FTSE 100 gives away post-Fed gains as euro zone worries resurface

* German export plunge latest sign of euro zone weakness

* British finance minister points to risk of crisis

* U.S. Fed minutes boost commodity prices and stocks

* Nomura downgrade depresses Vodafone (Recasts, adds quote, details)

By Alistair Smout

LONDON, Oct 9 (Reuters) - Britain's top equity index gave away early gains on Thursday in a broad-based sell-off as investors fretted over signs of serious weakening in the euro zone economy.

The falls offset strength in the mining sector, which was boosted after minutes from last month's Federal Reserve meeting suggested the U.S. central bank might keep interest rates lower for longer than expected, lifting metals prices.

The latest in a string of disappointing data releases from the euro zone's largest economy showed German exports plunged in August. Hours later, a group of leading economic institutes joined the International Monetary Fund in slashing forecasts for German growth.

British finance minister George Osborne said on Thursday the euro zone risks slipping back into crisis after recent weak economic data, adding that it was already affecting the British economy, which has performed strongly so far this year.

"It's economic factors that are hitting the market today," said Manoj Ladwa, head of trading at TJM Partners.

"After the IMF downgraded its forecasts a few days ago, with today's news the evidence is now mounting that Europe in particular is looking weak."

Mobile telecommunications (BSE: MOBILTEL.BO - news) group Vodafone was among top fallers, down 2 percent after investment bank Nomura cut its rating on the stock to "reduce" from "neutral", arguing Vodafone could be affected by greater competition within its sector.

The blue-chip FTSE 100 index was down 0.4 percent, or 27.92 points, at 6,455.32 by 1229 GMT, having given away strong early gains of as much as 0.8 percent that were prompted by the minutes of the Fed's Sept. 16-17 policy meeting.

They showed the U.S. central bank is concerned that the rising dollar could slow a needed rebound in inflation, signalling it will keep interest rates ultra-low and pursue its accommodative policy for longer.

The FTSE 350 Mining Index, which has fallen nearly 6 percent this year on worries over lower iron prices and a possible Chinese economic slowdown, outperformed to rise 1.9 percent as the Fed's tone lifted metals prices.

Leading the miners higher were precious metals specialists Randgold Resources and Fresnillo (Other OTC: FNLPF - news) which both rose more than 5 percent.

Traders said the Fed's concern over a strong dollar could boost appetite for "safe-haven" assets, such as gold.

Randgold was up 7.3 percent, also benefiting from an upgrade by Numis Securities to "buy" from "add".

"U.S. dollar strength has pummelled all the commodities," CMC Markets market analyst Jasper Lawler said. "People who have confidence in gold have come in looking for bargains, and the Fed minutes have really helped the cause" (Additional reporting by Sudip Kar-Gupta, Liisa Tuhkanen and Alasdair Pal in London; Editing by Catherine Evans)