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.

Petrogenetic Study of the Multiphase Chibougamau Pluton: Archaean Magmas Associated with Cu–Au Magmato-Hydrothermal Systems

“…In Archean greenstone belts, however, magma intrusions remain poorly documented and underexplored entities, as already noted in the 1980s [1]. The occurrence of porphyry-style mineralization during the Archean has now been recognized by many authors [14,123,124], but much remains to be done to refine the metallogenic model of these ancient deposits. To achieve this goal, ore-body descriptions must be completed, but attention must also be paid to the associated magma intrusions.…”

Intrusion-Associated Gold Systems and Multistage Metallogenic Processes in the Neoarchean Abitibi Greenstone Belt

“…They correspond to the Bachelor Au Mine associated with the O'Brien stock, the Cu-Au ± Mo mineralization associated with the East Sullivan stock, as well as the Cu-Au-Mo Upper Beaver and the Au-REE Lac Shortt deposits (Figure 4, Table 1). Most of these systems are described as porphyry-style mineralization [51][52][53] but formed during the syntectonic period and may then correspond to IRGS, according to the classification adopted here. The mineralization can also be structurally controlled (Bachelor and Lac Shortt), as the circulation of gold-bearing fluids may be coeval with major deformation events [51,54].…”

“…Nearby (Figure 4), the poorly documented Au-W-Ag Dolodeau mineralization is also spatially associated with a syenite-carbonatite complex [56,57]. The O'Brien stock comprises alkali-calcic to alkaline phases and is F-enriched, which explains the low temperature (650-700 • C) of its solidus [51]. The East Sullivan alkaline intrusion is also a polyphase pluton and displays anomalous Fcontents [52].…”

“…The East Sullivan alkaline intrusion is also a polyphase pluton and displays anomalous Fcontents [52]. The O'Brien and East Sullivan intrusions may correspond to sanukitoids [51,52]. The Upper Beaver complex also comprises a diversity of intrusive phases, such as monzonite, diorite, and granodiorite [53].…”

“…Alkaline magmatism is also generally considered oxidized, as indicated by petrogenetic studies [60] and direct measurements of the fO 2 parameter using zircon chemistry [11]. Such magma exsolved oxidized fluids, as indicated by characteristic alteration minerals, such as sulfates, magnetite, and hematite [51,53]. Indeed, in Archean contexts, oxidized fluids are rare and are generally exclusively exsolved from magmas [45].…”