The deal is expected to see the company through the next project de-risking phase, including a pre-feasibility study early next year. First Mining is aiming to complete project permitting by 2023.
The company said the deal came after a year of assessing non-dilutive financing alternatives.
It increases First Mining's strategic relationship with minority owner First Majestic, whose CEO Keith Neumeyer founded First Mining.
Payment will be made in three tranches: $10 million in cash and First Majestic scrip on closing, $7.5 million upon publishing the PFS, and $5 million upon completing a federal environmental assessment.
The 2019 preliminary economic assessment on Springpole guides for average silver output of 2.4 million ounces from years two through nine of the mine plan, for 22Moz total, of which First Majestic will buy half at 33% of the prevailing spot price, capped at $7.50.
First Mining retains the right to repurchase 50% of the stream for $22.5 million prior to production starting.
Springpole is one of Canada's largest undeveloped gold projects. It holds 4.67Moz gold indicated plus 24.19Moz silver.
Earlier this this week, First Mining sold its early-stage Goldlund project to Treasury Metals.
First Mining shares (TSXV: FF) closed slightly lower Thursday at C33c, valuing it at $209 million, while New York-quoted First Majestic (NYSE: AG) fell 9% to US$12.66, which values it at $2.65 billion.