Miner discovery? (4)

I think it could be either:

Which length description are you looking for?:

vein

(Other definitions for vein that I've seen before include "Irregular streak of colour" , "Vessel carrying blood towards the heart" , "Leaf rib; ore layer in rock" , "Deposit of mineral" , "Tube that carries blood to the heart" .)

I've seen this clue in The New York Times.

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