Answer:
Some are and some are not. Gold bars are generally produced as cast bars or minted bars.
Cast bars are large bars used for industiral purposes. They are generally not 24 karat.
Minted bars, such as Credit Suisse gold bars, are smaller bars used for investment purposes. They are generally 24 karat.
In both cases, the purity of the bar will be printed on it in the millesimal scale. Anything above 9990 is 24 karat fine gold.