Experimental Study on the Reaction of Cuprite (Cu₂O) with Acetate-Bearing Hydrothermal Fluids at 100-250 °C and 5-30 MPa

verfasst von

Dongmei Qi, Harald Behrens, Marina Lazarov, Roman Botcharnikov, Chao Zhang, Christian Ostertag-Henning, Stefan Weyer

Abstract

To improve our understanding of the formation of sedimentary copper deposits, the reaction of cuprite with 0.2 m HAc-KAc or pure H₂O solutions is studied systematically at 100-250 °C and 5-30 MPa. The experiments were carried out for periods of up to 72 h in a Parr autoclave, allowing for the in situ sampling of the fluid phase. The experiments conducted in this study demonstrate that cuprite (Cu₂O) underwent a series of changes: (i) simple dissolution, (ii) Cu(I) disproportionation to native Cu and Cu(II), and (iii) subsequent oxidation into tenorite (CuO). In pure water, only (i) and (ii) steps can be discerned, whereas all three processes have been observed in an acetate-bearing system. In HAc-KAc solutions, the maximum dissolved Cu content correlates inversely with temperature, i.e., 378 to 168 μg/g at 100 and 200 °C, respectively. However, equilibrium has not been reached in our experiments and these values may be treated as minimum cuprite solubility. In situ Cu isotope analyses have been carried out by laser ablation combined with a multicollector inductively coupled plasma-mass spectrometer. The data imply that copper isotope fractionation during cuprite replacement reactions is small. Both the microscopic observations on cross sections and the analytical data support the idea that the mineral replacement reaction is controlled by a coupled dissolution-reprecipitation (CDR) mechanism. This applies to both the deposition of metallic copper and the formation of tenorite. As suggested by the formation of pore spaces in the deposited layers, only a portion of the dissolved copper is redeposited directly in situ. The isotopic analyses of the solution and solid phases show that the partial transfer of copper into the surrounding solution is not associated with a significant isotopic effect, e.g., a measured difference between Cu and Cu₂O is within 0.32 ± 0.06‰. Our study indicates that acetate plays a dual role in copper transport and deposition. On one hand, the presence of acetate strongly enhances the Cu content in solution up to 400 μg/g, implying that acetate complexation can be responsible for metal transport in hydrothermal fluids. On the other hand, decarboxylation of acetate substantially decreases the dissolved Cu and aids the precipitation of tenorite. This may lead to the co-occurrence of Cu-bearing minerals with different oxidation valence states at low temperatures in a variety of geological settings such as supergene hydrothermal systems.

Organisationseinheit(en)

Institut für Mineralogie
AG Geochemie

Externe Organisation(en)

Xinjiang University
Johannes Gutenberg-Universität Mainz
State Key Laboratory of Continental Dynamics
Bundesanstalt für Geowissenschaften und Rohstoffe (BGR)

Typ

Artikel

Journal

ACS Earth and Space Chemistry

Band

8

Seiten

499-519

Anzahl der Seiten

21

Publikationsdatum

21.03.2024

Publikationsstatus

Veröffentlicht

Peer-reviewed

Ja

ASJC Scopus Sachgebiete

Geochemie und Petrologie, Atmosphärenwissenschaften, Astronomie und Planetologie

Ziele für nachhaltige Entwicklung

SDG 3 – Gute Gesundheit und Wohlergehen

Elektronische Version(en)

https://doi.org/10.1021/acsearthspacechem.3c00254 (Zugang: Offen)