Mexus owns 100% of NewCo until option is exercised...
Argonaut must complete the funding milestone amounts and dates as defined by JV agreement before they can exercise that 80% option. Unless there are some fine details not published, it does not look like any incremental ownership by Argonaut occurs over time, but rather it is structured as an all or nothing event which occurs after final milestone is met. Could be as late as DEC'17 or even DEC'18, but also at any time before then once the funding and investment targets have been met.
Therefore, it seems to me that all gold produced before the 80% option is exercised will belong entirely 100% to Mexus --- and with zero or close to zero cost. Nice upside for Mexus while the development is underway.