Arsenic in pyrite acts as a catalyst for dissolution-reprecipitation reaction and gold remobilization

• Published in: The American mineralogist Vol. 110; no. 1; pp. 82 - 92
• Main Authors: Sugiono, Dennis, Martin, Laure, Thébaud, Nicolas, Fougerouse, Denis, LaFlamme, Crystal K., Fiorentini, Marco, Petrella, Laura, Guagliardo, Paul, Reddy, Steve, Hu, Si-Yu, Suvorova, Alexandra
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.01.2025; Walter de Gruyter GmbH
• Subjects: Arsenic; Arsenopyrite; Boundaries; Catalysts; Chemical composition; Chemical precipitation; Chemisorption; Crystallography; Depletion; Dissolution; Dissolving; EBSD; Fluid infiltration; Fractures; Gold; gold inclusion; Inclusions; low-angle boundary; Orogeny; Precipitation; Pyrite; Trace elements; Zoning
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2023-9034

In orogenic systems, pyrite hosts both free-native and lattice-bound gold. The processes governing gold habit, however, remain uncertain and subject to widespread debate. In this study, we employ micro-to-nanoscale trace element mapping alongside crystallographic characterization of gold-rich pyrite from the Kanowna Belle deposit (Western Australia) to probe the mechanisms influencing gold precipitation. Our examination reveals a complex chemical zoning in mineralized pyrite samples, characterized by an As-depleted core (As ≤2.5 wt%; Au ≤149 ppm) and rim (As ≤2.5 wt%; Au ≤264 ppm), separated by an As-Au-rich banded mantle (As ≤4.5 wt%; Au ≤2251 ppm). Pyrite structure in the mantle domain shows that low-angle boundaries mostly follow the shape of As-rich oscillatory zones. Gold in the pyrite mantle domain occurs as: (1) lattice-bound Au associated with As-rich oscillatory zoning; (2) gold micro-inclusions formed in equilibrium with arsenopyrite inclusions hosted within convoluted As-depleted pyrite domain; and (3) Au-filled late-stage fractures that crosscut the pyrite mantle. The shift in pyrite chemical composition from Au- and As-poor pyrite cores to Au- and As-rich mantles suggests that lattice-bound Au may have been integrated into As-rich zones via chemisorption during rapidly changing fluid conditions. Conversely, we propose that the gold inclusions hosted within the As-depleted convoluted domain form through a coupled dissolution re-precipitation process driven by fluid infiltration along low-angle boundaries. This study underscores the significance of linking arsenic distribution and crystallographic characteristics to comprehensively understand the controls on both gold form and distribution in gold deposits.