Major copper miner Freeport (FCX) said this week it may re-activate its El Abra expansion project in Chile. Company officials noted that higher copper prices are once again making new capital projects attractive.

But in zinc the story appears to be different, with one of the world’s largest producing companies saying this week it has no intention of re-starting idled capacity — despite prices recently hitting their highest level in 10 years.

That’s Glencore (GLNCF). The firm’s chief operating officer Greg Ashe told local press in Australia that the timing is not yet right for a re-start of two of the world’s most important zinc mines: Mount Isa and Lady Loretta.

COO Ashe noted that although zinc prices are at decade-highs, Glencore sees the metal as too volatile to bet on. As he elaborated: “As little as 18 months ago we saw zinc at a five-year low and today it’s at a 10-year high.”

The decision not to restart these two mines is a critical one. Glencore’s move to close the two operations in 2015 is widely being credited as one of the major factors that’s since boosted zinc prices — which doubled following the closures, from below $0.75/lb to a current $1.49/lb.

Amid that kind of appreciation, many observers have been expecting a restart of Glencore’s big mines. But COO Ashe said the environment still isn’t right, noting that the Lady Loretta mine’s next 12 months of production will be ” the highest grade, highest margin material” ever produced by the operation.

With that valuable production hanging in the balance, Ashe said Glencore “need to make sure we get it right” in terms of timing on a restart. 

That’s somewhat of a double-edged sword — meaning either that Glencore sees a fall in prices ahead, or that the company believes prices will head higher from here, adding even more value to future production. But if this major zinc supply stays in the ground over the coming months, the likelihood of the latter scenario will rise. Keep an eye on prices.