Metals-related exchange traded funds have enjoyed a rally this year, but traders and the miners themselves are growing more pessimistic over the future outlook.

Over the past month, SPDR Gold Shares (NYSEArca: GLD) fell 1.8%, iShares Silver Trust ETF (NYSEArca: SLV) declined 4.7%, VanEck Vectors Gold Miners ETF (NYSEArca: GDX) dipped 0.2% and iShares MSCI Global Silver Miners Fund ETF (NYSEArca: SLVP) slipped 1.2%.

Related: 31 Gold ETFs Investors Should Size Up

Meanwhile, the broader SPDR Metals & Mining ETF (NYSEArca: XME) fell 5.4% and Materials Select Sector SPDR (NYSEArca: XLB) dropped 0.5% over the past month.

Mining executives warned that the weakness in the metals space could continue on uncertainty over Chinese growth, oversupply in many metals and resources and stronger U.S. dollar, reports Alex MacDonald for the Wall Street Journal.

“Although we have recently seen some positive signals…we are expecting another year or two of low copper prices,” Jean-Paul Luksic, chairman of Chilean copper producer Antofagasta PLC, said.

Related: Strengthening Dollar Plaguing Copper ETN

The iPath Dow Jones-UBS Copper Subindex Total Return ETN (NYSEArca: JJC) declined 7.2% over the past month.

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