* Sanford unfaithful to wife in affair with "dear friend"

* Quits as head of Republican Governors Association

* Miss. Governor Haley Barbour takes over as chairman (Adds publication of e-mails from governor, paragraph 18)

By Matthew Bigg

ATLANTA, June 24 (Reuters) - South Carolina Governor Mark Sanford tearfully admitted on Wednesday he had been unfaithful to his wife, likely ending any chance he might be a Republican contender for the U.S. presidency in 2012.

Sanford resigned as chairman of the Republican Governors' Association and was replaced by Mississippi Governor Haley Barbour, another possible 2012 candidate.

"Any aspirations for 2012, if he had any, are certainly out of the question," said Robert Oldendick, a political scientist at the University of South Carolina.

Sanford's confession at a tumultuous news conference ended days of speculation over his whereabouts. After he disappeared last week, his staff first said he was hiking on the Appalachian Trail. It later emerged he had traveled to Argentina to be with his lover, leaving his family over Father's Day weekend.

"I spent the last five days of my life crying in Argentina," he said.