The smoking — or in this case, oozing — gun is a layer of siliceous microfossils called diatoms. (sciencedaily.com)
3.48 billion year old siliceous rocks of the Dresser Formation, Pilbara craton, Western Australia, preserve evidence for having formed in hot springs on land — the earliest life on land yet known, and about the same age as when life may have been present on Mars (a wetter time in its past, with heating from once active volcanoes and bolide impacts). (astrobiology.nz)
Going into a little more detail, the diatomaceous earth is formed from algae with siliceous shells. (thebugsquad.com)