Analysis of the magnetic susceptibility well log in drill hole UE25a-5, Yucca Mountain, Nevada Test Site

Magnetic susceptibility measurements have been shown to be dependent upon the magnetite content of rocks with variations in rock susceptibility arising from changes in the shape, size, composition, and quantity of the contained magnetite grains* The present study was undertaken to determine the factor(s) responsible for the variation in magnetic susceptibility measurements from borehole UE25a-5 on the Nevada Test Site (NTS).The well logs and sample analyses presented in this paper form part of a larger geophysical welllogging project studying the physical properties of welded tuffs at NTS.The ash-flow sheets at NTS appear to be the products of single compositionally zoned magmas that tend, within a cooling unit, to erupt hotter, more mafic, and more crystal-rich with time.These factors, however, have little effect on the degree to which the tuffs become welded.Furthermore, zones of crystallization and alteration are superimposed upon the welded units.X-ray data show poor correspondence between the relative abundance of magnetite in a sample and the borehole magnetic susceptibility measurement associated with it.Curie balance experiments demonstrate no change in the magnetic mineralogy that could account for the susceptibility variation.» Thin-section observations corroborate the X-ray data, but indicate a proportional relationship between the borehole susceptibility measurements and the grain-size distribution of magnetite.The association of magnetic susceptibility anomalies with the crystal-rich zones of the welded tuffs will aid in the identification and correlation of the eruptive sequences at NTS.