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Microbial and Inorganic Depositional Processes in Travertine: Tools for Investigating a Possible Blind Geothermal System in Caribou County, Idaho

Sara R. Ohly, Michael O. McCurry, and John A. Welhan
Idaho State University

Travertine can be used as a proxy for hydrocarbon-forming microbial environments, as well as for geothermal exploration. Travertines from the Blackfoot Volcanic Field (BVF) in southeast ID were studied to determine depositional history and possible geothermal origin. Compositional variations in CaCO3 were used to infer travertine origin and water-rock interaction history, and petrographic microfacies studies allowed reconstruction of depositional history. Compositional data were gathered using SEM-EDS, XRD and solution ICP-MS. Morphological trends were determined by combining field observations with petrography and SEM-EDS. Travertines were grouped into 3 subtypes based on color, mineralogical, and morphological properties: white travertine (WT), red type 1 travertine (RT1), and red type 2 travertine (RT2). WT deposits are characterized by calcite shrubs and regions of secondary microcrystalline calcite with low to moderate intergranular, interparticulate, or moldic porosity. RT1 travertines contain alternating layers of Fe-stained calcite and hematite (and possibly oxyhydroxides) with trace fluorite. RT1 textures include clusters of skeletal calcite and aggregated rhombs with fenestral porosity. RT2 deposits include brecciated aragonitic botryoids cemented together with micritic calcite, with interparticulate pores partially filled with Fe/Mn oxyhydroxides, or peloids composed of micritic calcite and detrital clasts with meniscate cement. WT and RT1 travertines have trace element signatures indicative of water-rock interaction with highly porous Pleistocene basalts that cover most of the BVF. RT2 travertine signatures reflect older regional volcanics. CaCO3 in RT1 and RT2 travertines contains up to 5% Fe+Mg+Mn+Sr+Ba. WT travertine carbonates have concentrations of these elements of <1%. WT morphology indicates the presence of pond, proximal and distal slope facies with sparse laminated stromatolitic areas. Microtextures are suggestive of microbially influenced precipitation in ponds and laminated portions, and abiotic precipitation on slopes. RT1 and RT2 travertines are dominated by vent and apron facies, and microfacies indicate a predominantly abiotic precipitation mode. Depositional processes in BVF travertines appear to be controlled by proximity to vents. Areas proximal to vents with higher dissolved CO2 and flow rates tend to show abiotic features, and distal areas have evidence of microbial contributions to precipitation as well.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013