena

Bazarkina, Christophe Moeller, Gleb S. Pokrovski, et al.

► To cite this version:

Maria A Kokh, Stephan Klemme, Christian Schmidt, Jean-Louis F Hazemann, Denis Testemale, et al.. The role of S 3 •- in molybdenum transport by hydrothermal fluids revealed by in situ X-ray absorption spectroscopy. Goldschmidt 2023, Jul 2023, Lyon, France. . hal-05356085

HAL Id: hal-05356085

https://hal.science/hal-05356085v1

Submitted on 12 Nov 2025

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# The role of the trisulfur radical ion ($S_3^-$) in molybdenum transport by hydrothermal fluids revealed by in situ X-ray absorption spectroscopy (XAS)

Maria A. Kokh, Stephan Klemme, Christian Schmidt, J.-L. Hazemann, Denis Testemale, Clément Laskar, Manuela Borchert, Elena F. Bazarkina, Christoph Moeller, Gleb S. Pokrovski, and Max Wilke

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## Geological context

The knowledge of molybdenum speciation under hydrothermal conditions is a key for understanding the formation of porphyry Mo deposits [1-9]. A potential missing ligand for Mo complexation may be the $S_3^-$ radical ion [10,11,12], which is abundant in sulfate-sulfide rich acidic-to-neutral porphyry-like fluids under epithermal conditions.

Here we investigate molybdenite $MoS_2$ dissolution and its complexation with $S_3^-$ radical ion and $HS^-$ in experimental sulfur-bearing fluids at 300-350°C, 500 bar using in situ XAS and thermodynamic modeling.

Sample of quartz-pyrite-molybdenite vein (Toromocho copper deposit, Peru)

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## Experimental setup and sample environment

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ESRF synchrotron, (Grenoble, France); BM30B/FAME: bending magnet beamline [13]
An experimental cell for *in situ* XAS and hydrothermal autoclave equipped with Be windows operating under He pressure [14] Scan edge energy: 20 keV (Mo K edge)
Tunable energy [keV]: 19.8-20.8 keV Detectors: 3 diodes measuring x-rays scattered by a kapton foil in transmission mode; SSD fluorescence detector

EXAFS spectrum accumulated on an experimental fluid during molybdenite dissolution and a corresponding theoretical fit (3.8 ppm Mo)

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Fourier transform of two experimental spectra accumulated on $MoS_2$ bearing $S_3^-$ rich fluids, 300°C, 500 bar

3wt% S H₂S/HS-/SO₄²⁻/ S₃⁻; 300°C, 500 bars

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* Light blue and green lines – $MoS_4^{2-}$ and $MoOS_3^-$ aqueous species, extrapolation from [15]

* Dark blue line – $S_3^-$ aqueous species [11]

* Rose line – extra $Mo-S_3^-$ species, this study

* Triangles and squares – measured $MoS_2$ solubilities, *ex situ* experimental data

* Diamonds – measured $MoS_2$ solubilities, *in situ* experimental data

## Molybdenite solubility in a model $S_3^-$ rich fluid

Equations for isocoulombic extrapolation for reaction constants $K_T$ for thiomolybdate species using data obtained with UV-vis spectroscopy at 25°C [8,15]

$$ \log K_{T \text{exp}} = (T_{298K}/T_{\text{exp}}) * \log K_{T(298K)} $$

$$ MoO_4^{2-} + H_2S^0_{aq} = MoO_3S^{2-} + H_2O $$

$$ MoO_3S^{2-} + H_2S^0_{aq} = MoO_2S^{2-} + H_2O $$

$$ MoO_2S^{2-} + H_2S^0_{aq} = MoOS_3^{2-} + H_2O $$

$$ MoOS_3^{2-} + H_2S^0_{aq} = MoS_4^{2-} + H_2O $$

<figure><img src="image_8.png" /></figure>

## Conclusions

* The detection limit was improved for this experimental work by removing all Mo components of the heater. We could detect concentration as low as 0.1 ppm [Mo]

* *In situ* XAS experiments combined with thermodynamic modeling and classical ex-situ experiments revealed that Mo can form complexes with $S_3^-$ in acidic porphyry-like fluids. Work is in progress to strengthen the spectroscopic evidence by XANES calculations.

<figure><img src="image_9.png" /></figure>

## Funding:

Agence Nationale de la Recherche ANR-16-CE31-0017, Deutsche Forschungsgemeinschaft WI 2000/25-1, KL 1368/13-1, SCHM 2415/6-1, ESRF ES-1050, DESY I-20220251.

## References

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