The late Archean Hemlo gold deposit, Ontario, Canada: a review and synthesis

Abstract

The Hemlo gold deposit, northwestern Ontario, Canada, is situated within the late Archean Hemlo-Heron Bay greenstone belt of the Wawa Subprovince of the Superior Province. The greenstone belt represents an ancient volcanic island arc and has subsequently been subjected to a complex history of evolution, including two episodes of regional metamorphism, three main pulses of granitoid plutonism, and four generations of structures related to two major deformational events. The main (second) regional metamorphism was a low P T event and generally belonged to greenschist-amphibolite transition facies, but climaxed at middle amphibolite facies along both margins and the central axis of the greenstone belt. The Hemlo gold deposit is located within the central metamorphic zone of middle amphibolite facies. The main orebody contains approximately 80 Mt of ore averaging 7.7 g/t Au, and is 2900 m along strike, 1300 m down dip and 3 to 45 m thick. Additional gold mineralization extends sporadically and discontinuously westward, and includes the C zone of the Williams mine and the North, South and Highway zones of the Golden Sceptre Resources Limited property. The main orebody and North zone are contained within a major shear zone, proximal to a lithological contact between structurally underlying felsic rocks and overlying metasedimentary rocks. The C and South zones are entirely within the felsic rocks and commonly are present as intensively sheared and altered lenses. The Hemlo deposit is enriched in As, Ba, Hg, Mo, S, Sb, Te, Tl and V. Its economic gold concentrations correlate with the isotopic compositions of sulphide minerals but not their total abundances. The Hemlo gold deposit is characterized by intensive and extensive hydrothermal alteration, including potassic alteration, calc-silicate alteration, sulphidation, and, less significant silicification, tourmalinization, and carbonatization. Field occurrence, textural relationships, quantitative P-T-X-t data from mineral equilibria, oxygen isotope geothermometry, fluid inclusion microthermometry, and UPb and 40 Ar 39 Ar geochronology suggest a protracted history of hydrothermal activity, which commenced with an earlier calc-silicate alteration (550°C and 3–4 kbar; 2678-2676 Ma) immediately after the peak of the second regional metamorphism, followed by pervasive potassic alteration (mica schists and microcline-rich rocks; 450°–500°C; 2672-2670 Ma), and terminated with a later, low- to very-low-grade, calc-silicate alteration (200°–400°C, 1–2 kbar and 2643-2632 Ma). There remains uncertainty among investigators regarding the timing and style of gold mineralization of the Hemlo deposit. Consequently, many ore-forming processes have been proposed for its origin, including syngenetic, metamorphic replacement and porphyry models (suggesting gold mineralization either pre-dating or during the second regional metamorphism), and also structural and hot spring/epithermal models (indicating mineralization post-dating the second regional metamorphism). We suggest that the deposition of metals and sulphides in the Hemlo deposit is most consistent with a multistage late replacement model. The Hemlo Shear Zone was activated as a regional planar structure during the second regional metamorphism, and became a conduit for regional fluids during periods of dilatancy. Orogenic activity was protracted, and fluids focused along the conduit could have been variously of metamorphic and magmatic origin at different stages in the development of the Hemlo deposit. Gold mineralization most likely occurred during the late calc-silicate alteration event, about 40 Ma after the second regional metamorphism.