Sargent clearly showed an interest in mosaic floors in Italy. The two most obvious examples are the oil he did of the inside of San Marco in 1898 . . .
and the watercolor he did in Sicily.
Colorful Pavements, Sicily
If this is the floor of San Marco (in the painting Venetian Interior Pavement) the tonality is completely different than his beautiful The Pavement. Still, I'm just assuming he painted Venetian Interior Pavement that same year. He may not have. Though I find the color pallet is much lighter than most of Sargent's oils, I'm not sure it's unprecedented.
In Olson's book he talks about Sargent's interest in mosaic floors (p.216). And on my webpage of photos of the interior of San Marco, timeout.com talks about seeing the basilica at different times of the day to witness the "varying effects of light on the mosaics." The painting could have been another study at a different time, on a different day.