Archean Sanukitoid Gold Porphyry Deposits: A New Understanding and Genetic Model from the Lac Bachelor Gold Deposit, Abitibi, Canada

Abstract

In the Abitibi subprovince of the Superior craton, Canada, recognition of intrusion-related gold deposits is improving, but their formation remains poorly understood. The recently reopened Lac Bachelor deposit in the northeastern part of the Abitibi subprovince is located on the edges of a syenitic complex, and this deposit is an example of a highly fractionated, Fe-Mg–poor, quartz-syenite to alkali-granite centered deposit. Gold is associated with pyrite in localized alkaline, fluorine, and hematite-rich metasomatized zones in magnetite-rich host rock. Minor mineralization is present in a quartz vein stockwork on the edges of the syenitic complex. A genetic model is proposed, wherein alkaline-oxidized magma, which favors sulfur and gold solubility and transport, migrates through the crust via major and subsidiary faults. The presence of miaroles, transolvus, and low-temperature crystallization conditions confirms the shallow emplacement of the magma. The high fluorine content of the magma, which lowers both viscosity and solidus temperature, favored the shallow emplacement of the magma and permitted the development of a long-lived magmatic-hydrothermal system. Oxygen and hydrogen isotopes, miaroles filling, and mineralized zone paragenesis confirm that oxidized hydrothermal fluids were exsolved from the magma. The planar geometry of the metasomatized ore zones indicates that hydrothermal fluid circulation and metasomatism reactions occurred preferentially in high permeability pathways, such as preexisting faults in the host rock. The redox contrast between the oxidized magmatic-hydrothermal fluid and the intermediate volcanic host rock caused sulfide precipitation and gold deposition. This genetic model is applied to the Abitibi subprovince and other Archean cratons.