The Basalt Deltas should generate under some mountains

I am seeing Many people say there isn't enough variety in the underground biomes so I suggest a variant of the Basalt delta that uses deep slate instead of Blackstone and only can generate under mountain ranges. it would suggest that those mountains were formed by volcanic activity instead of tectonic activity which would be a neat little tidbit for children learning geology, help solve the issue of not enough diversity, and lets be honest hallowing out the inside of a mountain having a basalt delta at the bottom and a castle western air temple style in the middle would look so cool with a shader pack or rtx on bedrock.