Mining stocks were under heavy selling pressure on Friday afternoon as the sector pulls back after a decent performance over the past month.Even including today's 1.05% average drop, mining stocks have jumped around 10% over the last 30 days.Precious metals miner Fresnillo was hammered today, leading the fallers on the FTSE 100 with a 5% fall, after a downgrade by UBS from 'buy' to 'neutral' after the stock's recent strength. Since September 10th 2012, Fresnillo has gained 7% while silver prices have fallen 9%. This compares with the average fall for silver stocks (under UBS's coverage) of 2%. "We believe this outperformance was driven by increased investor appetite for high-quality assets (which FRES offers)," said analyst Chris Lichtenheldt. While Fresnillo is seen as "still a high-quality name", Lichtenheldt said that its valuation is now "full" with the shares trading at 1.26 times net asset value, 48% higher than the North American peer group.Sector peer Anglo American was also in the red after agreeing to sell its 70% interest in the Amapá iron ore mine in Brazil. The terms of the deal with Zamin Ferrous are confidential and subject to state regulatory approval, but the company said it was expected to complete in 2013. FTSE 100 stocks Randgold, Polymetal and Kazakhmys were also registering slight losses by the close of trade.One bright spot in the sector was AIM-listed Eurasia Mining after saying that its Monchetundra exploration licence for palladium mineralisation in the Kola Peninsula in Russia has been extended for another year. Shares soared by almost 30%.Top performing sectors so far todayMobile Telecommunications 3,743.19 +1.60%Fixed Line Telecommunications 2,839.11 +1.43%Tobacco 36,264.93 +1.39%Personal Goods 20,360.82 +1.16%Oil & Gas Producers 7,862.38 +1.06%Bottom performing sectors so far todayOil Equipment, Services & Distribution 24,039.85 -1.13%Mining 20,336.49 -1.05%Industrial Metals & Mining 2,953.41 -0.90%Chemicals 8,924.25 -0.81%Automobiles & Parts 5,496.79 -0.55%BC