The dollar gained in European trading yesterday after major European countries agreed in principle to support the United States with sanctions against Iran. Gold moved above the $500 an ounce mark.

In Zurich gold closed $5 higher at $502.50 an ounce. In London it rose to $505.50 an ounce from Monday's $497.50. In New York gold closed at $502 an ounce, up from $490.50 Monday. The COMEX settlement price was $501.80 an ounce, compared with $488.30.

Silver rose 30 cents to $13.80 an ounce on the cash market from $13.50 Monday, and silver speculators on the COMEX settled for $13.63 compared with $13.40.

European closing prices with late New York quotes in parenthesis:

Frankfurt, 1.8595 marks up from 1.8518 Monday (1.86); Paris, 4.32375 francs, up from 4.30375 (4.3250); Zurich, 1.7380 Swiss francs up from 1.7255 (1.7380); Brussels, 30.605 Belgian francs, up from 30.435 (29.76); Amsterdam, 2.04 builders, up from 2.035 (2.0407); Milan, 870.80 lire, up from 868.55 (870.50).

In London, however, the pound gained to $2.2375 from $2.230 and was quoted at $2.2350 in New York.