Very high column density and small reddening toward GRB 020124 at z=3.20

We present optical and near-infrared observations of the dim afterglow of GRB 020124, obtained between 2 and 68 hr after the gamma-ray burst. The burst occurred in a very faint (Rgreater than or similar to29.5) damped Lyalpha absorber (DLA) at a redshift of z=3.198+/-0.004. The derived column density of neutral hydrogen is log(N-Hi)=21.7+/-0.2, and the rest-frame reddening is constrained to be E(B-V)<0.065, i.e., A(V)<0.20 for standard extinction laws with R-V approximate to 3. The resulting dust-to-gas ratio is less than 11% of that found in the Milky Way but consistent with the SMC and high-redshift QSO DLAs, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. A gray extinction law (large R-V), produced through preferential destruction of small dust grains by the gamma-ray burst, could increase the derived A(V) and dust-to-gas ratio. The dimness of the afterglow is, however, fully accounted for by the high redshift: if GRB 020124 had been at z=1, it would have been approximately 1.8 mag brighter-in the range of typical bright afterglows.