The metallicity distribution of stars in the Fornax dwarf spheroidal galaxy is studied with infrared Ca II triplet spectroscopy for 117 red giants, obtained with FORS1 on the VLT. The results reveal a very wide distribution of metallicities in Fornax, including a surprising abundance of young, relatively metal-rich stars ([Fe/H]>-0.7). By combining these metallicities with the position of the stars in the colour-magnitude diagram, we derive an age-metallicity relation for Fornax, characterized by sustained star formation and chemical enrichement in the past few Gyrs. The results are discussed in the context of the predictions of evolutionary models of dwarf galaxies.