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

Abstract

The Chibougamau pluton is a Neoarchean multiphase intrusion that is related to Cu–Au porphyry-style deposits. In Archean greenstone belts, porphyries are marginal and poorly documented mineralizations. Such deposits are, however, important in the Chibougamau area, where the main historical mining camp (Central Camp) is a magmato-hydrothermal system. Understanding such systems requires documenting the related magmatic rocks. This contribution focuses on the petrogenesis of the Chibougamau pluton to elucidate how the intrusion participated in Cu and Au mineralized systems. Using field descriptions, whole-rock analyses, and petrographic observations, we describe the source, emplacement mechanism, and chemical evolution of the Chibougamau pluton. The Chibougamau pluton is a TTD (tonalite-trondhjemite-diorite) suite that contains more K than most plutons of similar age. This suite was produced from a heterogeneous source; i.e., a hydrated basalt and possibly a metasomatized mantle. These are rare (and thus prospective) characteristics for an Archean intrusion. In addition, differentiation may have been sufficiently prolonged in the diorite phase to concentrate metals and fluids in the evolved magma. These magmatic constraints must now be tested against a renewed understanding of the Cu-dominated mineralized systems of the Chibougamau area.

Tonalite-Dominated Magmatism in the Abitibi Subprovince, Canada, and Significance for Cu-Au Magmatic-Hydrothermal Systems

“…Most of the intermediate-felsic plutons of the Abitibi Subprovince are multiphase intrusions. Two or more intrusive phases with distinct composition and/or ages can generally be identified at the outcrop scale, e.g., the Chibougamau pluton [6]. At the scale of the Abitibi Subprovince, such areas are mapped as units made of two or more lithologies and the MERN map contains hundreds of unit types.…”

“…Some intrusive suites are clusters of tonalite and granodiorite plutons, while others are made of several phases emplaced in a restricted area to form multiphase plutons. Note that intrusive phases emplaced in the same area are not necessarily coeval, e.g., the Chibougamau pluton [6].…”

“…In greenstone belts, the main magmatic phase produced magmas with tholeiitic affinities (mafic lava flows and layered intrusions), as well as tonalite-and granodiorite-dominated large-volume batholiths [2][3][4]. In the Abitibi Subprovince, an additional rock type (diorite) is observed as part of tonalite plutons associated with Cu-Au magmatic-hydrothermal systems [5,6]. The petrogenetic evolution, as well as the geodynamic and economic significance of the tonalite, granodiorite, and diorite suite, is not fully resolved [4].…”

“…Less recognized mineralization corresponds to magmatic-hydrothermal systems related to intermediate-felsic intrusions of the synvolcanic period. These systems are considered as porphyry-style mineralization, and examples include the Central Camp Cu-Au and the Côté-Gold Au-(Cu) deposits associated, respectively, with the Chibougamau pluton and the Chester intrusive complex [5,6] (Figure 1). In greenstone belts, granitoid intrusions of the synvolcanic period are mostly tonalite-trondhjemite-granodiorite (TTG) suites [9,10], which are felsic (>64 wt% SiO 2 ), aluminous (>14-16 wt% Al 2 O 3 for 70 wt% SiO 2 ), and Fe-Mg-poor ([Fe 2 O 3T + MgO + MnO + TiO 2 ] < 5 wt%) intrusive rocks characterized by a low K 2 O/Na 2 O ratio (<0.6) [10].…”

“…Most TTG suites are also HREE-depleted which reflects their derivation from high-pressure partial melting of mafic crust-i.e., garnet amphibolite [10]. However, the Chibougamau and Chester intrusive complexes are too mafic to be classified as TTG suites, as they contain significant amounts of diorite, and thus, are referred to as tonalite-trondhjemite-diorite (TTD) suites [5,6].…”