 Although the Oppenheimers (De Beers) invested in CAST and its subsidiaries from the 1920s and had boardroom representation, CAST was independent of the Oppenheimer empire.SLST corporation which had exclusive mineral mining rights in Sierra Leone beginning in 1935 was scheduled to last for 99 years.In 1955, the SLST abandoned mineral mining and settled on mining the Yengema and Tongo Fields.