A. W. Rose
The effect of cuprous chloride complexes in the origin of red-bed copper and related deposits
Economic Geology and the Bulletin of the Society of Economic Geologists (October 1976), 71(6):1036-1048

Abstract:
Geological, mineralogical, and isotopic data indicate that red-bed copper deposits are introduced into their host rocks after sedimentation by flowing subsurface waters approximately in equilibrium with hematite, quartz, feldspar, and mica at temperatures less than about 75 degrees C. Precipitation of copper is caused by encountering a reducing environment. In most normal oxidizing terrestrial ground waters, solubility of copper is less than 1 ppm at reasonable pH values, so these waters are relatively ineffective in transport of Cu. However, cuprous ion (Cu (super +) ) forms the complex ions CuCl ₂ - and CuCl ₃ (super 2-) with formation constants of 10 (super 5.4) and 10 (super 5.8) at 25 degrees C. These complexes allow solubilities of about 100 ppm Cu in 0.5 m Cl (super -) at intermediate Eh in the stability field of hematite at pH 7.0, and solubilities of 6 ppm at pH 8.2. Weaker complexes of Cu (super 2+) with OH- SO ₄ (super 2-) , SO ₃ (super 2-) , and Cl (super -) are known but have comparatively little effect on copper solubility in most natural waters. Most red-bed copper and cupriferous sandstone-type uranium deposits are associated with evaporites which could have furnished chloride-rich ground waters. Escape of connate marine waters from underlying marine sediments may have furnished a copper-transporting fluid in other cases. Cuprous chloride complexing may also have been important in the origin of the Kupferschiefer and Zambian deposits, which also occur in association with red beds and evaporites.