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- Ultra basic: Igneous rocks containing less than 35% silica.
- Umpire assay:Verification of assay results by another assayer
- Unpatented claim: Usual lode, placer or mill site claim located under the mining laws.
- Up cast: A vertical raise to the ground surface form an underground mine.
- Upper lead: Pay gravel of pay streak in a gravel deposit that lies in strata well above bedrock.
- Upper Plate: The hanging wall of a fault.