LONDON – SolGold's coveted Ecuadorian copper-gold prospect may be 20 percent bigger than the mining group said earlier this week, its chief executive Nick Mather said on Thursday.

SolGold, in which Mather has a near 5% stake, said on Tuesday that its Alpala project in Ecuador's Cascabel region was around double the size and grade quality it was known to be in December 2017.

"We think there's still 20% to go," Mather said, adding he expected it to be around two years before work would start on building a mine at Alpala.

Over a career of three decades, Mather, a geologist, has been instrumental in founding and running 14 companies of which nine have been taken over, returning 5.7-billion Australian dollars ($4 billion) to shareholders.

Drilling continues to further assess the resource at Alpala, which easily counts as "tier-one", the mining industry term for large, long-life projects.

In what was seen as a huge vote of confidence, the world's biggest listed miner BHP paid a premium in October to double its stake in SolGold.

BHP also agreed that for a two-year period it cannot acquire any more unless SolGold agrees.

"For us to sell it (entirely to BHP), the price has to be right. There has been no discussion about that," Mather said.

"We're getting on with the job of building a copper/gold major company," added Mather, who describes copper as "a global must-have" as electric vehicles and demand for better grid networks drive demand and attract investment.

Despite wariness about the risks associated with the broader mining sector, SolGold shares have risen nearly 30 percent this year, while the sector is down more than 9 percent.

"We have interest expressed by people all the time. Whether we welcome them or not depends on what they're offering," Mather said. "We will do what's right by the shareholders."

Mather has a 4.89% stake, according to Refinitiv data, while BHP has 11.18% and Newcrest is the biggest investor with 13.41%.

SolGold has a 100% interest in 11 other copper gold targets in Ecuador, which it says is under-explored compared with Chile, the world's biggest copper producer, which also sits in the Andean copper belt.

Mather says Ecuador could hold even better finds than Alpala.

"There's no reason why it has to be the best. It just happens to be the first," he said.