  We present the results of observations taken with the X-shooter spectrograph
devoted to the study of quasars at z ~ 6. This paper focuses on the properties
of metals at high redshift traced, in particular, by the C IV doublet
absorption systems. Six objects were observed with resolutions ~27 and 34 km/s
in the visual, and 37.5 and 53.5 km/s in the near infrared. We detected 102 C
IV lines in the range: 4.35 < z < 6.2 of which 27 are above z ~ 5. Thanks to
the characteristics of resolution and spectral coverage of X-shooter, we could
also detect 25 Si IV doublets associated with the C IV at z>5. The column
density distribution function of the C IV line sample is observed to evolve in
redshift for z>~ 5.3, with respect to the normalization defined by low redshift
(1.5 < z <4) C IV lines. This behaviour is reflected in the redshift evolution
of the C IV cosmic mass density, Omega_CIV, of lines with column density in the
range 13.4 < log N(C IV) < 15, which is consistent with a drop of a factor of
~2 for z>~ 5.3. Considering only the stronger C IV lines (13.8 < log N(CIV) <
15), Omega_CIV gently rises by a factor of ~10 between z ~ 6.2 and z~ 1.5 with
a possible flattening toward z ~ 0. The increase is well fitted by a power law:
Omega_CIV = (2+-1)x10^{-8} [(1+z)/4]^{-3.1+-0.1}. An insight into the
properties of the C IV absorbers and their evolution with redshift is obtained
by comparing the observed column densities of associated C IV, Si IV and C II
absorptions with the output of a set of CLOUDY photo-ionization models. As
already claimed by cosmological simulations, we find that C IV is a good tracer
of the metallicity in the low-density intergalacitc medium gas at z ~ 5-6 while
at z ~ 3 it arises in gas with over density delta ~100.
