Boron metasomatism in the Alta stock contact aureole, Utah

The effects of fluid evolution and infiltration in the Alta stock contact aureole, Utah, were studied by evaluating chemical additions and depletions in igneous, replacement skarn, and carbonate whole-rock samples. Of the major and trace elements studied, boron proved to be the most interesting tracer of fluid, due in part to extensive borate mineralization (ludwigite and kotoite) in carbonates near the stock contact, and in part to the physical-chemical properties of boron which make it a useful element for isotopic studies and for spatial distribution analysis via alpha track mapping. Boron is enriched throughout the Alta aureole and is hosted by a variety of metamorphic and metasomatic silicate minerals (forsterite and humite group are the most significant). Aureole whole-rock B concentrations increase toward the contact, indicating that B originated in the stock and migrated down-temperature in exsolved magmatic fluids. Boron stable isotope measurements of carbonates, skarns and mineral separates range from $-$7.2 to +1.6$\perthous\ \delta\sp{11}$B. A value of ${-}6.0\perthous$ for a B-enriched igneous sill falls within this range and also indicates that the stock is the source of aureole B-enrichments