  We present a detailed spectral study (photoionization modelling and
variability) of the "Big Dipper" 4U 1624-490 based on a \chandra-High Energy
Transmission Gratings Spectrometer (HETGS) observation over the $\sim76$ ks
binary orbit of 4U 1624-490. While the continuum spectrum can be modeled using
a blackbody plus power-law, a slightly better fit is obtained using a single
$\Gamma=2.25$ power-law partially (71%) covered by a local absorber of column
density $N_{\rm H, Local}=8.1_{-0.6}^{+0.7}\times 10^{22} \rm cm^{-2}$. The
data show a possible quasi-sinusoidal modulation with period $43_{-9}^{+13}$ ks
that might be due to changes in local obscuration. Photoionization modeling
with the {\sc xstar} code and variability studies of the observed strong
\ion{Fe}{25} and \ion{Fe}{26} absorption lines point to a two-temperature
plasma for their origin: a highly ionized component of ionization parameter
$\xi_{\rm hot} \approx 10^{4.3} {\rm ergs cm s^{-1}}$ ($T\sim 3.0\times 10^{6}$
K) associated with an extended accretion disk corona of radius $R
\sim3\times10^{10}$ cm, and a less ionized more variable component of $\xi
\approx 10^{3.4} {\rm ergs cm s^{-1}}$ ($T\sim 1.0\times 10^{6}$ K) and $\rm
\xi \approx 10^{3.1} ergs cm s^{-1}$ ($T\sim 0.9\times 10^{6}$ K) coincident
with the accretion disk rim. We use this, with the observed \ion{Fe}{25} and
\ion{Fe}{26} absorption line variations (in wavelength, strength, and width) to
construct a viewing geometry that is mapped to changes in plasma conditions
over the 4U 1624-490 orbital period.
