According to the present invention there is provided a process for upgrading an ore or concentrate that contains metal sulphur minerals and gangue material. The process includes the stages of: a) selectively leaching the ore or concentrate using an ammoniacal solution containing ammonium carbonate that forms soluble metal ammine complexes; b) separating the solid and liquid phases formed in stage a) with the liquid phase forming a solution including soluble metal ammine complexes and the solid phase including at least in part the gangue material; c) removing ammonia and carbon dioxide from the liquid phase formed in step b) under conditions so as to selectively precipitate the valuable metal(s); and d) separating the solid and liquid phases formed in stage c) with the solid phase forming a more-concentrated source of valuable metal.