Ann: Gwalia Mass Extraction Feasibility Study, Guidance & webcast, page-74

I think you need to look deeper than a real basic level here. The hydraulic lift option has too much risk and probably wouldn't be able to produce enough ore. Ventilation and technical issues at 2000m below surface are major impediments.

What is the plan beyond 2021 and at what cost could gold be produced?

I worked with someone who was a specialist in slurry pumping about 3 years ago who said Gwalia had been investigating pumping from the underground for quite some time.

Well at the very least I guess you could say there is a lot of technical uncertainty in how they will achieve their plan. My first impression reading the announcement was that long term options were all on the table again. Basically it's back to the drawing board.