Shares of Asanko Gold Inc (AKG.TO) tumbled 13 per cent on Wednesday after U.S.-based investment firm Muddy Waters said it was shorting the stock due to a string of problems at the miner's Ghana operations and the "significant risk" of a 2018 liquidity crisis.

Muddy Waters said in a report that it expects Vancouver-based Asanko will run out of cash in 2018 as it tries to service a $165 million debt, reflecting erroneous investments in its Nkran, Esaase and other deposits, based on flawed geology.

Asanko's shares were halted on the Toronto Stock Exchange after the dive.

"This has been a very opportunistic ambush," said Asanko spokeswoman Alex Buck, adding that the company refutes the claims and would issue a formal rebuttal later on Wednesday.

The company will meet with its major shareholders beginning Monday, Buck added, on a previously organized two-week tour to discuss growth plans. The meetings will include updates on two projects that Asanko forecasts will boost annual production from Ghana to 450,000 ounces in 2020.

Asanko had estimated it will produce 230,000 to 240,000 ounces of gold in 2017, she said.

The company had a market value of $512.9 million before the fall.

The U.S.-listed shares of Asanko fell 31 per cent before being halted.

Muddy Waters noted that Asanko liquidity was limited to US$48 million of cash on hand, with no availability on its US$150 million revolver credit. Using "company-favorable assumptions," it estimated that Asanko will have a cash shortfall ranging from US$43 million to US$129 million in 2018, the report said, without specifying the currency.

Asanko is a development-stage gold miner that began commercial operations at its Asanko Gold Mine in April 2016.

The company issued a mineral resource estimate last December that reduced the estimated gold in its Nkran and Esaase deposits by US$581 million and "more in line with reality," but that estimate is still flawed and assumes a lofty gold price of US$2,000 an ounce, the Muddy Waters report said. The spot price of gold on Wednesday was US$1,266.21 an ounce.

Last June, Toronto hedge fund K2 & Associates Investment Management Inc took a short position in Asanko, saying its gold resources were overinflated, notably at its Nkran deposit, which Asanko acquired in 2014 from Resolute Mining Ltd.