Copper prices have been falling recently and today's sell off came after news that Chinese property developer Zhejiang Xingrun Real Estate has collapsed. China's property sector accounts for about 40% of copper demand.

Concerns about slowing Chinese growth and domestic bond defaults have weighed on copper. There have been concerns that copper-backed loans in China are expected to unwind. Some 60-80% of China's copper imports have been used as collateral.

Not everyone agrees however. From Goldman Sachs (via FT): "In contrast to some media reports, we find that the bulk of Chinese commodity financing deals are ongoing, facilitating ‘hot money’ inflows into China and providing a mechanism to import low cost foreign financing." They argue it remains a supply-demand story. With declining demand in China as supply stays strong.