According to Josina Anderson of ESPN, Brown approached Mayock to complain about a recent fine before calling him a "cracker" and using a barrage of "cuss words" directed at the first-year GM. A team source said Mayock "walked away because he saw it escalate" while other players held Brown back.
ESPN's Adam Schefter reported the team plans to suspend the star receiver.
Ian Rapoport of NFL Network added that the confrontation didn't get physical, but Brown threatened to hit Mayock in the face before punting a football.
This incident came after Brown complained about a $13,950 fine on Instagram on Wednesday for missing a team walkthrough Aug. 22 ahead of a preseason game against the Green Bay Packers in Winnipeg, Manitoba:
The receiver missed a practice Aug. 18 after filing a grievance with the NFL over a helmet issue, leading to a $40,000 team fine. Mayock was unhappy with the situation when discussing it with the media at the time.
"From our perspective, it's time for him to be all-in or all-out," he said.
Brown had also missed time at training camp because of a foot injury caused by misuse of a cryotherapy machine.
Though the Raiders haven't announced any discipline, the seven-time Pro Bowler was not at practice Thursday.
If the organization decides to suspend him for conduct detrimental to the team, it could release him without paying the $29.125 million guaranteed in his contract and a $1 million signing bonus, according to Mike Tannenbaum of ESPN.