By Sara Sjolin, MarketWatch

LONDON (MarketWatch) -- After the strongest rally in two months, European stock markets showed broad-based losses on Tuesday, as investors stayed cautious ahead of the outcome of the U.S. Federal Reserve meeting on Wednesday.

The Stoxx Europe 600 index dropped 0.4% to 312.34 after rising 1.3% on Monday.

Shares of CGG tanked 14% after the oil-services firm said it is "operating in difficult market conditions" and fourth-quarter earnings will be impacted by project delays.

Mining firms were also lower, tracking losses for most metals prices. Shares of BHP Billiton PLC (BHP) dropped 1.1%, Anglo American PLC fell 1.7%, and Glencore Xstrata PLC (GLCNF) slipped 0.3%.

Shares of Zurich Insurance Group AG gained 1.6% after the Swiss firm appointed the chief financial officer from fellow insurer Swiss Re AG , George Quinn, as its new financial boss. Swiss Re shares fell 0.5%.

More broadly, investors were still waiting for the all-important two-day Federal Reserve meeting kicking off later Tuesday. The main question remains whether the central bank will start cutting its asset purchases at this point or leave the tapering until next year.

After a recent string of solid data and a budget deal in Washington, analysts say the possibility of tapering at the December meeting has gone up, although they are still divided as to whether the stronger economic outlook will be enough for the Fed to reduce its bond buying. The decision is out Wednesday, after the European markets close.

Another central bank, Sweden's Riksbank, was in the spotlight after it cut its main interest rate by 25 basis points to 0.75%, in an effort to stave off lower inflation.

Elsewhere in Europe, the U.K.'s FTSE 100 index dropped 0.5% to 6,490.08, while France's CAC 40 index slid 1% to 4,078.08. Germany's DAX 30 index lost 0.4% to 9,127.87.

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