Comex copper contracts closed up 3.6 cents to $3.713 a pound as traders kept a watchful eye on labor negotiations at Chile's massive Escondida copper mine due to concerns that supplies of the red metal may be disrupted.

In the other precious metals, Nymex October platinum ended the session up $3.5 at $1,267.5 an ounce, while palladium traded up 95 cents to $335 an ounce by the close.

Shares of palladium miner Stillwater Mining ( SWC) were trading up almost 1% at $11.39. Meanwhile, stock of North American Palladium ( PAL) sank slightly despite the strong metal price and improved operating performance by the company. The firm saw a 20% increase in second-quarter palladium output, according to a company release made slightly before the opening bell.

"They are coming from a poor base," says Kevin Walkush, equity analyst with Morningstar. "They had nowhere to go but up," and also notes that NAP's management needs to deliver high-quality earnings on a consistent basis before he'd recommend the stock over Stillwater, his preferred pick of the two. Until then he remains in a wait and see mode.

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