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<p>In mountain ranges, crustal-scale faults localize multiple episodes of deformation. It is therefore common to observe current or past geothermal systems along these structures. Understanding the fluid circulation channelized in fault zones is essential to characterize the thermochemical evolution of associated hydrothermal systems. We present a study of a palaeo-system of the Pic de Port Vieux thrust fault. This fault is a second-order thrust associated with the Gavarnie thrust in the Axial Zone of the Pyrenees. The study focused on phyllosilicates which permit the constraint of the evolution of temperature and redox of fluids at the scale of the fault system. Combined X-ray absorption near-edge structure (XANES) spectroscopy and electron probe microanalysis (EPMA) on synkinematic chlorite, closely linked to microstructural observations, were performed in both the core and damage zones of the fault zone. Regardless of the microstructural position, chlorite from the damage zone contains iron and magnesium (<span class="inline-formula">Fe_total</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="527256ea34e0af356380afd605ccefc0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="se-15-1065-2024-ie00001.svg" width="8pt" height="14pt" src="se-15-1065-2024-ie00001.png"/></svg:svg></span></span> (<span class="inline-formula">Fe_total</span> <span class="inline-formula">+</span> <span class="inline-formula">Mg</span>) about 0.4), with <span class="inline-formula">Fe³⁺</span> accounting for about 30 % of the total iron. Chlorite in the core zone is enriched in total iron, but individual <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msup><mi mathvariant="normal">Fe</mi><mrow><mn mathvariant="normal">3</mn><mo>+</mo></mrow></msup></mrow><mo>/</mo><mrow class="chem"><msub><mi mathvariant="normal">Fe</mi><mi mathvariant="normal">total</mi></msub></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="59pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="be9f8a38828f3b736ccea5947e80ae8b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="se-15-1065-2024-ie00002.svg" width="59pt" height="16pt" src="se-15-1065-2024-ie00002.png"/></svg:svg></span></span> ratios range from 15 % to 40 %, depending on the microstructural position of the grain. Homogeneous temperature conditions about 280–290 <span class="inline-formula">°C</span> have been obtained by chlorite thermometry. A scenario is proposed for the evolution of fluid–rock interaction conditions at the scale of the fault zone. It involves the circulation of a single hydrothermal fluid with homogeneous temperature but several redox properties. A highly reducing fluid evolves due to redox reactions involving progressive dissolution of hematite, accompanied by crystallization of <span class="inline-formula">Fe²⁺</span>-rich and <span class="inline-formula">Fe³⁺</span>-rich chlorite in the core zone. This study shows the importance of determining the redox state of iron in chlorite to calculate their temperature of formations and to consider the fluid evolution at the scale of a fault.</p>