  Aim: We present early optical spectroscopy of the afterglow of the gamma-ray
burst GRB 060206 with the aim of determining the metallicity of the GRB
absorber and the physical conditions in the circumburst medium. We also discuss
how GRBs may be important complementary probes of cosmic chemical evolution.
Method: Absorption line study of the GRB afterglow spectrum. Results: We
determine the redshift of the GRB to be z=4.04795+/-0.00020. Based on the
measurement of the neutral hydrogen column density from the damped Lyman-alpha
line and the metal content from weak, unsaturated Sii lines we derive a
metallicity of [S/H] =-0.84+/-0.10. This is one of the highest metallicities
measured from absorption lines at z~4. From the very high column densities for
the forbidden Siii*, Oi*, and Oi** lines we infer very high densities and low
temperatures in the system. There is evidence for the presence of H$_2$
molecules with logN(H_2) ~ 17.0, translating into a molecular fraction of logf
\~ -3.5 with f=2N(H_2)/(2N(H_2)+ N(Hi)). Even if GRBs are only formed by single
massive stars with metallicities below ~0.3Z(solar), they could still be fairly
unbiased tracers of the bulk of the star formation at z>2. Hence, metallicities
as derived for GRB060206 here for a complete sample of GRB afterglows will
directly show the distribution of metallicities for representative star-forming
galaxies at these redshifts.
