Petroleum and natural gas dominated Oman's economy. Large deposits of copper, which supports a leading industry, have been discovered northwest of Muscat. The first evidence of copper production dated at Wadi Jizzi indicated that Oman has been a mineral producer for more than 5,000 years. Deposits at the Sohar mine were nearly exhausted by 1991; other deposits were discovered at Hajl al-Safi and at Rakah, in Ibri. More than half of Oman's copper production was taken from the Lasail Mine, with much of the remaining output from the Aarja surface mine. The Ministry of Petroleum and Minerals has reported proven copper reserves at 8 tons and proven chromite ore reserves at 1.6 tons. Gold was mined from the Rakah deposit, near Yanqui; the Rakah plant processed 100,000 tons per year of ore. In 2000, National Mining Co. of Oman explored for copper, gold, and silver near Sohar, and the Metal Mining Agency of Japan completed exploration for copper and gold in the South Batinaha area. Construction of an industrial port at Sohar to service the mineral industry continued in 2000, and a number of mineral-processing projects were under consideration, including an aluminum smelter, a number of ammonia and urea plants, a ferrochrome project, a methanol plant, and a unit to recover sulfur. Output of chromite in 2000 was 15,110 tons, down from 28,684 in 1998; sand and gravel, 22.45 million tons, up from 9.8 in 1998; gold, 604 kg; silver, 306 kg, up from 95 in 1998; and marble, 147,686 tons. Oman in 2000 also produced, and had large deposits of, limestone, manganese, gypsum, asbestos, and dimension stone.