bonanza

nouns

an especially rich vein of precious ore
bonanza
is a type of (narrower)
a layer of ore between layers of rock
mineral vein, vein
a sudden happening that brings good fortune (as a sudden opportunity to make money)
"the demand for testing has created a boom for those unregulated laboratories where boxes of specimen jars are processed like an assembly line"
bonanza, boom, bunce, godsend, gold rush, gravy, manna from heaven, windfall