  We examine the absorption line spectra of a sample of 31 luminous (M_UV=-23)
Lyman break galaxies at redshift z~6 using data taken with the FOCAS and OSIRIS
spectrographs on the Subaru and GTC telescopes. For two of these sources we
present longer exposure data taken at higher spectral resolution from ESO's
X-shooter spectrograph. Using these data, we demonstrate the practicality of
stacking our lower resolution data to measure the depth of various interstellar
and stellar absorption lines to probe the covering fraction of low ionization
gas and the gas-phase and stellar metallicities near the end of the era of
cosmic reionization. From maximum absorption line depths of SiII1260 and
CII1334, we infer a mean covering fraction of >0.85+/-0.16 for our sample. This
is larger than that determined using similar methods for lower luminosity
galaxies at slightly lower redshifts, suggesting that the most luminous
galaxies appear to have a lower escape fraction than fainter galaxies, and
therefore may not play a prominent role in concluding reionization. Using
various interstellar absorption lines we deduce gas-phase metallicities close
to solar indicative of substantial early enrichment. Using selected stellar
absorption lines, we model our spectra with a range of metallicities using
techniques successfully employed at lower redshift and deduce a stellar
metallicity of 0.4 +0.3/-0.1 solar, consistent with the stellar mass - stellar
metallicity relation recently found at z~3-5. We discuss the implications of
these metallicity estimates for the typical ages of our luminous galaxies and
conclude our results imply initial star formation at redshifts z~10, consistent
with independent analyses of earlier objects.
