Chemical characteristics of hydrothermal alteration at Bingham, Utah

Three chemical profiles, based on 31 analyses of intrusive igneous rocks, are used to characterize the relations of chemical changes to alteration mineralogy at Bingham, Utah. The chemical trends are reasonably coherent along the profiles, indicating a large-scale uniformity of alteration. Average compositions show cumulative effects of successive biotitic and sericitic alterations (slight loss of Al, substantial loss of Ca + Na, increase in Na: Ca ratios) that are similar in most respects to many porphyry copper deposits.Biotitic alteration of quartz monzonite occurred with only small gains in K; losses of Mg and ferrous iron were also small. The replacement of clinopyroxene and amphibole by hydrothermal biotite represents a high-temperature hydrolysis reaction in which K and Mg are largely recombined rather than lost from the system.A chemically and mineralogically distinct, northeast-trending zone of sericitic alteration was superimposed upon biotitized host rocks in the northern half of the orebody. This hydrolytic alteration occurred under conditions near the stability boundary between K-feldspar and sericite but on the mica side of the sericite-plagioclase boundary. Sulfide mineralization and alteration were integral parts of a multistage hydrothermal history confined to a broad zone of repeatedly fractured host rocks.