Introduction The Humboldt Range, Pershing County, Nevada, predominantly consists of Mesozoic igneous and sedimentary rocks that were modified several times by magmatism, metasomatism, and tectonism, and contain a variety of metallic (Ag, Au, Pb, Zn, Sb, W, Hg) and non-metallic (dumortierite, pinite, fluorite) mineral deposits (Knopf, 1924; Kerr and Jenney, 1935; Kerr, 1938; Cameron, 1939; Campbell, 1939; Kerr, 1940; Page et al., 1940; Johnson, 1977; Vikre, 1978; 1981; Crosby, 2012). Early Triassic Koipato Group volcanic rocks, which are widely exposed in the range, have been altered to quartz, muscovite (sericite), chlorite, pyrite, and other minerals during emplacement of Mesozoic intrusions and by crustal thickening. Most hydrothermal alteration of volcanic rocks and formation of mineral deposits involved externally derived water and other volatiles, although some volcanic strata were apparently altered by pore or dehydration water. Cospatial hydrothermal mineral assemblages and associations, produced by events widely spaced in time, are difficult to separate because of common mineralogy (quartz, sericite, and pyrite), partial to complete recrystallization, thermally compromised Ar geochronology, and lack of comprehensive investigations of volatile sources and deformational fabric. Distinguishing between metasomatic and metamorphic processes that affected rocks in the Humboldt Range is not straightforward.