|Author:||Dawson, 1966; Pilote, 2018|
|Type area:||NTS sheets 32C05 and 32D08|
|Geological province:||Superior Province|
|Geological subdivision:||Abitibi Subprovince|
|Lithology:||Granitoids (mafic to felsic intrusions)|
Table des matières
The first reports referring to geological aspects of this suite were those produced by Bancroft (1912), Cooke et al. (1931) and Tremblay (1950). These reports were followed by several township-wide mapping campaigns (see Imreh 1984, 1991), studies and numerous exploration projects (Hawley, 1931; Norman, 1945; Derry, 1949; Rowe, 1953; Siroonian et al., 1959). The first comprehensive studies encompassing this batholith and subdividing it into several individual batholiths were those produced by Dawson (1954, 1966). Subsequently came the work of Danis (1985), Bourne and Danis (1987), Boily et al. (1989, 1990), Rive et al. (1990), Boily (1992, 1995) and Mulja et al. (1995a, 1995b). Dawson (1966) was the first to combine the Preissac, La Motte and La Corne individual batholiths under the overall name Preissac-La Corne Batholith. It has been used for several decades. In consideration of the distinct geochemical nature and varying ages of these different batholiths, the name Preissac-La Corne Plutonic Suite is proposed (Pilote, 2018). This suite includes the three Preissac, La Motte and La Corne batholiths.
The Preissac-La Corne Plutonic Suite outcrops in the southern Abitibi Subprovince, about 40 km NE of the town of Val-d’Or (Tremblay, 1950; Dawson, 1966). This suite, syn-kinematic to late-kinematic, intruded in the Archean (U-Pb dating 2681-2643 Ma; Feng and Kerrich, 1991) into volcano-sedimentary rocks metamorphosed to the greenschist to lower amphibolite facies of the Caste Formation (Imreh, 1984), Malartic Group and Kinojévis Group. The Caste Formation is contained in the Manneville Thrust Zone (Daigneault et al., 2002, 2004).
The work of Bourne and Danis (1987), Boily et al. (1992, 1995) and Mulja (1995a and 1995b) allows for the subdivision of the Preissac-La Corne Plutonic Suite into two distinct magmatic suites, namely: 1) an early dioritic to granodioritic suite, metaluminous, containing numerous metasedimentary and metavolcanic xenoliths; and 2) a late peraluminous monzogranitic suite, moderately to non-foliated, containing few xenoliths and associated with a pegmatitic aureole mineralized in Li, Be, Ta and Mo. The monzogranitic suite is partially to completely represented by the three Preissac, La Corne and La Motte batholiths, whose surface exposure ranges from satisfactory to very good.
Oxygen and neodymium isotopic data, combined with major and trace element composition, indicate that biotite monzogranite is derived from partial melting of a quartzo-feldspathic source (Boily et al., 1992, 1995). It is not the residual liquid resulting from fractional crystallization of a granodioritic cognite of the early suite. However, various factors control differentiation of granitic magma from more primitive magmas (biotite) to more evolved magmas (muscovite-garnet), such as fractional crystallization, thermogravitational diffusion, transport by volatiles, or a combination of these. These processes are responsible for depletion in Fe, K, Ba, Sr, Zr, light rare earth elements (LREE) and Th, and enrichment in Nb, Ta, U and Mo that characterize the evolution of the peraluminous suite. However, low Li and Cs contents of the muscovite-garnet monzogranitic facies (with respect to the biotite-muscovite monzogranitic facies) suggest that monzogranite comes from a magmatic chamber subject to an important chemical and thermal gradient. It is proposed that the formation and migration of chloride complexes with Li, cs, Be, Ta and Nb contributed to enrichment of the apical portion of this magmatic chamber at the expense of its median portion (Boily et al., 1992, 1995).
The first granitoid bodies to crystallize, biotite-muscovite monzogranites, form a carapace under which residual liquids accumulate. They expel spodumene pegmatites (with high contents of Cs, Rb, Ta, Nb and Be) in the walls (Boily et al., 1992, 1995). These residual liquids gradually evolve towards biotite-muscovite monzogranites that generate granitic pegmatites emplaced in fractures and joints of the biotite monzogranite carapace. These pegmatites, which come from the median portion of the magmatic chamber not enriched in lithophile elements, form non-economic beryl and colombo-tantalite pegmatites. This model (Boily et al., 1992, 1995) can explain the coarse zonation of the pegmatitic shell of the Preissac-La Corne Plutonic Suite (i.e., lithium pegmatites in the walls and non-economic within monzogranite plutons). The last residual liquids that cannot pierce the granite carapace are emplaced on the pluton periphery, in contact with surrounding rocks, to form the muscovite-garnet monzogranitic facies. Low temperature (<350°C) and molybdenum-rich aqueous fluids are expelled during consolidation and circulate through monzogranite and walls. These fluids will then precipitate into quartz-feldspath-muscovite-molybdenite veins and veinlets in fractures (Boily et al., 1992, 1995).
The La Motte Batholith is relatively poorly exposed in outcrop. It intrudes into biotite schists of the Caste Formation. The majority of this batholith consists of muscovite-biotite monzogranite and monzonite, granite and granodiorite, aplite and granitic pegmatite (unit nAmo), and granodiorite and early quartz monzonite with hornblende or biotite (unit nAmo2). These lithologies have an E-W-oriented magmatic foliation, varying in intensity from moderate to strong. There are some outcrops of muscovite-garnet monzogranite on the eastern, southern and western edges of the pluton. Monzogranites are cut by granitic pegmatite and aplite dykes, but these are concentrated in a 500 m area on the northern edge and south of this pluton. Several pegmatites contain beryl and colombo-tantalite; however, very few of them contain spodumene. The density of pegmatites in and around the La Motte Batholith, however, does not reach the level observed inside the La Corne Batholith.
Unlike the La Corne Batholith, an external well-developed pegmatite aureole is not recognized in this batholith, but this is probably due to the low exposure rate of surrounding rocks. Molybdenum prospects and showings are mainly located on the western edge of the pluton and were explored during the 1910s to 1920s. However, there are few outcrops. Molybdenite mineralized quartz-muscovite-feldspar veins are visible on the southern edge of the batholith, in contact with biotite schists. These veins are located near pegmatites mineralized in colombo-tantalite, highlighting their otherwise genetic, at least spatial association (Boily et al., 1992, 1995).
Located in the SW part of Preissac-La Corne Plutonic Suite, the Preissac Batholith outcrops relatively well. The two main units that make up this batholith are represented by a whitish muscovite-garnet monzogranite (nAprs2, western part) and a pinkish muscovite-biotite monzogranite (nAprs1, eastern part). Both facies are medium to coarse grained. Monzogranite, whitish to pinkish, has an extremely homogeneous structure and outcrops as moderate-relief hills. Contact with surrounding rocks, biotite schists and mafic metavolcanic rocks, is very sharp, although there is a contact metamorphism zone in metasedimentary rocks that extends over a distance of a few metres (marked by the emergence of cordierite and staurotide). Monzogranites of the Preissac Batholit contain very few xenoliths.
The La Corne Batholith is located in the eastern part of the Preissac-La Corne Plutonic Suite, it is elongated along an N-S axis. Tectonic foliation, marked by orientation of biotite and muscovite flakes, is evident on the margins of the La Corne Batholith; this foliation, however, becomes less pronounced in its center. The whole batholith is surrounded by an aureole of lithium granitic pegmatites, mainly developed in the western and northern parts, and within which many mineralized showings and prospects have been recognized (see deposit records cited in Pilote ; Dawson, 1966; Boily et al., 1989; Boily, 1995). The Québec Lithium Mine is located on the northern contact of the La Corne Batholith and is part of this aureole. Lithium pegmatites also contain varying amounts (trace-2%) of colombo-tantalite, molybdenite and beryl.
The work of Bourne and Danis (1987), carried out almost exclusively in the SW portion of the La Corne Batholith, proposed subdividing the early suite into two distinct plutonic facies. The first, called the inner zone (unit nAlac3), has compositions ranging from brecciated biotite-hornblende gabbro to monzonite, it contains less than 5% modal quartz. The second facies, called the outer zone, contains more than 5% modal quartz and consists of biotite-hornblende quartz diorite (nAlac5) that progressively evolves towards biotite-hornblende granodiorite (nAlac1). The transition between each facies is relatively steep, but continuous. These two facies are reversely zoned, i.e. cores are mafic and margins are more felsic. A strip of metasomatized granodioritic rocks appears on contact with the outer suite and surrounding sedimentary rocks. This strip is characterized by myrmekitic plagioclase and albitic-margin plagioclase, as well as higher quartz and biotite contents than other plutonic rocks in the outer zone.
The early dioritic to granodioritic suite covers almost all of this batholith. In particular, it outcrops along a N-S-oriented strip located between the villages of Vassan and La Corne on contact with surrounding volcano-sedimentary rocks. A few outcrops also appear on the eastern and western shores of Malartic Lake (sheet 32D08). In general, dioritic to granodioric intrusive masses consist of small, shallow outcrops with poor exposure compared to monzogranitic phases.
For the purpose of simplification and in line with recent mapping in these areas (Pilote, 2017; Pilote and Lacoste, 2016; Pilote et al., 2016, 2017), the eight (8) lithologies recognized within this batholith have been renamed, grouped and distributed within five (5) units (nAlac1 to nAlac5). The early suite is represented by units nAlac4 (formerly hornblendite or diorite-gabbro), nAlac3 (formerly hornblende-biotite monzodiorite), nAlac5 (formerly biotite-hornblende quartz diorite; hornblende-biotite tonalite) and nAlac1 (formerly hornblende-biotite quartz monzodiorite); the late suite is represented by unit nAlac2 (formerly hornblende monzonite; hornblende-biotite monzonite; biotite-hornblende granodiorite).
The Preissac-La Corne Plutonic Suite covers most of sheets 32D07, 32D08 and 32D05. Their E-W axis reaches nearly 60 km, while their N-S axis exceeds 12 km.
The biotite-muscovite monzogranitic phase of the La Motte Batholith gave an age of 2647 ±2 Ma. For the Preissac Batholith, the different phases gave ages ranging from 2681 to 2660 Ma (Ducharme et al., 1997).
Datings performed to date show a significant time gap between early and late magmatic suite emplacements. For example, ages obtained for the early suite range from 2695 +65/-25 Ma (Steiger and Wassenburg, 1969), 2675 ±24 Ma (Feng and Kerrich, 1991; site LC-23), 2681 to 2660 Ma (Ducharme et al., 1997) and 2681 ±2 Ma (David, personal communication, 2017). For the late intrusive suite, Feng and Kerrich (1991) obtained an age of 2643 ±4 Ma (site LC-30) on a single zircon, while Ducharme et al. (1997) obtained an age of 2647 ±2 Ma.
These plutonic lithologies intruded into metasedimentary units of the Caste Formation. The La Motte Batholith has an E-W-oriented magmatic foliation, with a moderate northward dip subparallel to the general attitude of the Manneville Thrust Zone (Daigneault et al., 2002, 2004). This feature indicates that the post-tectonic La Motte Batholith borrowed this pre-existing structure to intrude and cut adjacent volcano-sedimentary lithologies.
Does not apply.