http://en.wikipedia.org/wiki/Heavy_metal_music
Being a fledgling music junkie at that time, the term Heavy Metal was definitely used in regard to Sabbath and Zeppelin. I seemed to remember Lester Bangs was the main purveyor of the term in Creem and Rolling Stone magazine, and wikipedia confirms that.
Surprising to read that Steppenwolf's "
Born to Be Wild" first used the phrase in song lyrics.
("... heavy metal thunder")