An exsolution origin for "low-temperature" sulphides at the Hemlo gold deposit, Ontario, Canada

Powell, W.G., and Pattison, D.R.M.

1997, Economic Geology, v. 92, pp. 569-577

Many aspects of the timing and genesis of the Archean Hemlo gold deposit are unresolved; pre-, syn- and postmetamorphic models have been proposed. The abundance of low-temperature sulfide minerals within highly deformed, sillimanite-grade host rocks have been used to support a post-metamorphic origin for the deposit. However, the present Sb-As-Hg ore assemblage, including cinnabar-realgar-orpiment assemblages developed through a sequence of exsolution events during postpeak metamorphic cooling. At peak metamorphism, most of the As and Hg, along with Tl, Cu, Zn and minor Fe, was incorporated into a high-temperature antimonian sulfosalt. In the lower amphibolite facies or upper greenschist facies, impure (Hg, Zn)S exsolved from the high-temperature phase. An impure chalcopyrite and native antimony exsolved from the host sulfosalt soon after, leaving behind an Sb₂S₃-As₂S₃ solid solution. Further cooling resulted in the exsolution of cinnabar and mercurian sphalerite from the (Hg, Zn)S. Impurities in the cinnabar and chalcopyrite exsolved to form aktashite, arsenopyrite and native antimony. Eventually, orpiment exsolved from the arsenic-bearing stibnite. Late, retrograde events converted the free orpiment to realgar and remobilized the realgar over short distances, along with cinnabar and native antimony. The history of sequential sulfide exsolution, along with the deformed nature of the Sb-As-Hg-bearing veins, and their metamorphosed alteration selvages, suggests that the metals were introduces prior to, or during, peak metamorphism.