• Li, Jian Wei
• Rempel, Kirsten
• Wu, Ya Fei
• Evans, Katy
• Williams-Jones, Anthony E.

Abstract

<p>Invisible gold hosted by pyrite represents a large proportion of gold resources worldwide. Gold is enriched in pyrite relative to hydrothermal fluids by five orders of magnitude, but controls on the hyperenrichment of gold in pyrite remain unclear. Here, we present the first micrometer- to nanometer-scale evidence for a colloidal iron sulfide phase that forms a precursor to pyrite and shows a remarkable capacity to scavenge gold from ore fluid. In chalcedony cement of breccia ores from the world-class Daqiao epizonal orogenic gold deposit, China, we find that numerous iron sulfide colloids, together with silica colloids and amorphous carbonaceous matter, formed from a highly supersaturated ore fluid in response to rapid fluid depressurization. Mechanisms that favor gold incorporation into iron sulfide colloids include large specific surface areas, abundant structure defects, negatively charged surfaces, high reactivity, and sequestration by silica colloids. These iron sulfide colloids subsequently aggregated and transformed to spherical cryptocrystalline pyrite aggregates. This process may be common during hydrothermal gold mineralization. Co-precipitation of amorphous carbonaceous matter plausibly enhanced the stability and dispersibility of iron sulfide colloids, facilitated sequestration of iron sulfide by silica, and adsorbed minor amounts of gold. Iron sulfide colloids represent a previously unrecognized mechanism for hyperenrichment of invisible gold in pyrite under rapidly changing and potentially non-equilibrium conditions. This study emphasizes that the iron sulfide colloids may be an important component of conceptual models for hydrothermal gold mineralization within Earth's crust.</p>

Topics

• impedance spectroscopy
• surface
• amorphous
• phase
• gold
• cement
• defect
• precipitation
• iron