Syria's mineral resources were not extensive, but deposits of iron, petroleum, and phosphate have been exploited in recent years; Syria was the fifth-leading exporter of phosphate rock. Petroleum was the leading industry in 2002—the oil and gas industry accounted for nearly three-quarters of Syria's export earnings and more than one-third of its GDP—and phosphate rock mining was its sixth-leading industry. Production of phosphate rock (gross weight) was 2 million tons in 2001, down from 2.5 in 1998; 74% was exported, most to Europe. Other mineral deposits included asphalt, salt, chromite, and marble. Marble and salt were mined in commercial quantities, although no marble was produced in 2001. Output of salt totaled 106,000 tons. The country also produced hydraulic cement, refractory-grade dolomite, natural gas, natural gas liquids, gravel and crushed rock, gypsum, nitrogen, phosphatic fertilizers, phosphoric acid, construction and industrial sand, steel, dimension stone, sulfur, and volcanic tuff. No metal was mined in 2001, and no marble was produced. Deposits of silica sand in al-Qaristyn had resources of 150 million tons. The mineral industry was owned and controlled by the government. In 2001, the government announced its intentions to open the mineral industry to local and foreign private investors. The rapid expansion of the construction sector in the near future was expected to increase Syria's demand for cement, gypsum, limestone, gravel, sand, and steel.