* TSX down 90.5 points, or 0.55 percent, at 16,257.48
Seven of the TSX's 10 main groups move lower
TORONTO, Jan 23 (Reuters) - Canada's main stock index fell in morning trade on Tuesday, with a broad retreat led by base metal miners as copper prices hit a one-month low and a gold exploration company dropping sharply after announcing its 2018 production and cost forecast.
* 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. (Reporting by Alastair Sharp; Editing by Bill Trott)