The latter is reportedly in the final throes of raising $500-600 million for a portfolio that includes the Capricorn operation in Queensland, located 7km from Castillo's key Mt Oxide ground holding.
Castillo plans to test multiple targets at Mt Oxide over the next few months, including the Big One, Arya and Sansa prospects.
It also said it would now be able to "expedite" exploration programs in Zambia.
Castillo, which has ambitions to become a mid-tier copper producer, said the fundraising meant it now has "significantly higher confidence that 2021 will be a transformative year".
New shares being issued in the fundraising were priced at 4.2c each.
Shares in Castillo were down 10% to 4.6c in early trade, capitalising the company at $46 million.
The stock was at levels around 2.6c 12 months ago, while cash at the start of the current quarter totalled $2.4 million.
