RAMAN SPECTROSCOPY OF OOLITIC HEMATITE: EVIDENCE OF BIOSIGNATURES IN IRON OXIDE RICH ROCKS

Despite the extensive distribution of oolitic hematite, the origin and conditions of their formation is still under considerable debate. Currently, it is postulated that the source of iron, aluminum, and silica for ooid formation comes from exhalative fluids rising through the substrate. However, nothing is known about the temperature of the fluids, and whether biological or non-biological processes occur to induce the precipitation and hence formation of these extensive deposits. In order to shed light on the formation conditions of these deposits, we undertake a systematic approach to elucidate the mineralogy and chemistry of oolitic hematite samples found mainly in the USA; such areas include Alabama. Raman spectroscopy reveals the presence of hematite, quartz, apatite, rhodochrosite, siderite, magnesite, low Mg calcite, and calcite. The hematite composing this formation varies from very poor crystallinity, and poor to moderate crystallinity, which is demonstrated by the Raman spectroscopic analysis. Interestingly however, the Raman spectroscopic analyses showed the presence of sp² carbonaceous materials, presumably derived from diagenetically altered biological material. Due to the high spatial resolution that Raman spectroscopy affords, it can be gleamed that the sp² carbonaceous materials were associated with larger size ooids. Typically, ooids seen in hand sample are less than 1 mm and show little evidence of associated sp² carbonaceous materials. The spectra that exhibit sp² carbonaceous materials are typically acquired from ooids varying from 1 – 5 mm and on rare occasion 10 mm scale ooids. Another common trait that can be seen in hand sample is the color variation. The larger ooids have a darker hue of dark brown versus the light brown matrix. The ooids are rounded ellipsoids with no specific orientation. Further, all the spectra of sp² carbonaceous materials from various samples of oolitic hematite are representative of the organic material undergoing 60-140^o C which is consistent with thermally immature diagenetic alteration.