Characteristics of stable isotopes of alunite in high sulfidation epithermal deposit system

The epithermal gold deposit can be divided into two types according to the characteristics of mineral assemblage and alterations,i.e.high sulfur(HS) type and low sulfur(LS) type.Alunite is the typical mineral in high sulfidation gold deposit.Acid sulfate alteration of high sulfidation can be generated in four principal geologic environments,i.e.magmatic hydrothermal environment,magmatic steam environment,steam-heated environment and supergene environment.Magmatic hydrothermal alunites in the deposits have δD values close to those of magmatic water,δ18OOHvalues are larger than those of magmatic water,δ34S values are 16 to 28 per mil larger than those of associated pyrite,δ18OSO4values are usually 8 to 16 per mil larger,and Δ34Salunite-pyvalues are generally suitable for temperature determinations.Alunites of magmatic steam origin have δ34S values near δ34SΣS values and δD and δ18OSO4 values near magmatic values,and δ18OOH values are in equilibrium with magmatic values.Alunites of steam-heated origin have δ34S values the same as those of precursor H2S,and δD values similar to those of local meteoric water,and δ18OSO4 and δ18OH2O values also reflect the degree of exchange of the meteoric fluids with wall rock.Supergene alunites have δ34S values virtually identical to precursor sulfides,δD values are little lower than those of local meteoric water,δD and δ18OOH values of supergene alunites fall in a broad zone parallel to the meteoric water line,and Δ18OSO4-OH values are almost negative.The characteristics of stable isotopes of alunite may well reflect the formation environment and fluid source of high sulfidation epithermal hydrothermal deposits,which plays an important role in the instructions for the exploration work.