NEW YORK—Gold prices jump more than $20 Thursday after the dollar weakened and crude oil spiked—inflationary moves that boosted demand for safe, alternative investments.

Other commodities also traded higher as part of a broad rally in futures markets, with silver, copper, corn and soybeans all gaining sharply.

The dollar fell Thursday amid concerns over a possible government bailout of mortgage finance companies Freddie Mac and Fannie Mae. The 15-nation euro rose to $1.49 in morning trading in New York from $1.4768 late Wednesday.

Gold for December delivery ...