高硫成矿系统中明矾石的稳定同位素特征

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

根据矿物组合及蚀变特征,浅成低温热液型金矿可划分为高硫化型和低硫化型,明矾石是高硫化型金矿床中典型矿物。高硫化型矿床中的酸性硫酸盐蚀变共分为4种环境：岩浆热液环境、岩浆蒸汽环境、蒸汽-加热环境和表生环境。岩浆热液环境中明矾石的δD值接近岩浆水,δ^18OOH值大于岩浆水,δ^34S值比共生的黄铁矿高1.6%-2.8%,δ^18OSO4值通常为0.8%-1.6%,Δ^34Salunite-py值可很好地反映成矿温度;岩浆蒸汽环境中明矾石的δ^34S值接近δ^34SΣS值,且δD和δ^18OSO4值接近岩浆流体,δ^18OOH值与流体平衡;蒸汽-加热环境中明矾石的δ^34S值与初期H2S相同,δD值与当地大气水相同,δ^18OSO4值与δ^18OH2O值反映大气水与围岩交换的程度;表生环境中明矾石δ^34S值与早期硫化物相同,而δD值比当地地表水略小,δD值和δ^18OOH值位于与大气水平行的区域,Δ^18OSO4-OH值通常为负值。明矾石的稳定同位素特征可以很好地反映高硫型浅成低温热液型矿床的形成环境和流体来源,对找矿工作有着重要的指示作用。

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,δ^18OOHvalues are larger than those of magmatic water,δ34S values are 16 to 28 per mil larger than those of associated pyrite,δ^18OSO4values are usually 8 to 16 per mil larger,and Δ^34Salunite-pyvalues are generally suitable for temperature determinations.Alunites of magmatic steam origin have δ^34S values near δ34SΣS values and δD and δ^18OSO4 values near magmatic values,and δ^18OOH 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 δ^18OSO4 and δ^18OH2O values also reflect the degree of exchange of the meteoric fluids with wall rock.Supergene alunites have δ^34S 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 Δ^18OSO4-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.