Correlation Between Redox States, Source Regions and Metallogenesis of Middle Tertiary Lavas of the Western United States

ABSTRACT
This study compares and contrasts the oxidation state, mineralogy,
and chemical compositions of 128 Tertiary lava flows from
Nevada, Utah, and Montana. The FeO content of each sample was
obtained using standard titration methods and Fe2O3 was determined
by difference from x-ray fluorescence total Fe analyses. An empirical
equation relating mole fraction of Fe2O3 and FeO to absolute
temperature and liquid composition at 1 bar was utilized to estimate
oxygen fugacity for each sample. This enabled a comparison
between the empirical equation with data from analyzed ilmenite
and magnetite pairs. A comparison of the two methods indicates that
magmatic oxygen fugacities calculated using measured whole-rock
FeO/Fe2O3 ratios are usually in agreement with oxygen fugacities
determined from magnetite-ilmenite pairs as long as the rock contains
more than 5 weight % total iron as FeO.
The resulting data suggest a spatial relationship between
whole-rock trace and major element composition, location, and
inferred magmatic oxygen fugacity. Whole-rock Fe2O3, K2O and
Na2O and inferred magmatic oxygen fugacity all appear to increase
eastward through Nevada and into Utah. Similar eastward increase
in Cu-, Ni-, Pb-, Sr-, and Ba-contents of lavas may be attributed to
the increased abundance, melting or assimilation of phlogopite-rich,
metasomatic veins in the mantle. Such veins may be more abundant
in the Proterozoic lithospheric mantle east of an inferred suture zone
located approximately beneath the Roberts Mountain Thrust