Paper No. 12
Presentation Time: 11:45 AM


WILLIAMS, Amy J.1, PHAN, Athena T.1, SUMNER, Dawn Y.1, ALPERS, Charles N.2, CAMPBELL, Kate M.3 and NORDSTROM, D. Kirk4, (1)Geology, University of California, Davis, One Shields Avenue, Davis, CA 95616, (2)U.S. Geological Survey, California Water Science Center, 6000 J St, Placer Hall, Sacramento, CA 95819, (3)Water Resources Division, U.S. Geological Survey, Boulder, CO 80303, (4)U.S. Geological Survey, Boulder Labs, 3215 Marine St, Boulder, CO 80303,

We documented microbially-mediated mineral biosignatures in hydrous ferric oxide (HFO) and ferric oxyhydroxysulfates (FOHS) in three environments at Iron Mountain, California: 1) actively precipitating FOHS-HFO scale in pipes carrying acidic mine water, 2) much older, naturally weathered in situ gossan, and 3) remobilized iron deposits, which are small deposits of recycled in situ gossan containing lithified clastics and zones of HFO precipitate.

The pipe scale formed through microbial oxidation of Fe(II) to Fe(III) in acidic (pH 2.5 to 3.0) mine water with subsequent precipitation of schwertmannite with minor goethite. SEM images of the scale showed mineral spheres (diameters 0.8–3.8 μm), unmineralized microbial filaments (diameter 0.5 μm), and FOHS filaments (diameter ~2.4 μm) coated by <1 μm particles with central lumina 0.5 μm wide. Goethite filaments associated with gossan were coated with <1-μm-wide particles. Weathered in situ gossan samples included HFO filaments (diameters 2.4–31.8 μm, mode=9.8 μm) with colloform textures, euhedral mineral spheres that nucleate on HFO filaments or fill void space (two populations average 5.7 and 20.5 μm wide) and bladed structures (elongated crystals that radiate from a central point to form lamellar aggregations). In remobilized iron deposits oriented fabrics consisted of smooth-textured HFO filament masses (diameters 1.0–19.7 μm, mode=7.8 μm) with 0.5-μm-wide central lumina. Computed tomography reconstructions indicate porosity of ~1.1% for in situ gossan rocks, and ~6.0% in remobilized iron deposits.

Mineral filaments preserved microbial textures and the microenvironment controlled biosignature morphology in these environments. Older gossan HFO filaments were morphologically similar to mineralized microbial filaments that formed rapidly from acid water in the pipe scale. In situ gossan HFO precipitation took the form of HFO filaments, mineral spheres, and bladed structures. Remobilized iron deposits had mm-scale, oriented fabrics of HFO filaments, where HFO precipitation was faster relative to in situ gossan. Gossans have been proposed as Martian environmental analogs; this research is being applied to the search for similar mm-scale mineral biosignatures detectable with the Mars Curiosity Rover cameras.