Ireland was a leading European producer of zinc in 2001, and an important producer of lead, alumina, and peat. Interest in gold, lead, and zinc has revitalized exploration activity in the last few years, resulting in the development of new zinc and lead mines. In 2001 Ireland had its 10th consecutive year of economic growth, during which GDP grew at an average rate of 8%. Mineral production in 2001 included zinc, 225,136 tons, compared to 262,877 in 2000 and 180,951 in 1998; lead, 44.5 million tons, compared to 67 in 1996, 36.5 in 1998, and 57.8 in 2000; and alumina, 1.2 million tons. Other commercially exploited minerals were silver, barite, hydraulic cement, clays for cement, fire clay, granite, gypsum, lime, limestone, marble, sand and gravel, rock sand, silica rock, shales, slate, dolomite, diatomite, building stone, and aggregate building materials. Zinc production centered on three zinc-lead mines, the Lisheen (a joint venture of Anglo American PLC and Ivernia West PLC), the Galmoy (Arcon International Resources PLC), and the Tara (Outokumpu Oyj), three of Europe's most modern mines. Outokumpu announced that because of low zinc prices, it was closing the Tara Mine (at Navan, County Meath), the largest lead-zinc field in Europe, and putting it on care and maintenance; the Tara came into production in the late 1970s. The Galmoy Mine was producing 650,000 tons per year of ore at target grades of 11.3% zinc and 1% lead, and the Lisheen Mine, which mined its first ore in 1999 and began commercial production in 2001, initially planned to produce 160,000 tons per year of zinc concentrate, to be increased to 330,000 tons per year of zinc concentrate and 40,000 tons per year of lead in concentrate at full production; both were on the Rathdowney Trend mineralized belt, southwest of Dublin. Cambridge Mineral Resources PLC continued diamond and sapphire exploration work, identifying numerous diamond indicator minerals and recovering significant quantities of ruby and sapphire. Gold was discovered in County Mayo in 1989, with an estimated 498,000 tons of ore at 1.5 grams per ton of gold. There was a marked increase in mining exploration beginning in the early 1960s, resulting in Ireland becoming a significant source of base metals.