Petrology, Mineralogy and Geochemistry of the Beattie Syenite and Country Rocks, Abitibi Greenstone Belt, Québec

Abstract

The Beattie syenite is composed of a series of lenticular bodies of syenitic rocks, situated immediately north of the Porcupine-Destor fault zone in the town of Duparquet, approximately 32 km north of Rouyn-Noranda in the Abitibi Subprovince. The principal body is 3.3 km long and 470 m in width and is flanked by a series of smaller lenses to the south and northeast. The intrusion has been zircon dated at 2682±1Ma and 2682.9±1.1 Ma and hosted the major part of the Au-mineralization of the now defunct Beattie mine, which was an important producer of gold in the area from 1933 to 1956 (9.66 Mt at 4.88 g/t Au). A total of 5 petrographic units are defined here, on the basis of field relationships, macroscopic textures, petrology and mineralogy:

1- The porphyritic Beattie syenite unit is composed of 2% to 10% of tabular sub- to anhedral feldspar phenocrysts about 2 mm to 10 mm in size, set in a red feldspathic and aphanitic matrix. This unit can be strongly cataclastic with abundant hydrothermal minerals.

2- The equigranular magnetite-bearing syenite unit includes few feldspar phenocrysts, about 2 mm to 10 mm in a fine-grained matrix. It is characterized by unaltered titanite, epidote, hornblende porphyroblasts and is the only unit with actinolite replacing clinopyroxene.

3- The porphyritic Central Duparquet syenite unit contains between 2% - 25% of coarse equant euhedral feldspar phenocrysts, about 5 mm to 16 mm in size, in a red or sometimes grey aphanitic matrix.

4- The megaporphyritic syenite unit is composed of very coarse alkali feldspar phenocrysts, typically 1 cm to 6 cm across, in a red aphanitic matrix. The phenocrysts often form a glomeroporphyritic texture.

5- The lath syenite unit occurs as numerous ~m-width thin dykes, which cross-cut all other petrographic units. These dykes display a characteristic trachytic texture, as defined by the preferential alignment of alkali feldspar laths, which are typically 1 cm to 3 cm in a grey or red aphanitic matrix.

Petrology and geochemistry investigations revealed that the syenite intruded into the older 2720-2718 Ma volcanic Deguisier Formation and the <2687±2 Ma sediments of the Porcupine Group. Detailed geochemical analysis revealed that the Deguisier Formation is composed of three units, forming a complete tholeiitic suite. The Deguisier Formation is proposed here to have been produced in a volcanic arc setting.

Detailed petrological, mineralogical and geochemical studies indicate that, the syenite is part of the alkaline series and is metaluminous to peraluminous. The syenite intruded at shallow depths through a sequence of injections, yielding the five comagmatic units.

The syenite and country rocks were then subjected to a series of hydrothermal alterations and brittle deformation events, combined with metamorphism to the greenschist facies. The Au-mineralization in this region is attributed to the hydrothermal alteration with brittle deformation.