The Role of Sulfate-Sulfide-Oxide-Silicate Equilibria in the Metamorphism of Hydrothermal Alteration at the Hemlo Gold Deposit, Ontario

摘要

The Hemlo Au (-Mo) deposit is an example of atypical, greenstone-hosted, mesothermal mineralization from the Superior province of north-central Ontario. The ore is shear hosted and consists primarily of disseminated native gold in several mineralized zones near the contact between quartz-feldspar porphyry and metasedimentary rocks. Metsasedimentary rocks (i.e., pelite, graywacke, arenite, marl, mafic fragmental rocks, and baritic sediment) host most of the ore. The dominant alteration is potassic and is characterized by a central zone of granoblastic microcline-quartz-(barite-muscovite-biotite-pyrite) rocks, surrounded by muscovite-quartz-pyrite schists. Regional, amphibolite-facies metamorphism (~630°C, 5–7 kbars) overprinted the ore zones and recrystallized the alteration assemblages. Unaltered metasedimentary wall rocks contain the assemblages biotitegarnet-staurolite-kyanite and/or sillimanite (metapelite) and amphibole-plagioclase (mafic fragmental rocks and marly sediment). As the ore zones are approached, the proportions of muscovite and K-feldspar and the mole fraction of pyrope in garnet increase, garnet, staurolite, and kyanite gradually disappear, biotite becomes Mg rich, ilmenite is replaced by rutile, and pyrrhotite is replaced by pyrite. In altered and mineralized mafic fragmental rocks, biotite replaced amphibole, which indicates that amphibole was present prior to alteration and that gold mineralization occurred at pressure-temperature conditions equivalent to those of the greenschist facies (400°–500°C, 3–5 kbars).