El Salvador, 90% of which is of volcanic origin geologically, is less well endowed with mineral resources than other Central American country. Mineral production contributed less than 1% to the GDP. The country has produced gold and silver in the past. In 2001, output for both was zero. Recent activity has been limited to exploration, including at El Dorado, whose indicated resources were estimated at 23,100 kg of gold and 172,000 kg of silver. A 1,050 m diamond drill program on the Aldea Zapote project was also completed in 2001. Industrial minerals, especially limestone mined for domestic cement plants, were the primary commodities of the industry. Marine salt output rose from 89,000 tons in 1998 to 710,000 in 2001. Gypsum, copper, lead, zinc, and sands containing titanium and ilmenite were also found in El Salvador.