Mineralogical and geochemical features of high-alumina fenites of the Mont Saint-Hilaire alkaline complex, Québec, Canada

Yakovleva O.S.*, Pekov I.V.*^,**, Bryzgalov I.A.*, Horváth L.***, Yapaskurt V.O.*, Guseva E.V.*

¹Moscow State University, Faculty of Geology, Moscow, Russia;

²Institute of Geochemistry and Analytical Chemistry of RAS, Moscow, Russia;

Á³Hudson Heights, Québec, Canada.

The Mont Saint-Hilaire complex, part of the Monteregian Hills alkaline province, is situated 40 km east of Montréal, Québec, Canada, forming a roughly circular intrusion emplaced in Ordovician shales and limestones. Total area of the massif is about 8 km². The Mont Saint-Hilaire complex was formed by three separate igneous intrusion phases, which resulted in three distinct suites of intrusive rocks; the oldest (western) one is represented by basic and alkaline ultrabasic rocks such as several varieties of gabbros and jacupirangites; the second suite includes nepheline gabbros, diorites and monzonites and biotite trachygabbro; the youngest (eastern), alkali-rich intrusive suite consists of nepheline syenites (including their peralkaline varieties), their pegmatites, igneous breccias and various xenoliths. Thick hornfels aureole surrounding the massif is a product of the contact metamorphism (Currie et al., 1986). High-alumina rocks found at Mont Saint-Hilaire have not been studied before, only the occurrence of corundum, columbite and hercynite were noted. This paper is the first systematic characterization of these rocks, considered by us as specific fenites.

High-alumina rocks, uncovered in the Poudrette quarry, form small bodies at the contact of alkaline pegmatites with hornfels and granites. Also they replace granite veins. These fenites are fine- to medium-grained rocks consisting substantially of feldspars (albite, K,Na feldspar and potassic feldspars are abundant, albite-oligoclase is subordinate). Hercynite (in all the rocks discussed below) and biotite (in almost all rocks) are the other rock-forming minerals. Typically biotite is high-alumina annite close to siderophyllite. Sporadically nepheline becomes a rock-forming mineral.

We have subdivided the high-alumina oxide-silicate rocks of Mont Saint-Hilaire to four types:

(1) A rock with significant biotite content (to 15%). Isolated spessartine crystals (up to 1 cm) are typical for this rock. Concentrically-zonal polymineral rims surround spessartine crystals. Internal part of a rim consists of biotite with Mn-rich hercynite inclusions, whereas the outer part is composed of pyrrhotite with chalcopyrite and pyrophanite inclusions. Corundum is a typical accessory mineral.

(2) A rock with corundum-hercynite intergrowths surrounded by annite rims (sometimes with muscovite). Siderite is typically observed in cores of hercynite grains. The rock contains rare impregnation of pyrrhotite and manganocolumbite.

(3) A rock with Zn-rich hercynite grains with annite rims and intergrowths of biotite and muscovite. A lot of zoned well-shaped and skeletal zircon crystals occur there. The rock is strongly altered and contains different carbonates: sideriteãmagnesite series members, ankerite, dolomite; calcite, mckelveyite, Ca,REE fluorocarbonates are subordinate; late natrolite and fluorite are also present. Accessory minerals are members of the ferrocolumbiteãmanganocolumbite series, ilmenite, Nb-bearing rutile (var. ilmenorutile), uedaite-(Ce), pyrrhotite, monazite-(¶È), xenotime-(Y), and a Ti-free REE niobate, probably fergusonite-(Ce). Isolated inclusions consisting of quartz (60%) and biotite (typically annite) (40%) are observed in a rock. Some biotite crystals contain inclusions of metacrystals of Ti-bearing spinelides of the magnetite-chromite series.

(4) A rock with equal impregnation of Zn-rich hercynite (up to 20%), accessory zircon and ilmenite (with ferrocolumbite microinclusions). Sporadically, hercynite forms aggregations containing clusters of zircon with thorite inclusions. There are big (up to 6 mm) equant idiomorphic crystals of Fe-rich spessartine, as well as isolated corundum crystals (up to 1 mm) intergrown with muscovite in the rock. Pyrophanite, pyrrhotite, chalcopyrite, sphalerite, galena and rhodochrosite are also present in this rock.

Similar rocks are known in large fenitized xenoliths in the Khibiny massif (Kola Peninsula, Russia) (Shlyukova, 1986; Yakovenchuk et al., 2005; Yakovleva et al., 2006). High-alumina rocks in two agpaitic massifs, Mont Saint-Hilaire and Khibiny, have a lot of common mineralogical and geochemical features. These rocks are Si-poor in silica and, as a result, oxides became rock-forming minerals: hercynite at first and corundum in some cases (Khibiny). The main rock-forming minerals in both Mont Saint-Hilaire and Khibiny are alkali feldspars, hercynite, biotite (typically annite), quite often muscovite; corundum and Al-garnets occur and accessory minerals are similar. Pyroxenes and amphiboles are absent in Mont Saint-Hilaire high-alumina rocks that is also typical for the majority of the Khibiny high-alumina fenites.

ÁBesides the common mineralogical features, we note some differences. Majority of Khibiny high-alumina fenites contain nepheline that is not typical for the high-alumina Mont Saint-Hilaire rocks. Carbonates are widespread at Mont Saint-Hilaire unlike at Khibiny.

The oxide-silicate rocks of Mont Saint-Hilaire and Khibiny are characterized by high contents of Al, Fe, Na, and K while concentrations of Ca and Mg are low. Similar rare-element speciality of high-alumina rocks of both massifs should be noted: the majority of accessory minerals contain Nb (+Ta), Zr, Ce (+ other LREE), and Th, less Y (+HREE), Sr, Ba, and U.

When the geochemistry of high-alumina rocks of both alkaline massifs are compared, we noted the higher alkalinity (especially Na) for Khibiny and relative enrichment in CO₂ and Mn for Mont Saint-Hilaire rocks. Content of S in high-alumina rocks of Mont Saint-Hilaire is lower compared to Khibiny, while contents of rare elements are close. The important common feature at both localities is the reducing conditions of mineral formation, that is shown by the abundance of hercynite (both massifs), and the presence of native iron (Khibiny) or Fe²⁺ carbonates (Mont Saint-Hilaire).

Geochemically, the high-alumina fenites contrast sharply with relatively Al-poor host agpaitic rocks. It can be explained by the nature of a protholite that coud be initially Al-rich. In Khibiny, the protholite is most likely metamorphic rocks (muscovite shales or gneisses) formed after clays, whereas in Mont Saint-Hilaire it was hornfels or granite. High-alumina rocks of Khibiny were altered by alkaline fluids more strongly compared to their Mont Saint-Hilaire analogues. In the Khibiny xenoliths, nepheline-bearing rocks are typical products of the fenitization unlike the majority of Mont Saint-Hilaire high-alumina rocks, in which almost all Na is concentrated in feldspars. Note that nepheline-free varieties of high-alumina fenites, very similar to Mont Saint-Hilaire ones, are common also at Khibiny.

This study was supported by grant of President of Russain Federation No. 863.2008.5, grant of RFBR No. 08-05-00077-a and grant of Russian Science Support Foundation (I.V.P.).

References

Currie K.L., Eby G.N., Gittins J. The petrology of Mont St-Hilaire complex, southern Quebec: An alkaline gabbro-peralkaline syenite association // Lithos, 1986, 19, pp. 65-81.

Shlyukova Z.V. Mineralogy of Contact Formations of the Khibiny Massif. Moscow: Science , 1986, 97 pp. (in Russian).

Yakovenchuk V., Ivanyuk G., Pakhomovsky Ya., Men'shikov Yu. Khibiny. Apatity: Laplandia Minerals, 2005, 468 pp.

Yakovleva O.S., Pekov I.V., Kononkova N.N. Hercynite from alkaline alumina rocks of fenitized xenoliths in the Khibiny massif // Proc. All-Russian Conference Geochemistry, Petrology, Mineralogy and Genesis of Alkaline Rocks, Miass, 2006, pp. 310-313 (in Russian).