Uncertainty about the global economy was weighing heavily on the mining sectors on Tuesday, as concerns over the demand outlook prompted investors to scale back their appetite for risk.Fears of a hard landing in China have dampened sentiment today after JPMorgan Chase & Co cut its economic growth forecasts for the country, while a fund manager survey by Bank of America Merrill Lynch showed increasing caution by investors surrounding the region.Meanwhile, Barclays was providing further downwards pressure on stocks after cutting the European mining sector from 'neutral' to 'negative', saying that the "building blocks that made up the supercycle are gently being removed".In other news, stock markets are beginning to contemplate a possible 'exit strategy' from Federal Reserve stimulus after a Wall Street Journal article this weekend suggested that the central bank is starting to map out plans about how to scale back its $85bn-a-month asset purchase programme.Investors were also digesting the news of a whopping $1.6bn write-down on the European division of Indian steelmaking giant Tata Steel.EVRAZ, Fresnillo, Vedanta, Anglo American and Glencore Xstrata were among the worst performers on the FTSE 100 on Tuesday.Along with mining stocks, copper prices were firmly lower today on worries over demand and as stockpiles (as measured by the London Metal Exchange) increased the most in four weeks.Shares in Lonmin, the world's third largest platinum producer, took a hit today after the group suspended all work at its Marikana mine following a refusal by employees to go underground, despite arriving at work. Elsewhere, AIM-listed Minera IRL plunged after the Latin America-focused gold miner reporting a 13% fall in sales and production in the first quarter.Top performing sectors so far todayGas, Water & Multiutilities 6,195.33 +1.99%Aerospace and Defence 5,063.59 +1.53%Chemicals 9,492.38 +1.30%Household Goods & Home Construction 10,517.07 +1.27%Automobiles & Parts 6,810.79 +1.22%Bottom performing sectors so far todayIndustrial Metals & Mining 1,810.85 -3.60%Mining 16,851.10 -1.38%Personal Goods 22,036.83 -1.01%Health Care Equipment & Services 4,263.01 -0.92%Mobile Telecommunications 4,484.70 -0.75%BC