Mars has extensive iron oxide ore. Much of it on the surface. Not readily extracted, but extractable with the correct smelting process.
It has silicates, as well; this allows for glass.
It has carbon dioxide; with a solar panel farm, and a cracking solution, that's oxygen for the taking.
And that's just materials available readily at surface. Mars will have other metal resources available via mining, and other non-metal resources as well. What it lacks is free oxygen and surface water; water can be extracted from Mars' polar "ice" (most of which isn't actually water).
Mars has sufficient resources to be able to expand for a considerable time; it's general composition is thought to be similar to Earth's, though with fewer heavy elements, and having cooled sufficiently to have a solidified core (and thus a very weak magnetosphere - 1/(10^4) that of Earth).
Mars may or may not have sufficient resources for a permanent self-sustaining colony, but it does have a plentitude of resources that could make a colony viable in an interim colonial mode.