* TSX down 65.80 points, or 0.56 percent, AT 11,714.75
* Weak gold, base metals prices weigh on TSX
* Five of 10 main groups lower (Adds details, quote)
By Jennifer Kwan
TORONTO, June 3 (Reuters) - Toronto's main stock index sank at midday on Thursday, led lower by weakness in mining shares due in part to soft bullion and base metal prices.
Key names on the downside included Barrick Gold ABX.TO, which fell 2.3 percent to C$43.80, Goldcorp G.TO, down 3 percent at C$44.56, and Teck Resources TCKb.TO, which dropped 4.1 percent to C$34.59.
Fertilizer giant Potash Corp of Saskatchewan Inc POT.TO was down 2 percent at C$101.09, while the broader materials group sank 2.3 percent.
"You've seen a drop in gold prices so there's some profit-taking in the gold sector on the back of that decline in gold," said Elvis Picardo, an analyst and strategist at Global Securities in Vancouver.
Gold prices eased as risk sentiment overseas improved on investor optimism about the economic recovery, with global equities moving higher. Base metals reversed earlier gains on lingering concerns over European sovereign debt levels and monetary tightening in China. [GOL/] [MET/L]
At 12:40 p.m. (1640 GMT), The Toronto Stock Exchange's S&P/TSX composite index .GSPTSE was down 65.80 points, or 0.56 percent, at 11,714.75, with half of its 10 main groups lower.
Earlier, the resource-heavy index followed global and U.S. equities higher, supported by strength in energy issues as oil prices climbed above $73 a barrel. [O/R
However, oil prices turned negative, dragging the energy sector down to the break-even level, while U.S. stocks lost steam as investors digested mixed U.S. economic data. [.N] [ID:nN0360959]
Gainers included Canadian Natural Resources CNQ.TO, up 1.4 percent at C$37.18, and Enbridge Inc ENB.TO, which rose 1.8 percent to C$48.79, while Canadian Pacific Railway CP.TO was up 2.4 percent to C$59.58.
($1=$1.04 Canadian) (Reporting by Jennifer Kwan; editing by Rob Wilson)