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Julio Cesar Chavez Jr. was deported from the United States to Mexico ... but the star boxer isn't roaming free in his home country -- instead, he's cooling his heels in a jail cell after American authorities turned him over to officials south of the border.

Chavez Jr. was deported from an undisclosed location in America on August 18 ... after 46 days in U.S. custody. Shortly before noon, he was transferred to a high-security prison in Hermosillo, Sonora.

JCC Jr. remains in Mexican custody.

Of course, the 39-year-old former WBC middleweight boxing champ and son of legendary pugilist, Julio Cesar Chavez Sr. , was initially arrested in Southern California on July 2 by Immigration, Customs, and Enforcement agents after Mexico issued a warrant for his arrest. U.S. Department of

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