Sept. 6, 1961 - Richard Mason, 30-year-old British explorer and member of the Royal Geographic Society, was slain by native tribesmen while he was on an expedition in central Brazil, it was reported today. Mr. Mason, two other British explorers, and three Brazilian members of the expedition had been surveying an area around the Cachimbo range since the middle of May. Mr. A few days ago, Mr. Mason had gone off by himself in search of food and supplies when an unknown indigenous hunting party laid an ambush and killed him with 6-foot-long bamboo poison arrows and clubs. His body was found lying on the main supply trail a few kilometers from the explorers' camp. He had been hit by eight arrows, and his skull and thigh were smashed by club blows.