Metal mobility during hydrothermal breakdown of Fe-Ti oxides: Insights from Sb-Au mineralizing event (Variscan Armorican Massif, France)

Abstract

Hydrothermal alteration related to Sb-Au mineralization is widespread in the Variscan Armorican Massif, but mineral replacement reactions are not well characterized, in particular the hydrothermal breakdown of ilmenite-titanohematite. Based on petrography, electron probe micro-analyzer and laser ablation-inductively coupled plasma-mass spectrometer analyses, we document mineralogical change at rock-and mineral-scale and the redistribution of Sb and others trace elements during the recrystallization of ilmenitetitanohematite to hydrothermal rutile. Hydrothermal alteration is mainly potassic with associated carbonation. The replacement mechanism is interpreted to be an interface-coupled dissolution-reprecipitation process. Results show that Mn, Zn, Co, Ni, Sn, Mo and U are released during hydrothermal alteration, whereas Sb and W are incorporated in newly-formed hydrothermal rutile from the hydrothermal fluid. Furthermore, the concentration of Sb evolves through time suggesting a change in fluid composition likely related to an enrichment of fluid in Sb during rutile crystallization. Considering that Fe-Ti oxides breakdown during hydrothermal alteration is common within epithermal and mesothermal/orogenic Au-Sb mineralizing systems, results report in this study yield important constraints about metal mobility and exchanges in hydrothermal gold systems.

Variscan Sb-Au mineralization in Central Brittany (France): A new metallogenic model derived from the Le Semnon district

“…(1) in the crystal lattice of arsenopyrite, with a mean content of about 200 ppm (Gloaguen et al, 2016b), (2) in native form in quartz veins, arsenopyrite and stibnite, or (3) in aurostibite within stibnite (Chauris et al, 1985). Hydrothermal alteration is characterized by carbonation, as revealed by the destruction of calcic plagioclase and clinopyroxene and by the presence of ankerite and calcite in the groundmass, by potassic metasomatism, as highlighted by sericitized plagioclase and hydrothermal illite in the groundmass of the hydro-thermally altered dyke, and by a weak silicification (Pochon et al, 2017). All the magmatic Fe-Ti oxides, represented by ilmenite and il-menite-hematite solid solution, are totally replaced by rutile and leucoxene at the beginning of hydrothermal event (Chauris et al, 1985;Pochon et al, 2017).…”

“…Hydrothermal alteration is characterized by carbonation, as revealed by the destruction of calcic plagioclase and clinopyroxene and by the presence of ankerite and calcite in the groundmass, by potassic metasomatism, as highlighted by sericitized plagioclase and hydrothermal illite in the groundmass of the hydro-thermally altered dyke, and by a weak silicification (Pochon et al, 2017). All the magmatic Fe-Ti oxides, represented by ilmenite and il-menite-hematite solid solution, are totally replaced by rutile and leucoxene at the beginning of hydrothermal event (Chauris et al, 1985;Pochon et al, 2017). During the breakdown of the ilmenite-hematite assemblage into rutile, rutile incorporated Sb (∼100 ppm) suggesting that the fluid was already enriched in Sb at the early stage of hydro-thermal alteration (Pochon et al, 2017).…”

“…All the magmatic Fe-Ti oxides, represented by ilmenite and il-menite-hematite solid solution, are totally replaced by rutile and leucoxene at the beginning of hydrothermal event (Chauris et al, 1985;Pochon et al, 2017). During the breakdown of the ilmenite-hematite assemblage into rutile, rutile incorporated Sb (∼100 ppm) suggesting that the fluid was already enriched in Sb at the early stage of hydro-thermal alteration (Pochon et al, 2017). Sb-Au mineralization is con-sidered to be syn-to post-kinematic by Marot (1984) because some Sb-veins are affected by the regional cleavage.…”

“…Rare apatite grains have been ob-served inside the Sb-bearing quartz veins close to the wall and per-pendicular to blocky elongate quartz. Rutile comes from the breakdown of the magmatic ilmenite-hematite assemblage during a hydrothermal event (Chauris et al, 1985, Pochon et al, 2017 and mainly consists of irregular grains with sizes ranging from 10 to 80 μm found within the external preserved shape of its parent mineral (Fig. 10e).…”

Polygenic chamosite from a hydrothermalized oolitic ironstone (Saint-Aubin-des-Châteaux, Armorican Massif, France): crystal chemistry, visible–near-infrared spectroscopy (red variety) and geochemical significance

“…Later, this ironstone was hydrothermally altered by early Variscan polyphase fluid flow (Gloaguen et al , 2007; Tartèse et al , 2015) related to a 360 Ma mafic magmatism (Pochon et al , 2016a, 2016b). This fluid flow induced specific mineralizations (Pochon et al , 2017, 2018), including the Saint-Aubin-des-Châteaux sulfide ore (Pochon et al , 2019). In this deposit, the oolitic ironstone was partly transformed into pyritized lenticular bodies and associated chlorite alteration haloes.…”