GTNH Ore Veins
Ores in GTNH are spawned in singular small ores and in large veins that are 3x3 chunks in size.
Here what the world looks like with just the ores (the red lines are the borders of a 3x3 chunk area):
Each 3x3 chunk area has a chance to spawn an ore and will at a specific height range. Here's the GTNH Ore vein chart.
The best method is to mine down in a chunk until you find an ore. Once you do, mine out 2 horizontal tunnels to find the center chunk. All the other ore veins will be 3, 6, 9, 12, etc. chunks away. Ore veins do not overlap so if you find one, there will not be another below it except maybe the edge of one (although there might be small ores which can be useful).
Ore veins are made up of few different ores and are 6-8 blocks tall.
Here's a few more pics of ore veins.
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