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DOE Yucca Mountain Project
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Deposits of secondary minerals, mostly calcite and opal, are present as coatings on open fractures and in lithophysal cavities in unsaturated-zone tuffs at Yucca Mountain.

Outermost layers of calcite and opal have radiocarbon ages of 16,000 to 44,000 years before present and thorium-230/uranium ages of 28,000 to more than 500,000 years before present, much younger than the 13-million-year age of the tuffs.

The physical and isotopic data from calcite and opal indicate that they formed from solutions of meteoric origin percolating through connected fracture pathways in the unsaturated zone.

image of an opal deposit from the floors of lithophysal cavities in the Exploratory Studies Facility (ESF)tunnel, Yucca Mountain, Nevada.
Image is a cross-sectional slab of 2.5 to 3 millimeter-thick mineral coating consisting mostly of elongated calcite crystals (bluish white) growing outward from the base with multiple layers of opal (green) in the interior of the coating and small, thin layers of opal at the outer surface.

  

image is a side and top views of very thin (less than .5 millimeter), fragile bladed calcite showing banded growth layers.
side view of very thin (less than 0.5 millimeter), fragile, bladed calcite showing banded growth layers and septer-head blade tops.

The calcite and opal typically are present only on footwall surfaces of a small fraction of fractures and only on floors of a small fraction of lithosphysal cavities. The similarities in the carbon, oxygen, and strontium isotopic compositions between fracture calcite and soil-zone calcite support a genetic link between the two depostional environments. Older deposits contain oxygen isotope compositions that indicate elevated temperatures of mineral formation during early stages of deposition; however, in the youngest deposits these values are consistent with deposition under geothermal gradients similar to modern conditions.

  
Cumulative evidence from calcite and opal deposits indicates that the growth of minerals within the fracture network at the potential repository horizon has remained relatively uniform over long periods of time, implying a relatively stable hydrologic environment in the lower part of the Yucca Mountain unsaturated zone.      
 

image shows a magnified calcite blade tip with the latest scepter-head calcite overgrowth covering an opal hemisphere deposited on the former blade tip.
Image is a blade tip showing the latest scepter-head calcite overgrowth covering an opal hemisphere deposited on the former blade tip.

    

Text and images from Paces, J.B., Neymark, L.A., Marshall, B.D., Whelan, J.F., and Peterman, Z.E., 2001, “Ages and Origins of Calcite and Opal in the Exploratory Studies Facility Tunnel, Yucca Mountain, Nevada”: U.S. Geological Survey Water-Resources Investigations Report 01-4049, 95p..

    


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