Geochemical implications of chrysocolla-bearing alluvial gravels

Chrysocolla-bearinõ alluvial õravels, associated spatially with oxi-dized copper sulfide ore deposits, indicate the competence of aqueous sur-face solutions to transport larõe quantities of copper over distances of sev-eral kilometers. Their features sugõest hat such transport occurs after the formation of a distinct "oxide zone " resultinõ from in situ alteration of the primary sulfides. Hydrolysis equilibria involvinõ the rock-forminõ silicate minerals rep-resent the most probable controls of the release of silica to such solutions, and the solubilities of the secondary copper minerals control the avail-ability of copper. The process by which chrysocolla forms in the õravels is not known. Reaction with solids present in the õravels may increase the pH of the transportinõ solutions thus favorinõ the reaction of dissolved silica and copper and the subsequent precipitation of chrysocolla. Alternatively, chrysocolla may form by solution-solid reactions involvinõ silicate min-erals already present in the õravels. Using albite as an example, this mechanism may be described in terms of the following overall reaction' Cu ++ q- 9H20-Jr- 2NaA1SiaOs(albite) = 2Na + q- 3Si(OH)4 q- H4A12Si209(kaol.) q- CuSiOa ' H 20 (amorphous