  We differentiate between the metal enrichment of the gas in virialized
minihalos and that of the intergalactic medium at high redshift, pertinent to
cosmological reionization, with the initial expectation that gas in the high
density regions within formed dark matter halos may be more robust thus
resistant to mixing with lower density intergalactic medium. Using detailed
hydrodynamic simulations of gas clouds in minihalos subject to destructive
processes associated with the encompassing intergalactic shocks carrying
metal-enriched gas, we find, as an example, that, for realistic shocks of
velocities of 10-100km/s, more than (90%,65%) of the high density gas with
rho>500 rhob inside a minihalo virialized at z=10 of mass (10^7,10^6)Msun
remains at a metallicity lower than 3% of that of the intergalactic medium by
redshift z=6. It may be expected that the high density gas in minihalos becomes
fuel for subsequent star formation, when they are incorporated into larger
halos where efficient atomic cooling can induce gas condensation hence star
formation. Since minihalos virialize at high redshift when the universe is not
expected to have been significantly reionized, the implication is that gas in
virialized minihalos may provide an abundant reservoir of primordial gas to
possibly allow for the formation of Population-III metal-free stars to extend
to much lower redshift than otherwise expected based on the enrichment of
intergalactic medium.
