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What: Brexit fears slammed shares of Freeport-McMoRan (NYSE:FCX) on Friday, sending them down more than 10% by 3:00 p.m. EDT. Those same concerns also weighed on fellow mining heavyweights BHP Billiton (NYSE:BHP) and Rio Tinto (NYSE:RIO), which were down 7% and 8%, respectively, by the mid-afternoon on Friday.

So what: Investors were left shell-shocked after the unexpected vote by Britain to leave the EU. That decision stoked investor fears of increased global economic and political uncertainty, and caused them to flee riskier assets like oil and copper for safe havens like gold. Copper's sell-off pushed its price down more than 3%, which is weighing on copper producers Freeport-McMoRan, BHP Billiton, and Rio Tinto.

Investors are worried that this decision could lead to weaker economic growth, not just in Britain, but across the eurozone. That could reduce the demand for copper across the continent, which would be bad news for copper producers. While China is the biggest global consumer of copper, Europe is, by far, the commodity's second-largest consumer. Because of that, any demand destruction due to a weakening European economy could cause copper prices to keep falling.

It is this potential for weakening demand that is driving producers lower today. Freeport-McMoRan, BHP Billiton, and Rio Tinto all have a rather bullish outlook for the copper market, which is why all three are investing heavily in bringing new copper projects online. Freeport-McMoRan, for instance, is spending to add capacity at its Cerro Verde project in Peru, as well as its Grasberg mine in Indonesia.

Likewise, Rio Tinto is investing billions in a Mongolian mining project, while BHP Billiton has several copper-expansion projects in the works. However, if global copper demand is weaker than expected as a result of the fallout of Britain leaving the EU, it could lead to weaker returns from these projects.

Now what: As it typically does, the market is taking a sell-first-ask-questions-later approach to the Brexit vote. It sees Europe's economic risks increasing, which could weigh on its demand for copper. That would be bad news for copper producers, given their investments to bring new supplies online to meet the demand that had been projected to grow sharply in the years ahead.

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