What: Shares of Freeport-McMoRan (NYSE:FCX) were down more than 11% by 3:15 p.m. ET on Thursday. Fueling the sell-off was another drop in the price of oil as well as continued weakness in the price of copper.

So what: Crude oil fell another 2% today and is now under $34 per barrel. That puts the price of crude more than $10 per barrel below the cash flow breakeven point of Freeport-McMoRan's oil and gas business for 2016.

The continued slide in crude is widening the gap between cash flow and capex, given that every $5 per barrel change in the price of oil impacts its cash flow by $170 million. The big concern here is that with oil now well below its breakeven point, the company might have to pay for a portion of its oil and gas capex with its balance sheet, which is already stretched to the limits. This is one of the reasons why its rating agency recently put the company on review for a potential credit rating downgrade.

Making matters worse, the price of copper fell to its worst level in nearly seven years due to the continued weakness in China. The country is by far the largest market for copper as it consumes 45% of global output. So, with its economy continuing to slow it suggests that its demand could weaken further, which is bad news for the price of copper and Freeport-McMoRan. That's because copper is the biggest driver of its cash flow with each $0.10-per-pound change in its price having a $330 million impact on its cash flow, which is a huge percentage for a company that had expected to produce $4.4 billion in operating cash flow this year.

Now what: With the price for Freeport-McMoRan's two key commodities falling, it's no surprise to see its stock price tumbling. The company's downward slide will likely continue until commodity prices rebound.