The abundance of metals in the hot, gaseous X-ray haloes of galaxy clusters depends crucially on the evolution of the constituent galaxies and their associated stellar populations. The metal abundances in X-ray clusters at high redshifts should therefore provide important insights into the nature and epoch of galaxy formation. Here we report the detection of an extended X-ray source in the direction of the lensed quasi-stellar object MG2016 + 112 (refs 1, 2). Although deep optical searches have failed to reveal a galaxy duster at the lens position(3,4), the X-ray emission is consistent with thermal bremsstrahlung radiation from a hot metal-rich, diffuse gaseous halo, as observed in nearby galaxy dusters. This is the most distant galaxy duster discovered in X-rays so far. Furthermore, the mass of the duster derived from this emission is consistent with that implied by lensing models of the system(5). Given that the duster apparently comprises few galaxies, yet contains a large amount of iron, a new type of astronomical object is implied by our results. A revision of theoretical models of the metal enrichment process in galaxy dusters may therefore be required.