The dollar fell sharply against most major foreign currencies yesterday as the 2.7 percent drop in the September index of leading U.S. economic indicators was steeper than European traders had expected. Gold fell in Europe but gained in New York.

New York dealers said trading was thin with uncertainty about the economy and interest rates making traders reluctant to take positions.

In Zurich gold fell $7 an ounce to $424.50. In London it eased to $424 from $431.

In New York gold rose $5.50 to $429, and the New York Commodity Exchange settled it at $428.90, up $4.90. Silver gained 27 cents to $9.27 and settled at $9.27 on the Comex, also a 27-cent advance.

The pound rose slightly in London at $1.8320 against $1.8255 and rose to $1.85 in late New York trading.

In Tokyo, the dollar edged up to 235.25 yen from 235.05 yen Wednesday. It opened at 235 and traded between 234.80 and 235.50.

OCCIDENTAL VENTURE: Italy's state energy group Ente Nazionale Idrocarburi (ENI) and Occidental Petroleum Corp. signed definitive agreements yesterday to a form a billion-dollar joint venture in coal and chemicals, the two companies announced.

The agreements for the venture, called ENOXY, were signed at ENI's Rome headquarters by Armand Hammer, Occidental chairman, and Alberto Grandi, chairman of ENI. A preliminary agreement was signed in July.

The venture involves pooling 60 of ENI's chemical plants in Italy with four of Occidental's coal mines in West Virginia and Kentucky. Business operations will begin Jan. 1.

IATA PRESIDENT: The president of Swissair, Armin Baltensweiler, yesterday was elected president of the International Air Transport Association at the closing of its 37th general assembly.

Baltensweiler succeeds Air France President Pierre Giraudet.

The next IATA congress will be held in Geneva Nov. 8 to 11, 1982.