Nanoscale mineralogical evidence confirms Cu transport as chloride complexes in brines in the Central European Kupferschiefer district Open Access

High-grade sediment-hosted stratiform copper (SSC) deposits will be critical for securing future supplies of Cu. However, current genetic models for SSC deposits have few constraints on the physicochemical properties of ore fluids and timing of ore-stage mineralization. In the Central European Kupferschiefer district, SSC deposits are assumed to have formed following basin-scale circulation of metal-bearing chloride-rich brines, but direct evidence of Cu transport in these brines is lacking. We use transmission electron microscopy (TEM) to investigate the pore-scale mineralogy of Cu-mineralized mudstones from the Saale subbasin (eastern Germany). Detailed TEM investigations showed Cu_xCl_y(OH)_z mineral phases, including Cu₂Cl(OH)₃ (atacamite) and, to a lesser extent, CuCl (nantokite) nanoparticles (⁓5−20 nm), broadly intergrown with illite in pore spaces surrounding disseminated Cu-(Fe) sulfides. Cu_xCl_y(OH)_z likely formed within, and between, the illite layers due to the adsorption of soluble Cu(I) chloride complexes on illite edges. The preservation of the Cu_xCl_y(OH)_z-illite assemblage in pore spaces adjacent to ore-stage bornite provides important boundary conditions for mineral systems models; specifically, that Cu was transported as chloride complexes in low-temperature brines that reacted with host units undergoing burial diagenesis.