Cook County officials tell us Dontay Banks Jr. -- DThang's government name -- was found deceased on the 7900 block of S. Loomis Blvd. in Chicago, and has been assigned a case number. An autopsy still has to be done to figure out the cause and manner of death.

Reports say DThang was shot in the head outside of a nightclub and died on the spot ... citing alleged witnesses who say they saw the shooting go down.

DThang was a rapper himself ... but wasn't nearly as big as Durk is, who's blowing up right now. Still, he's got a string of songs out -- although it doesn't look like they ever collab'd.

This is just the latest tragedy to strike Durk's inner circle -- he's lost a handful of friends and loved ones over the years ... including, famously, the death of his labelmate King Von ... who was shot and killed last year. A producer he worked closely with named Turn Me Up Josh also died just last week ... something Durk himself commemorated with a post.

Durk hasn't commented on his brother's passing yet, but condolences have been pouring in all over social media. DThang was 32.