We present sensitive near–infrared spectroscopic observations for a sample of five z \sim 6 quasars . These quasars are amongst the most distant , currently known quasars in the universe . The spectra have been obtained using ISAAC at the VLT and include the C iv , Mg ii and Fe ii lines , which are powerful probes of the chemical enrichment and the black hole masses in these objects . We measure the Fe ii/Mg ii line ratio , as an observational proxy for the Fe/ \alpha element ratio . These elements are produced by different types of supernovae and their abundance ratio can therefore serve as a cosmological clock . We derive a ratio of 2.7 \pm 0.8 for our sample , which is similar to that found for lower redshift quasars , i.e. , we provide additional evidence for the lack of evolution in the Fe ii/Mg ii line ratio of quasars up to the highest redshifts . This result demonstrates that the sample quasars must have undergone a major episode of iron enrichment in less than one Gyr and star formation must have commenced at z \geq 8 . The linewidths of the Mg ii and C iv lines give two estimates for the black hole masses . A third estimate is given by assuming that the quasars emit at their Eddington luminosity . The derived masses using these three methods agree well , implying that the quasars are not likely to be strongly lensed . We derive central black hole masses of 0.3 - 5.2 \times 10 ^ { 9 } M _ { \odot } . These include the lowest black hole mass ever measured at z \sim 6 , suggesting that we probe a more typical quasar population ( with lower masses/luminosities ) than examined before . We use the difference between the redshift of Mg ii ( a proxy for the systemic redshift of the quasar ) and the onset of the Gunn Peterson trough to derive the extent of the ionized Strömgren spheres around our target quasars . The derived physical radii are about five Mpc . Using a simple ionization model , the emission of the central quasars would need of order 10 ^ { 6 } -10 ^ { 8 } yr to create these cavities in a surrounding intergalactic medium ( IGM ) with a neutral fraction between 0.1 and 1.0 . As the e–folding time scale for the central accreting black hole is on the order of a few times 10 ^ { 7 } years , it can grow by one e–folding or less within this time span .