BlackRock World Mining narrowed its discount to net assets in November and thinks mining stocks at these levels are a good buying opportunity, especially with further takeover activity likely.The trust's discount to net asset value (NAV) is down to 15% - the lowest in three months - from 17.2% in October. Managing director and portfolio manager Evy Hambro says the medium to long term outlook for the mining sector appears "robust", with miners struggling to meet an increase in demand from emerging markets."As concerns over a double dip recession ease and commodity prices remain robust, our outlook for the market is positive," Hambro said in his monthly portfolio update."The sector continues to trade near historically low valuations on a variety of metrics, suggesting current levels could represent an attractive entry point to the market for investors. At these levels, we would not be surprised to see further M&A activity within the mining sector."The trust grew net assets by 3.9% in November and the shares continued their rally, adding almost 7% over the month to 711.5p.