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If you wanna change the amount of ore which genereate you have a few options.
(World Generation.cfg) http://ftb.gamepedia.com/GregTech_5/Mining_and_Processing
cassiterite {
B:Asteroid_true=true (Spawn in Asteroid's =true)
I:Density_5=5 (How dense the Ore's spawn; Smaller values = small...