EXCLUSIVE

Ozzy Osbourne's right hand man in Black Sabbath -- bassist Geezer Butler -- got busted for a bar fight, and spent a night in jail.

Sources tell us Butler was in a bar at the Furnace Creek Ranch in Death Valley Tuesday ... when the brawl broke out -- and apparently the ol' Geezer's still got some fight left in him.

Inyo County Sheriff's deputies, clearly believing the Butler did it ... arrested the 65-year-old for battery, vandalism, and disorderly conduct.

Butler famously wrote most of Sabbath's lyrics ... while Ozzy got all the glory as the band's frontman.

Has he lost his mind? Can he see or is he blind?

Who knows? But cops tell us Butler was released from jail Wednesday morning after drying out.