Chapter 4: Hemlo Gold System, Superior Province, Canada

Hemlo combines several rare to unique features in the spectrum of Archean greenstone gold deposits. It is an isolated, approximately 800-metric ton (t) gold system in a region of otherwise limited known gold endowment. The geology of Hemlo is dominated by deformed and metamorphosed sedimentary, felsic volcanic, and volcaniclastic units, a premineral coherent felsic porphyry, and a swarm of mainly postmineral, intermediate, feldspar-phyric dikes. Ore is dominantly in the form of gold-bearing lenses of pyritic, feldspathic schist derived from deformation of both the clastic rocks and the felsic porphyry. The deposit and its host rocks were metamorphosed at moderate pressures to assemblages diagnostic of the mid-amphibolite facies, followed by progressive retrogression to those of the greenschist facies. The result is a wide range of silicate mineral species in ambiguous textural relationships. The gold system itself is known for ore and related alteration minerals with significant concentrations of Mo-As-Sb-Hg-Tl-V-Ba-K-Na. The inferences derived from lithologic mapping, structural chronology, U-Pb geochronology, and mineral paragenesis favors an interpretation of Hemlo as a deformed and metamorphosed gold system formed from oxidized hydrothermal fluids in an upper crustal setting. Uncertainty remains as to the exact nature and geometry of that ore-forming hydrothermal system, however, and the role subsequent metamorphism and deformation have played in the local remobilization of ore constituents into their present paragenetically late structural sites.