Naruto is Wind and Sasuke is Lightning if I'm not wrong. What would Sakura's Chakra element be? Was it mentioned in the series before?
From what I can tell, it was never announced officially. While she is shown to have some main skills--medical ninjutsu and strong resistance to genjutsu--her chakra element isn't mentioned.
Everything that I have found (NarutoHurricane.com, Naruto Wikia, etc.) either lists it as a ? or doesn't list it at all. Naruto Wikia lists her chakra transformations (Earth Release, Water Release, Yin Release and Yang Release) but doesn't give her element.
According to the Naruto Fourth Databook: "Although she doesn't usually use standard ninjutsu, Sakura's natures include Earth, Water, Yin, and Yang Release."