Biogenic Iron Mineralization at Iron Mountain, Ca, with Implications for Detection with the Mars Curiosity

Introduction: Microbe-mineral interactions and biosignature preservation in oxidized sulfidic ore bodies (gossans) are prime candidates for astrobiological study. Such oxidized iron systems have been proposed as analogs for some Martian environments [1]. Recent studies identified microbial fossils preserved as mineral-coated filaments [2,3,4,5,6,7,8,9]. This study documents microbially-mediated mineral biosignatures in hydrous ferric oxide (HFO) and ferric oxyhydroxysulfates (FOHS) in three environments at Iron Mountain, CA. We investigated microbial community preservation via HFO and FOHS precipitation and the formation of filamentous mineral biosignatures. These environments included 1) actively precipitating (<3 yr) FOHS-HFO scale in pipes carrying acidic mine water, 2) much older (>1000's yrs), naturally weathered HFO from in situ gossan, and 3) remobilized iron deposits, which contained lithified clastics and zones of HFO precipitate. We used published biogenicity criteria [6,10,11] as guidelines to characterize the biogenicity of mineral filaments. These criteria included A) an activelyprecipitating environment where microbes are known to be coated in minerals [2,5,6,8,13], B) presence of extant microbial communities with carbon signatures, C) structures observable as a part of the host rock, and D) biological morphology, including cellular lumina, multiple member population, numerous taxa, variable and 3-D preservation, biological size ranges, uniform diameter, and evidence of flexibility. This study explores the relevance and detection of these biosignatures to possible Martian biosignatures. Similar filamentous biosignatures are resolvable by the Mars Hand Lens Imager (MAHLI) onboard the Mars Science Laboratory (MSL) rover, Curiosity, and may be identifiable as biogenic if present on Mars. Methods: We collected samples of pyrite, HFO, and quartz boxwork from surface gossan during the winter of 2010–2011, and samples of FOHS-HFO scale from a mine water effluent pipe in 2012. We characterized microbially-associated HFO and FOHS precipitation by identifying 1) mineral phases with XRD, reflected light microscopy, and energy dispersive x-ray spectrometry via SEM and 2) mineralcoated microbial textures and morphology with SEM and optical microscopy. We evaluated mineral filament biogenicity using published criteria [6,10,11]. Results & Interpretations: Mineral Morphologies: Pipe Scale. 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 and minor goethite [12]. The SEM images of the scale showed mineral spheres (diameters 0.8–3.8 μm, measured with ImageJ), unmineralized microbial filaments (diameter ~0.4 μm), and FOHS filaments (diameter ~2.4 μm) coated by <1 μm particles with central lumina ~0.6 μm wide (Fig. 1A). Mineral Morphologies: Surface Gossan. Mineral filaments associated with gossan were coated with <1 μm wide HFO crystals. In situ gossan samples included HFO filaments (diameters 2.4–31.8 μm, mean = 9.8 μm) with colloform texture (Fig. 1B), mineral spheres either nucleated on HFO filaments or relic quartz boxwork (two populations average 5.7 and 20.5 μm wide, respectively), and bladed crystals (elongated crystals that radiate from a central point to form lamellar aggregations). In remobilized iron deposits, oriented fabrics consisted of muted-colloform HFO filament masses (diameters 1.1–19.7 μm, mean = 7.8 μm) that form masses of oriented filaments with ~0.5 μm wide central lumina (Fig. 1C).