* Union accepts mediation for Escondida strike
* Dollar rebounds on Yellen rate hike comments
* Protest in north China over building of aluminium plant
* Data indicates Chinese credit growth remains strong (Updates with closing prices)
By Eric Onstad
LONDON, Feb 14 (Reuters) - Copper slipped into the red on Tuesday on hopes strike talks would restart at the world's biggest copper mine in Chile while aluminium touched its highest in 21 months on renewed concerns about potential closures of Chinese smelters to cut pollution.
Also pressuring base metals was a stronger dollar, which rebounded after Federal Reserve Chair Janet Yellen said it would be unwise to wait too long to raise U.S. interest rates.
Benchmark copper CMCU3 on the London Metal Exchange closed down 1.4 percent at $6,021, pulling back from a session high of $6,190 and after hitting its highest since May 2015 on Monday at $6,204.
Striking workers at Chile's massive Escondida copper mine have accepted a government invitation to try to resume dialogue with mine operator BHP Billiton BHP.AX BLT.L as the strike entered its sixth day on Tuesday. said the news hit a market worried that funds who have taken bullish positions might be ready to sell on any signal that the strike may be concluded.
"The markets are very nervous and when it looked like someone sold a large amount, that just swept the market down. Any move down will attract some panic sellers," a trader in London said.
There was no guarantee, however, that the two sides would make progress in the mediation.
"There's quite a wide gap between what workers are wanting and what BHP is offering," Warren Patterson, commodities strategist at ING in Amsterdam, said.
Offering some support to copper were persistent supply issues in Indonesia, where an export ban remained in place.
Freeport-McMoRan Inc FCX.N has halted production of concentrate at Grasberg in Indonesia, the world's second-largest copper mine, in a long-running dispute over investment guarantees. CMAL3 bucked the weaker trend and ended up 0.9 percent at $1,887, paring gains after touching a session peak of $1,907, a high since May 13, 2015.
In a sign of growing public anger about pollution, hundreds of residents in a northeastern Chinese city protested against the building of an aluminium processing plant. came a day after Reuters reported a draft policy document showed that the world's biggest metals consumer was considering forcing steel and aluminium producers to cut more output as Beijing intensifies its war on smog. the proposed measures are applied this should impact a significant proportion of current China steel and aluminium output," Alastair Munro at broker Marex Spectron said in a note.
Elsewhere, lead CMPB3 dropped 2.7 percent to close at $2,355.50, zinc CMZN3 dipped 0.5 percent to $2,903, nickel CMNI3 added 0.4 percent to $10,770 and tin CMSN3 fell 0.4 percent to $19,925.
PRICES
Three month LME copper
CMCU3
Most active ShFE copper
SCFcv1
Three month LME aluminium
CMAL3
Most active ShFE aluminium
SAFcv1
Three month LME zinc
CMZN3
Most active ShFE zinc
SZNcv1
Three month LME lead
CMPB3
Most active ShFE lead
SPBcv1
Three month LME nickel
CMNI3
Most active ShFE nickel
SNIcv1
Three month LME tin
CMSN3
Most active ShFE tin
SSNcv1