Prices of major traded-metals tumbled to multi-year lows Friday as weak Chinese manufacturing data added to the brew of excess supply and a strong dollar that has put pressure on several commodities this week.

Three-month copper futures fell to $5,202.50 a metric ton on the London Metal Exchange, their lowest level in six years, as a preliminary gauge of China’s manufacturing activity dropped to a 15-month low in July. Prices of three-month aluminum on the LME, another heavily-traded base metal, also dropped to a six-year low at $1,634 per ton.

“It is a double whammy for metals,” says Avatar Sandu, senior manager of commodities at Singapore-based Phillip Futures. “On the one hand you have excess supply from mines, and on the other weak demand from China.”

China’s consumption of industrial metals fueled a more-than decade-long rally in prices after the turn of the century. But concern over the nation’s slowing economic growth has caused metals prices to start sliding over the last twelve months.

The question for investors now is how prolonged the commodity market malaise will prove to be.

The decline in copper prices could continue through to at least 2018 due to high supply growth as well as weakness in China’s property sector, a major consumer of the metal, Goldman Sachs said in a note this week.

The slowing Chinese economy has also piled pressure on aluminum prices. The latest Chinese customs data show the country’s export of unwrought aluminum, or semi-finished products, surged 35% year-on-year in the first half of the year. Chinese aluminum producers have been stepping up their overseas sales this year, adding to global excess supply of the metal used in automaking and packaging.

“While we believe the aluminum prices have found a bottom, the supply glut and the expectation of interest rate hikes may continue to keep aluminum prices low,” said Helen Lau, analyst with Hong Kong-based Argonaut Securities in a research report.

Currency market moves have put further pressure on dollar-priced metals, as they become more expensive for foreign buyers when the greenback rises.

“The dollar has also been inching up in value, which is not supportive of metals,” said Phillip Futures’ Mr. Sandhu.

Iron ore’s price is down by nearly 30% since the beginning of the year at $50.6 per ton for grades with 62% iron content at China’s Tianjin Port. The supply glut in iron ore has worsened with leading Brazilian producer Vale S.A. boosting iron ore production in the last quarter to the second-highest levels in its history, said a report by Australia and New Zealand Banking Group Ltd. Other major producers BHP Billiton PLC and Rio Tinto PLC this week said they were sticking to plans to raise output.

“We don’t see a price recovery at least in the next month or two,” said Daniel Hynes, commodity analyst with ANZ Bank. “It’s a seasonally weak period.”

Gold prices slumped to their lowest level in five years earlier this week amid growing expectations that the U.S. Federal Reserve will raise interest rates before the end of this year, which will likely add to the dollar’s strength.

On Friday, the precious metal was hovering close to the low hit on Monday, and briefly dipped below $1,080 per troy ounce.

Analysts say gold could fall below $1,000 per ounce, a level at which becomes unprofitable for a number of mines to keep producing.

“As the Fed leads the way back towards normality, the gold price might return to the levels of around $850 [per ounce] seen at the end of 2007,” according to a Capital Economics report.