Salt print bleached in sun

A few weeks ago I submitted a salt print which had been left in full sun (well... partial UK sun!) with half of it covered intending to test my salt process and expected odd colours or anything odd. The outcome was the portion in the sun bleached.
1 comment and my own curiosity wondered what would happen to a gold toned print. This print has been gold toned and is just the same.
Any viewer with knowledge of chemistry might like to offer a suggestion as to why.