Mining stocks were bearing the brunt of the sell-off on Wednesday as a tumble in the price of copper hammered share prices across the sector.Miners were lower across the board but those with exposure to copper, such as Kaz Minerals and Vedanta Resources, were faring the worst. Kaz and Vedanta were down 22% and 16% in afternoon trade respectively.Commodities trader and mining giant Glencore, meanwhile, fell 10% to a record-low level of 242.25p. Glencore, whose copper operations account for 45% of its operating profits, is the third-largest producer of the metal in the world.Other miners with copper activities including Anglo American, BHP Billiton, Rio Tinto and Antofagasta were also registering losses of at least 5%.Copper prices on the London Metal Exchange fell 5% to $5,552.50 a metric tonne, having earlier slumped to $5,353, its lowest since July 2009.Demand concerns were weighing heavily on copper after the World Bank lowered its forecasts for global economic expansion for this year and the next due to "sputtering" recoveries in the Eurozone and Japan, and a slowdown in China.Analyst Connor Campbell from Spreadex said copper replaced oil as the FTSE's Achilles' Heel on Wednesday. "The UK index has been punished by the metal hitting new six-year lows, as its mining stocks provided a reproduction of the kind of trading the FTSE's energy-stocks have seen in the past few weeks."Gold prices, down 0.5% at $1,228.40 an ounce, were also weaker as they pulled back from a four-month high reached the previous session.Gold miners such as Randgold Resources, Centamin and Fresnillo were all trading in the red. Top performing sectors so far todayFood & Drug Retailers 3,052.06 +0.31%Industrial Metals & Mining 1,444.81 +0.14%Industrial Transportation 2,779.42 +0.07%Fixed Line Telecommunications 4,553.36 +0.01%Bottom performing sectors so far todayMining 12,812.56 -6.50%Gas, Water & Multiutilities 5,990.22 -3.08%Oil & Gas Producers 6,620.04 -3.06%Food Producers & Processors 7,981.65 -2.90%Automobiles & Parts 8,110.75 -2.77%