RIO DE JANEIRO (Reuters) - Sigma Lithium ( SGML ), the biggest player in Brazil's booming lithium industry, told regulators late last year it had found likely mineral reserves on two plots of disputed land, about a month after it said they held "no mineral reserves and resources," according to previously unreported regulatory documents.

The documents, which were filed by Sigma with Brazil's mining regulator in November and have not been made public, show the firm had found "probable" mineral reserves on the land. Reuters could not determine at what date that finding was made.

Since last year, the mineral rights for that land have been caught up in a legal dispute between Sigma Chief Executive Ana Cabral and her ex-husband Calvyn Gardner, a former co-CEO.

The legal battle that was touched off by the couple's divorce proceedings rattled Sigma's share price in September, when Reuters first reported on a Brazilian court injunction halting the sale or mining of the land where Sigma has been planning open pits while exploring options to sell itself.

The existence of lithium strengthens Gardner's case that the land might hold value, according to his lawyers, who have asked the injunction that limits Sigma's control over the land to be upheld and for a formal geological assessment of it to be made.

Sigma did not reply to questions about the matter. In a letter sent to Reuters, outside lawyers working for the miner said "documents filed and allegations made by Mr. Gardner and his counsel are part of a legal record that will be proven and/or disproven during the course of the litigation."

Gardner declined to comment on the ongoing legal dispute.

Sigma has previously downplayed the importance of the dispute, saying in comments sent to Reuters in September that the plots "contain no mineral reserves or resources" and that it was contracting a third-party valuation of the disputed land, "which we anticipate being not material."

LITHIUM-RICH LAND

The disputed plots are in a lithium-rich part of Brazil, where Sigma hopes to develop phase 2 and 3 of its flagship Grota do Cirilo mining project. When complete, the project is set to nearly triple the 270,000 metric tons of lithium concentrate Sigma has estimated it will produce this year.

In April, the Vancouver-headquartered firm backed off efforts to sell itself amid sliding lithium prices and said it would invest $100 million in its phase 2 expansion.

While Sigma said in September that the disputed plots were likely of no material value, Gardner's lawyers said in a June 2023 court filing they could be worth 2.9 billion reais ($540 million) due to lithium reserves there.

Two geological studies dated Oct. 26 and filed by Sigma to Brazil's mining regulator ANM in November suggest that there could be lithium in the plots.

The documents were included in Gardner's lawsuit in April. Alongside the studies, Gardner's lawyers asked the court to order a geological assessment of the land, which the judge has yet to rule on.

The study relating to one plot, which is inside Sigma's phase 2 expansion, says the area has 4,000 tonnes of "measured" lithium resources, 130,000 tonnes of "indicated" resources and 1.4 million tonnes of "inferred" resources. The document also indicates the plot has around 110,000 tonnes of "probable" reserves.

    The study for the other plot, inside Sigma's phase 3 expansion, showed around 1.2 million tonnes of "indicated" lithium resources and 590,000 tonnes of "probable" reserves.

ANM did not immediately reply to a request for comment.

The documents show that there may be lithium in the ground, two mining experts told Reuters, but more studies are needed to confirm the volume and commercial viability, they said.

($1 = 5.3582 reais)