Boron metasomatism of the Alta stock contact aureole, Utah: Evidence from berates, mineral chemistry, and geochemistry

Geochemical study of the Alta stock and adjacent contact aureole rocks prov ides information concerning the source, composition, and physical-chemical conditions of infiltrating fluids. Special emphasis was given to boron (B) as a tracer of fluid-rock interactions due to the occurrence of berate mine rals (ludwigite, kotoite, and szaibelyite) in skarn deposits around the sto ck. In addition, thin section alpha-track mapping implies significant B enr ichments in fluid-altered minerals within the stock, stockwork veins and re lated selvages, igneous sills near the stock, contact skarns, and in marble s up to 500 m from the stock. Forsterite. clinohumite, lizardite, and malac hite contain between 50 and 1200 ppm B. Diopside. calcite, clintonite, phlo gopite, brucite, hedenbergite, tremolite, and other minerals host B to a le sser extent. Aureole B enrichments correlate well with major and other trac e-element enrichments, and support existing models of element transport in magmatic fluids with lateral down-temperature flow. Large variations in min eral B concentrations reflect changes in B concentrations of these fluids t hrough rime. Mass-balance calculations indicate that magmatic fluids emanat ing from the Alta pluton could supply most B in the Alta aureole. It is est imated that the emplaced magma had an initial B concentration between 7-10 ppm; indicated exhalative losses of B from the pluton are on the order of 5 0%. We estimate that the exsolved fluids had a time-integrated B concentrat ion of 160 +/- 40 ppm, although much higher concentrations may have attende d local berate mineralization.