  We present the analysis of two BALs seen in the spectrum of the QSO SDSS
J160501.21-011220.0. Our UVES spectrum shows two well detached BALs at z(abs)=
4.685 and 4.855. The system at z(abs)= 4.855 covers the background source
completely suggesting that the gas is located outside the BLR. On the contrary
the system at z(abs)= 4.685 has a covering factor of ~0.9. The observed H I
absorption line together with the limits on C II and Si II absorptions suggest
that 16<log N(H I) (cm^{-2})<~17 in z(abs) = 4.855 absorption system.
Comparison with models show that the observed column densities of N V, Si IV
and C IV in this system require that N is underabundant by more than a factor 3
compared to Si if the ionizing radiation is similar to a typical QSO spectrum.
This is contrary to what is usually derived for the emission line gas in QSOs.
We show that the relative suppression in the N V column density can be
explained for Solar abundance ratios or abundance ratios typical of Starburst
abundances if an ionizing spectrum devoid of X-rays is used instead. Thus, if
the composition of BAL is like that of BLR then it is most likely that the
cloud sees a spectrum devoid of X-rays similar to what we observe from this
QSO. This is consistent with the fact that none of our models have high Compton
optical depth to remove X-rays from the QSO. Similar arguments lead to the
conclusion that the system at z(abs)= 4.685 as well is not Compton thick. The
estimated black hole mass is 8 10^8 solar mass suggesting the accretion onto
the seed black hole must have started as early as z = 11.
