no one knows but i think its wind and he trained after jiriya and it said in the series that he tried to fuse the element with the rasengan so i think its wind like naruto
i would guess lightning since he probably uses that element in Hiraishin no Jutsu "Flying Thunder God Jutsu".
The name Hiraishin is also the phonic of the Japanese word 避雷針, which means "lightning rod"; this may suggest the reasoning as to why the technique is restricted towards a seal, because lightning is attracted to lightning rods.
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