* Lead and zinc producer says Q3 core profit drops 74 pct
* Shares tumble 11 pct after the results
* No decision yet on how to tackle company's debt
* Nyrstar says a bond buyback may be one option (Adds comment by CFO)
Oct 30 (Reuters) - Metals producer Nyrstar NV said on Tuesday that third-quarter core profit dropped 74 percent, hit by a combination of falling zinc prices, rising energy prices and higher mine operating expenses.
The Belgian company, whose biggest shareholder is Swiss-based global commodities trader Trafigura, had already issued a profit warning last month and its shares slid 11.1 percent following Tuesday's results.
A weak dollar also hurt third-quarter earnings, it said.
"Over the course of the third quarter, Nyrstar was exposed to adverse market conditions (which led to) poor financial results for the group," Chief Executive Hilmar Rode said in a statement.
Second-quarter underlying earnings before interest, tax, amortisation and depreciation (EBITDA) totalled 13 million euros ($14.8 million) on revenues of 1 billion euros, the zinc and lead producer said.
Nyrstar increased zinc metal production by 9 percent in July-September from a year earlier, but said the average zinc price fell 22 percent in the quarter compared to the average for the first half of the year.
The company, whose relatively high debt levels are also a source of investor concern, warned last month that its second-half underlying EBITDA would be materially lower than the first half, although it left forecasts for production, capex and positive free cash flow unchanged.
It confirmed the free cash flow outlook on Tuesday.
For January-September, underlying EBITDA dropped 17 percent from a year earlier to 134 million euros.
Moody's cut Nyrstar's credit rating last month, citing concerns about its liquidity and ability to refinance 350 million euros' worth of bonds maturing next year.
Nyrstar's CFO Michel Abaza said that no decision had yet been taken on how to tackle the company's debt levels but that the board would come back with a decision in the coming months.
"A bond buyback will be contemplated as an option among others... it will be part of the review of the capital structure of the group," Abaza said.
The company said its net debt at the end of September, excluding zinc metal prepay and perpetual securities, had fallen 61 million euros from the end of June.
Abaza added that discussions were ongoing to extend a $250 million working capital facility from Trafigura. The facility is due to expire at the end of 2019.
$1 = 0.8789 euros Reporting by Piotr Lipinski in Gdynia; Julia Payne in London; Editing by Darren Schuettler and Susan Fenton