For decades, the risky world of early-stage mining ventures was the playground of institutional funds and commodity trading houses. But after a more than 900% rally in a little-known Canadian potash developer last year, a new class of investors is moving in: family offices.

Millennial Potash Corp. has attracted a group of ultra-high-net-worth clans that are increasingly bypassing traditional funds to take direct stakes in critical resources. Potash, a potassium-rich salt, is a fertilizer for crops ranging from cereals to potatoes, helping boost yields and plant resilience. The Vancouver-area miner is developing a potash project in Gabon and has no other revenue.