Source of Dresser Formation 3.5 Ga dating I've seen many papers and sites that give the dating of Australia's Dresser Formation as 3.5 Ga (variously 3.48 or 3.49 Ga), but I can't seem to find the source of this.

Where does the dating come from, and what is it based on?
From Australian Government- Geoscience AustraliaAustralian Stratigraphic Units Database: 

Panorama Formation:  Paleoarchean min age: 3427 MA Age method: isotopic, U/Pb-Pb/Pb ion probe

Dresser Formation: Min Age: Ma: 3477 Age method: inferred, overlying unit

McPhee Formation:  Min Age: 3477 Age method: isotopic, U/Pb-Pb/Pb ion probe