* Hearing was due to start on Monday
* Government blocking use of North Mara tailings dam from Saturday
* Acacia says majority owner Barrick has undervalued it
* Barrick has no comment
By Barbara Lewis and Yadarisa Shabong
LONDON, July 17 (Reuters) - Acacia Mining ACAA.L said on Wednesday it was seeking a stay of international arbitration proceedings against Tanzania, a step that could ease tensions between the two sides locked in a dispute following a $190-billion tax bill.
However, that was tempered by the company's statement that it had been notified of an imminent ban on the use of a storage dam that would prevent its North Mara gold mine - its main source of revenue - from operating.
The offer to postpone arbitration, which was scheduled to start on July 22, precedes a looming July 19 deadline for Acacia's majority owner Barrick Gold Corp ABX.TO GOLD.N , holder of a 63.9% stake, to make a firm bid to buy out Acacia.
Acacia and many of its shareholders have opposed the offer, saying it is too cheap. appears to be a conciliatory move by Acacia after the government of Tanzania effectively threatened to shut down the company over the past few days," analyst Tanya Jakusconek at Scotiabank, wrote in a note. "It's unclear at this point how the (government of Tanzania) will respond to this gesture."
International arbitration is generally a last resort for mining companies in disputes with governments.
But some Acacia shareholders have said it has a good chance of winning, which would increase the company's value and be preferable to accepting too low an offer from Barrick.
Barrick declined to comment on Wednesday.
It has been leading negotiations with Tanzania over a $190 billion tax bill - about four times the country's gross domestic product - handed to Acacia in 2017 for allegedly under-reporting output. Acacia denies the allegation.
If the Tanzanian government agrees to the stay, the arbitration hearing would be postponed, allowing time to complete settlement discussions with Barrick, Acacia said in a statement.
Acacia shares, which opened down 2.6%, pared losses to trade marginally lower at 181.3 pence at 2:58 pm GMT in London. Barrick shares advanced 2.5% to C$21.56 in morning trade in Toronto, ahead of the benchmark NYSE Arca Gold Miners index's 1% gain.
ENVIRONMENTAL RULES
Tanzanian allegations that Acacia has broken environmental regulations have ratcheted up pressure it already faces.
Acacia said it had received notice from Tanzania's National Environment Management Council ordering its North Mara mine to stop using its tailings storage facility by Saturday morning, citing a breach of environment rules.
Acacia said it would request the data and reports that had spurred the government notice for the North Mara mine to stop using the storage facility.
While the mine was issued an Environmental Protection Order and fine in May 2019 for alleged deficiencies at a tailings storage facility, Acacia said it had never received any reports that would justify the ban on use.
It added the North Mara technical team had been working "constructively and collaboratively" with the Tanzanian government.
Analysts at Berenberg, which rates Acacia a "hold," said the postponement request should be received positively by Tanzania.
But they said a prolonged outage at North Mara would have a "very significant" impact on finances, given the mine makes up 70% of group revenue, and cannot operate without a tailings dam.