Nchanga Mines, to prevent them falling under US control. However, its main concern was to receive royalties. Ancient surface copper workings were known at Kansanshi (near Solwezi), Bwana Mkubwa and Luanshya, all on what later became known as the Copperbelt, and BSAC exploration in the 1890s indicated there were significant deposits in the area. However, they could not be commercially exploited until a railway had been built. A railway bridge across the Zambezi was constructed in 1903 and the line was continued northward, reaching Broken Hill in 1906, where the lead and zinc vanadium mine was opened, and reaching the Belgian Congo border in 1909. At that time, mining had started in Katanga, where rich copper oxide ores occurred near the surface. In Northern Rhodesia, the surface ores were of poorer quality, and copper was only worked intermittently at Bwana Mkubwa, until in 1924 rich copper sulphide ores were discovered about 100 feet below the surface. In 1922, the Southern Rhodesian voters rejected the option of inclusion in the Union of South Africa and opted for responsible government: the Northern Rhodesian settlers were not consulted. The BSAC wanted to give up responsibility for administering Northern Rhodesia, but to preserve its mining and land rights by negotiating a settlement with the British government for both parts of Rhodesia. For Northern Rhodesia, the most important provision of that agreement was that the Crown would recognise that BSAC was the owner of the mineral rights acquired under the concessions obtained from Lewanika in North Western Rhodesia Certificates of Claim issued by Harry Johnston in North Eastern Rhodesia. Under the Northern Rhodesian settlement, the company dropped its claim for reimbursement of a £l.6 million administrative deficit: in return the British Government agreed to give the BSAC half the net revenue from certain rents and land sales and recognised (or appeared to recognise) the company as the owner of Northern Rhodesia's mineral rights in perpetuity. The British Government could have bought out these rights by paying BSAC £l.6 million to meet its reimbursement claim, but declined to make the money available. This agreement was criticised then and later time by both African and European inhabitants of the territory. The elected unofficial members of the Legislative Council pressed for the royalties issue to be referred to the Privy Council, as the BSAC's title to unalienated land in Southern Rhodesia had been. Instead, for forty years up