  Recent analyses of samples from asteroid (162173) Ryugu returned by JAXA's
Hayabusa2 mission suggest that Ryugu and CI chondrites formed in the same
region of the protoplanetary disk, in a reservoir that was isolated from the
source regions of other carbonaceous (C-type) asteroids. Here we conduct
$N$-body simulations in which CI planetesimals are assumed to have formed in
the Uranus/Neptune zone at $\sim15$--25 au from the Sun. We show that CI
planetesimals are scattered by giant planets toward the asteroid belt where
their orbits can be circularized by aerodynamic gas drag. We find that the
dynamical implantation of CI asteroids from $\sim15$--25 au is very efficient
with $\sim 5$\% of $\sim 100$-km planetesimals reaching stable orbits in the
asteroid belt by the end of the protoplanetary gas disk lifetime. The
efficiency is reduced when planetesimal ablation is accounted for. The
implanted population subsequently evolved by collisions and was depleted by
dynamical instabilities. The model can explain why CIs are isotopically
distinct from other C-type asteroids which presumably formed at $\sim5$--10 au.
