Basically, each tile of those asteroid has a chance to contain a random ressources, with varying probabilities. Not too common, but spawn in belts, can allow to get some rarer ores.
Then, we could have mixed ore asteroids, wich can have two or more ressources in them. Scordite asteroids, could contain a lot of steel with a bit of sulfur, Pyroxeres could contain sulfur with copper, and Veldspar contain Steel and tri-steel (You can tell I was inspired with the names because I stole them from eve online)