At 9:44 a.m. EST (1444 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 90.5 points, or 0.55 percent, at 16,257.48. Seven of its 10 main groups were in negative territory, with decliners outnumbering advancers by 1.4-to-1 overall.
The most influential movers on the index included First Quantum Minerals Ltd, which was down 3.6 percent to C$18.35, and Teck Resources Ltd, off 3.6 percent at C$34.95.
Both companies produce copper, which fell to the lowest in a month after inventories surged, while First Quantum also was hit by a downgrade to "neutral" from Credit Suisse.
Copper prices declined 2.0 percent to $6,924.5 a tonne.
The materials group, which includes precious and base metals miners and fertilizer companies, lost 1.4 percent.
Pretium Resources Inc slumped 18.9 percent to C$10.88 after the gold exploration company provided an operational update for its Brucejack mine in northern British Columbia and Credit Suisse downgraded the stock.
The heavyweight financials group fell 0.6 percent, while industrials fell 0.7 percent.