AN EXSOLUTION ORIGIN FOR LOW-TEMPERATURE SULFIDES AT THE HEMLO GOLD DEPOSIT, ONTARIO, CANADA

Many aspects of the timing and genesis of the Archean Hemlo gold depos it are unresolved; pre-, syn- and postmetamorphic models have been pro posed. The presence of abundant low-temperature sulfide minerals withi n highly deformed, sillimanite-grade host rocks has been used to suppo rt a postmetamorphic origin for the deposit. However, the present Sb-A s-Hg ore assemblage, including cinnabar-realgar-orpiment assemblages, developed through a sequence of exsolution events during postpeak meta morphic cooling. At peak metamorphism, most of the As and Hg, along wi th Tl, Cu, Zn, and minor Fe, was incorporated into a high-temperature antimonian sulfosalt. In the lower amphibolite facies or upper greensc hist facies, impure (Hg,Zn)S exsolved from the high-temperature phase. An impure chalcopyrite and native antimony exsolved from the host sul fosalt soon after, leaving behind an Sb2S3-As2S3 solid solution. Furth er cooling resulted in the exsolution of cinnabar and mercurian sphale rite from the (Hg,Zn)S. Impurities in the cinnabar and chalcopyrite ex solved to form aktashite, arsenopyrite and native antimony. Eventually , orpiment exsolved from the arsenic-bearing stibnite. Late, retrograd e events converted the free orpiment to realgar and remobilized the re algar over short distances, along with cinnabar and native antimony. T he history of sequential sulfide exsolution, along with the deformed n ature of the Sb-As-Hg-bearing veins, and their metamorphosed alteratio n selvages, suggests that the metals were introduced prior to, or duri ng, peak metamorphism.