GregTech disables all ore generation, and replaces them with its own ore generation patterns. There are two types of ore generation. Small Ores, and Mix Veins.
Small ores generate as single blocks. These ores are intended to get started and acquire initial resources. When broken they drop either a impure pile of ore dust, Crushed Ore or sometimes a gem. They may also drop dust or dirty dust of the stone they were found in. You can’t use Silk touch on small ores, but you can use fortune.
Mix Veins comes in four parts. Primary, Secondary, Inbetween and Sporadic. The upper 2-4 levels of a vein will almost all be primary ore. The lower 2-4 levels will almost be secondary ore, and the middle 2-4 levels will be mostly inbetween ore.
Sporadic ore appears randomly interspersed in the vein. In total a vein will equal portions of primary and secondary ore, and a little less of inbetween ores, and much less of sporadic ore.
Ore vein generations works like this. First at the center of every 3×3 chunk, a random ore mix is chosen. If that ore mix cannot generate in its current dimension, the generator will try again. If the chosen ore vein is allowed, a random height will be chosen within the mix of veins Min/Max height values. In some areas you will find none of the mix veins, but there will be small ores.
A full list of the ores, and in which Min/Max Height they spawn in can be seen here. It is sorted by the highest Max Height, to the lowest: