The latter is reportedly in the final throes of raising A$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 were priced at A4.2c each. The issue was supported by sophisticated investors in Australia and the UK.
The company reported cash of A$2.4 million at the start of April.
Shares in Castillo dropped to A4.4c in Australian trade. The stock last traded at 2.5p in London.
