Secondary Mineral Deposits and Evidence of Past
Seismicity and Heating of the Proposed Repository
Horizon at Yucca
Mountain, Nevada
By Joseph F. Whelan
USGS Water-Resources Investigations Report 03-4321, 12 p., 8 figs.
Online Only
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The citation for this report, in USGS format, is:
Whelan, J.F., Secondary Mineral Deposits and Evidence of Past Seismicity
and Heating of the Proposed Repository Horizon at Yucca Mountain, Nevada:
U.S. Geological Survey Water-Resources Investigations Report 03-4321,
12 p.
Abstract
The Drift Degradation Analysis (DDA) (BSC, 2003) for the proposed high-level
radioactive waste repository at Yucca Mountain, Nevada, describes model
simulations of the effects of pre- and post-closure seismicity and waste-induced
heating on emplacement drifts. Based on probabilistic seismic hazard analyses
of the intensity and frequency of future seismic events in the region
(CRWMS M&O, 1998), the DDA concludes that future seismicity will lead
to substantial damage to emplacement drifts, particularly those in the
lithophysal tuffs, where some simulations predict complete collapse of
the drift walls.
Secondary mineral studies conducted by the U.S. Geological Survey since
1995 indicate that secondary calcite and silica have been deposited in
some fractures and lithophysal cavities in the unsaturated zone (UZ) at
Yucca Mountain during at least the past 10 million years (m.y.), and probably
since the tuffs cooled to less than 100°C. Tuff fragments, likely
generated by past seismic activity, have commonly been incorporated into
the secondary mineral depositional sequences.
Preliminary observations indicate that seismic activity has generated
few, if any, tuff fragments during the last 2 to 4 m.y., which may be
inconsistent with the predictions of drift-wall collapse described in
the DDA. Whether or not seismicity-induced tuff fragmentation occurring
at centimeter to decimeter scales in the fracture and cavity openings
relates directly to failure of tuff walls in the 5.5-m-diameter waste
emplacement drifts, the deposits do provide a potential record of the
spatial and temporal distribution of tuff fragments in the UZ. In addition,
the preservation of weakly attached coatings and (or) delicate, upright
blades of calcite in the secondary mineral deposits provides an upper
limit for ground motion during the late stage of deposition that might
be used as input to future DDA simulations. Finally, bleaching and alteration
at a few of the secondary mineral sites indicate that they were subjected
to heated gases at approximately the temperatures expected from waste
emplacement. These deposits provide at least limited textural and mineralogic
analogs for waste-induced, high-humidity thermal alteration of emplacement
drift wall rocks.
Contents
Abstract
Introduction
Site Geology
Secondary Mineral Deposits
Vapor-Phase and Fumarolic Mineral Deposition
Low-Temperature Secondary Mineral Deposition
Thermal History
Evidence of Past Seismicity and Heating
Tuff Fragmentation
Coating Displacement or Damage to Delicate Mineral Textures
Heating Effects and Alteration of Fracture Surfaces
Summary
References Cited
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