The Mining Effort
First, Scarlett will have to get rid of the Delvers that infest the mine. Since she was already in the copper mine looking for Moonstone, this means finishing exploring all the tunnels and eliminating the remaining Delvers. (She doesn't have to open the closed off area by the abandoned entrance to do that.)
Scarlett can only complete this quest once she gets to Venice. In the Skullbreak Tavern in the Arsenal District there are three workers, one of them is glad to talk to Scarlett about this opportunity and promises to go to San Pasquale.