POROSITY, BULK DENSITY, AND ROCK-PARTICLE DENSITY OF LITHOSTRATIGRAPHIC COMPONENTS IN LITHOPHYSAL ROCKS OF THE TOPOPAH SPRING TUFF AT YUCCA MOUNTAIN, NEVADA

Rock properties such as porosity, bulk density, and rock-particle density (zero porosity) are commonly measured with hydrogeologic, thermal, and mechanical properties in the densely welded, crystallized, lithophysal and nonlithophysal tuff of the proposed nuclear-waste repository host rocks at Yucca Mountain, Nevada. More than 1,500 measurements of bulk-rock properties were made on core samples from the host horizon. These measurements are used in calculating similar properties from geophysical logs and for scaling and modeling the distribution of various properties. Lithostratigraphic features, or components, of the host rock include the matrix-groundmass (MG), lithophysal cavities, rims (R), and spots (S; similar to rims, but without cavities). Rock descriptions were not made on most individual core samples, so to test the hypothesis that amounts of lithostratigraphic features relate to variations in rock properties, 47 core samples from upper and lower lithophysal zones were cut into MG, R, and S component parts and analyzed. Properties of R and S samples are indistinguishable and are combined. Analyses of samples with greater than 90 percent of the end-member components are summarized below.

Lithophysal		No. of	Porosity (cm3/cm3)			Rock-particle density (g/cm3)
Zone	Feature	Samples	Range	Mean	Std. Dev.	Range	Mean	Std. Dev.
Upper	MG	11	0.081 to 0.117	0.103	0.012	2.50 to 2.57	2.52	0.03
Lower	MG	28	0.092 to 0.138	0.104	0.012	2.52 to 2.61	2.56	0.02
Upper	R&S	05	0.274 to 0.311	0.288	0.015	2.52 to 2.54	2.53	0.01
Lower	R&S	30	0.234 to 0.372	0.302	0.031	2.51 to 2.66	2.57	0.03

Conclusions are: (1) Rock property values of MG and R&S samples are consistently smaller or larger than most previously published values; therefore, they are viable end-member component values for mixing models. (2) End-member component values can be used with quantitative rock descriptions to provide estimates of bulk-rock properties. (3) Similar rock-particle densities for MG and R&S in each lithophysal zone indicate similar compositions of these features. (4) These results indicate slightly larger mean values of porosity and rock-particle density for lithostratigraphic features in rocks of the lower lithophysal zone compared to those in the upper lithophysal zone.