  Limited constraints on the evolution of the Lyman Continuum (LyC) escape
fraction represent one of the primary uncertainties in the theoretical
determination of the reionization history. Due to the intervening intergalactic
medium (IGM), the possibility of observing LyC photons directly in the epoch of
reionization is highly unlikely. For this reason, multiple indirect probes of
LyC escape have been identified, some of which are used to identify
low-redshift LyC leakers (e.g. O32), while others are primarily useful at $z>6$
(e.g. [OIII]/[CII] far infrared emission). The flux ratio of the resonant MgII
doublet emission at 2796$\dot{\rm A}$ and 2803$\dot{\rm A}$ as well as the MgII
optical depth have recently been proposed as ideal diagnostics of LyC leakage
that can be employed at $z>6$ with JWST. Using state-of-the-art cosmological
radiation hydrodynamics simulations post-processed with CLOUDY and
resonant-line radiative transfer, we test whether MgII is indeed a useful probe
of LyC leakage. Our simulations indicate that the majority of bright,
star-forming galaxies with high LyC escape fractions are expected to be MgII
emitters rather than absorbers at $z=6$. However, we find that the MgII doublet
flux ratio is a more sensitive indicator of dust rather than neutral hydrogen,
limiting its use as a LyC leakage indicator to only galaxies in the optically
thin regime. Given its resonant nature, we show that MgII will be an exciting
probe of the complex kinematics in high-redshift galaxies in upcoming JWST
observations.
