DURING the commodity “supercycle”, prices largely marched up and down in unison, fuelled by the strength (or weakness) of demand in China. Since last year commodities have again been on a tear, but for more idiosyncratic reasons. In the case of copper, strikes and supply disruptions in two of the world’s largest mines have helped push prices this week to their highest level in 20 months. This fits into a narrative of longer-term potential supply shortages that has investors licking their lips over prospects for the red metal.

A strike that began on February 9th at Escondida in Chile, the world’s largest copper mine, has been compounded by a dispute between operators of Grasberg, another huge copper mine, located in the Indonesian province of Papua, and the government. That led to a halt in copper-concentrate production there, too, on February 10th. The two account for 9% of mined copper supply.

Robert Edwards of CRU, a consultancy, says a one-month shutdown at both mines would remove about 140,000 tonnes, or 0.7% of the world’s output this year. He adds that labour contracts amounting to 14% of production are up for renewal this year, raising the spectre of further strikes. The possibility that disruptions in 2017 could increase from 2016, at a time of robust Chinese demand, has pushed up prices recently (see chart).

In Chile, BHP Billiton, operator of Escondida, has clashed with the workers’ union over benefits. This week, both sides were toing and froing over whether to take part in informal mediation talks convened by the government. The union wants to preserve benefits from the previous labour contract and extend them to new workers. BHP is resisting.

Juan Carlos Guajardo, a Chilean analyst, says the stakes are raised by the introduction of a new labour code in April that will dismantle curbs on the power of unions and protect existing benefits. Both sides want the best possible deal before the new law takes effect. The union also wants compensation for the hardships of the past few years of falling prices, while BHP seeks to bring the labour productivity of the mine up to rich-world standards.

The Indonesian stand-off could be just as fractious. On January 12th the government said that if Freeport-McMoRan, an American firm that operates Grasberg, wanted to keep an exemption allowing it to export copper concentrate despite a 2014 ban on ore exports, it would have to convert its decades-old “contract of work” into a new mining licence. Freeport says it will do so as soon as Indonesia attaches to the licence the same guarantees of fiscal and legal stability that the current contract affords. The two sides remain at loggerheads, so Freeport has started sending Grasberg workers home.

Analysts believe that the government’s pressing need for tax revenues means it may seek a compromise. But damage has already been done. Rio Tinto, Freeport’s partner in Grasberg, says it is reconsidering the option to increase its interest in 2021.

In both Chile and Indonesia, swift resolutions are as likely as long-term disruptions. But in the meantime, they bolster the case of those who believe the red metal has a stellar future. On February 16th McKinsey Global Institute, a consultancy, joined the fray, singling out copper as a commodity for which demand could grow strongly over the next two decades, because of Chinese demand and its importance to electric vehicles and wind- and solar-energy units. It also predicted that supply would be constrained by the depletion of copper ores after 2025. Copper bulls will be snorting with excitement.