  We study the constraints that future multifrequency Cosmic Microwave
Background (CMB) experiments will be able to set on the metal enrichment
history of the Inter Galactic Medium at the epoch of reionisation. We forecast
the signal to noise ratio for the detection of the signal introduced in the CMB
by resonant scattering off metals at the end of the Dark Ages. We take into
account systematics associated to inter-channel calibration, PSF reconstruction
errors and innacurate foreground removal. We develop an algorithm to optimally
extract the signal generated by metals during reionisation and to remove
accurately the contamination due to the thermal Sunyaev-Zel'dovich effect.
Although demanding levels of foreground characterisation and control of
systematics are required, they are very distinct from those encountered in
HI-21cm studies and CMB polarization, and this fact encourages the study of
resonant scattering off metals as an alternative way of conducting tomography
of the reionisation epoch. An ACT-like experiment with optimistic assumtions on
systematic effects, and looking at clean regions of the sky, can detect changes
of 3%-12% (95% c.l.) of the OIII abundance (with respect its solar value) in
the redshift range $z\in$ [12,22], for reionization redshift $z_{\rm re}>10$.
However, for $z_{\rm re} <10$, it can only set upper limits on NII abundance
increments of $\sim$ 60% its solar value in the redshift range $z\in$ [5.5,9],
(95% c.l.). These constraints assume that inter-channel calibration is accurate
down to one part in $10^{4}$, which constitutes the most critical technical
requirement of this method, but still achievable with current technology.
