User blog comment:Unownshipper/Metallurgists Wanted: Rapture's Natural Resources/@comment-3967731-20140810212653/@comment-75.36.142.216-20140811034115

The high concentrations are in sedimentary precambrian rock or by groundwater concentration near hot springs (on land). The problem with  the volcanic iron is that is spread out too much in the matrix rock and is much more difficult/expensive to concentrate for use. Not impossible, just much more difficult and requiring lots of energy and other materials to purify it and processing of massive amounts of rock with fairly low returns.

The likely  well avialable Basalt with its reasonable aluminium/magnesium/iron content might just have to be processed expensively - unfortunate though that for construction that is ALOT of the metal required.