So, the local aliens deserted the city because they were running short on minerals?
That might be so.
Scientist Enkuush has completed research on the third level of that alien tech-tree that we discovered in a research lab on El Dorado, Advanced Lasers. Possibly we can turn this into an effective mid-range PD weapon at some point.
We have found ANOTHER abandoned research lab, and another free tech: Enhanced Radiation Warheads for missiles. This is a planetary bombardment tech, only useful against living opponents (and it makes a mess of the world).
EDIT: A comparison between the Advanced and normal Lasers at similar calibres shows that Advanced Lasers give +20% damage and +20% range, for the same size.