  We study cosmological simulations of early structure formation, including
non-equilibrium molecular chemistry, metal pollution from stellar evolution,
transition from population III (popIII) to population II (popII) star
formation, regulated by a given critical metallicity, and feedback effects. We
investigate the properties of early metal spreading from the different stellar
populations and its interplay with primordial molecular gas. We find that,
independently of the details about popIII modeling, after the onset of star
formation, regions enriched below the critical level are mostly found in
isolated environments, while popII star formation regions are much more
clumped. Typical star forming haloes show average SN driven outflow rates of up
to 10^{-4} Msun/yr in enriched gas, initially leaving the original star
formation regions almost devoid of metals. The polluted material, which is
gravitationally incorporated in over-dense environments on timescales of 10^7
yr, is mostly coming from external, nearby star forming sites ("gravitational
enrichment"). In parallel, the pristine-gas inflow rates are between 10^{-3} -
10^{-1} Msun/yr. However, thermal feedback from SN generates turbulence and
destroys molecules within the pristine gas, and only the polluted material,
incorporated via gravitational enrichment, can continue to cool by atomic metal
fine-structure transitions on time scales short enough to end the initial
popIII regime within less than 10^8 yr.
