Mining and energy accounted for 1% of Burundi's GDP in 1999. The country has been known to produce columbium (niobium)-tantalum ore, gold, kaolin (china clay), tin, and tungsten ore, mostly for export, and limestone, peat, sand, and gravel for domestic consumption. Burundi had significant deposits of feldspar, kaolin, nickel, phosphate, platinum-group metals, quartzite, rare-earth metals, vanadium, and limestone for cement. There were gold deposits at Mabayi, Muyinga, Cankuzo, and Tora-Ruzibazi, where artisanal mining took place. After waning in the early 1990s, gold production rose to 1,000 kg in 1994 and 2,200 kg in 1996, then dropped to 1,500 in 1997–2000. The government has tried to transfer technical skills to artisanal miners, to raise productivity and increase state revenues. The Burundi Mining Corp., a government–private venture, was exploring the possibility of producing gold on a commercial basis at Muyinga, where resources were estimated at 60 tons of gold. Deposits of cassiterite, columbite-tantalite, and wolframite associated with pegamatites were found in Kayanza and Kirundo provinces. Nickel reserves, found in 1974, were estimated at 370 million tons (3%–5% of the world's total); high transportation costs, low world market prices, and political instability have delayed their exploitation. Since 1993, foreign investment and development of Burundi's resources have been hindered by civil unrest, social strife, and economic sanctions imposed by regional states; the economy contracted by 23% in the period 1993–96. Although the sanctions were lifted in 1999, internal strife continued to hurt the economy. In 2000, Burundi joined with 19 other nations to form Africa's first free-trade area, and the World Bank and other international donors pledged to give $440 million in reconstruction aid to Burundi. In 2000, production of columbite-tantalite (gross weight) was 42,000 kg, and of peat, 14,700 tons, up from 10,300 in 1998. Kaolin, lime, and tin were also mined in 2000, in small amounts.