EMERGING MARKETS-Brazil stocks erase early gains as CSN weighs

* Brazil Bovespa down 0.46 pct, Mexico IPC falls 0.16 pct SAO PAULO, Jan 21 (Reuters) - Brazilian stocks gave up early gains on Tuesday, led by shares of steelmaker CSN, on concerns the company could be forced to disburse up to $3 billion to buy out partners in its Namisa iron ore unit. Mexico's IPC index fell for the third session in four, while Chile's bourse edged lower. After initial gains of as much as 0.8 percent, Brazil's benchmark Bovespa stock index was down 0.46 percent b