There doesn't seem to be much about it. the American heritage says it comes from the 1500s:
A very kind and good nature, as in Bill is very generous; he has a heart of gold. This expression alludes to gold in the sense of "something valued for its goodness." [Late 1500s]
Source: The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company.