* FTSE 100 down 0.3 pct at close
* Standard Life at 15-month high on Aberdeen merger
* Ultra Electronics Holding top STOXX gainer after results
* Miners drag index lower (Updates prices at close, adds detail)
By Kit Rees and Helen Reid
LONDON, March 6 (Reuters) - Britain's blue-chip FTSE 100 index edged down on Monday as weakness in mining stocks outweighed the positive impact of a potential 11 billion pound merger between Scottish fund managers Standard Life and Aberdeen Asset Management.
The index was down 0.3 percent at its close, in line with losses in the broader European index.
Standard Life was the top gainer, jumping 5.7 percent and holding at a 15-month high on news of a merger with mid-cap peer Aberdeen Asset Management. Aberdeen shares were up 4.2 percent.
The funds, seeking synergies and cost savings to fight back against cheap, index-tracking funds, said they expected to save 200 million pounds through the merger.
"The deal makes perfect sense as a defensive play. The explosive growth in passive investing trends has heaped pressure on active managers like Aberdeen and Standard Life and consolidation had to be on the cards," said Neil Wilson of ETX Capital.
Shore Capital warned of "grave concerns" over the proposed board structure of the combined company, particularly the appointment of co-CEOs.
Glencore, Anglo American, Antofagasta and Rio Tinto were all top fallers on the blue-chip index, down 2.1 to 3.5 percent as copper prices slipped due to risk-off sentiment in broader markets.
Strong gains from Aberdeen and Ultra Electronics Holding helped the mid-cap index outperform, ending flat.
Ultra Electronics Holding hit an all-time high, up 4.8 percent, the top mid-cap gainer and among the best-performing on the STOXX 600, after it posted an increase in full-year profit, and order intake up 22 percent.
Acacia Mining was the top mid-cap faller, down 10 percent after a 13.5 percent slide on Friday when the Tanzanian Ministry of Energy and Minerals issued a surprise ban on gold/copper concentrate exports.
Acacia operates three gold mines in Tanzania, accounting for 30 percent of its revenues.
The miner said in January it was in preliminary merger discussions with Canada's Endeavour Mining, but analysts said the export ban could jeopardise a deal.
"With no clarity over a type or timing of a resolution, we fear Acacia shares will be suppressed," said Jefferies analysts, downgrading the stock from 'buy' to 'hold'.
"In our opinion, with a spike in jurisdictional risk in Tanzania, merger discussions with Endeavour are at the very least going to be delayed," said Jefferies. (Reporting by Kit Rees and Helen Reid; Editing by Hugh Lawson)