  We investigate the abundance and distribution of metals in the high-redshift
intergalactic medium and circum-galactic medium through the analysis of a
sample of almost 600 SiIV absorption lines detected in high and intermediate
resolution spectra of 147 quasars. The evolution of the number density of SiIV
lines, the column density distribution function and the cosmic mass density are
studied in the redshift interval 1.7 <= z <= 6.2 and for log N(SiIV) >= 12.5.
All quantities show a rapid increase between z~6 and z< 5 and then an almost
constant behaviour to z~2 in very good agreement with what is already observed
for CIV absorption lines. The present results are challenging for numerical
simulations: when simulations reproduce our SiIV results, they tend to
underpredict the properties of CIV, and when the properties of CIV are
reproduced, the number of strong SiIV lines (log N(SiIV) > 14) is
overpredicted.
