Discovery of a new alkaline complex in the Urban-Barry belt

Pierre Rhéaume, Daniel Bandyayera et Kamal N. M. Sharma
Direction de Géologie Québec

Recent fieldwork by Géologie Québec in the Abitibi region has outlined a new alkaline complex, the economic potential of which remains to be determined. This discovery was made in the course of a metallogenic synthesis of the Urban-Barry belt and a mapping survey in the Lac Lagacé area, under the 2003-2006 three-year plan for the Abitibi and Baie James regions.

The occurrence consists of an isolated area of outcrop exposed on an island on Lac Lacroix (PDF Format, 85 Kb). This zone is located within an elliptical positive magnetic anomaly of 3 km by 4 km in size, bounded to the north by the Ranan fault and to the south by the Lacroix fault (PDF Format, 103 Kb).

The outcrop area shows an assemblage of sodic alkaline rocks that contain 20 to 30% nepheline, 10 to 20% albite, 20 to 25% phlogopite and green biotite, and 20 to 25% carbonates (calcite).

Click for enlargement

Sample of carbonate-nepheline-bearing alkaline rock (A),
showing an enclave rimmed by an alteration zone (B)

The abundance of coarse-grained carbonate crystals and of nepheline, as well as the sodic nature of the rock, are typical of carbonatites. Moreover, these alkaline rocks exhibit cm-scale compositional layering defined by important variations in the biotite and phlogopite content. They also exhibit a tectonic foliation, at an angle of 20 to 30º relative to the compositional layering. The degree of deformation suggests the rocks are Archean in age, similar to other known carbonatites in the Abitibi such as Dolodau, Lac Shortt, Grevet and Douay. Information available to date is not sufficient to assess the distribution of these alkaline rocks, although it may quite likely coincide with the elliptical magnetic anomaly described above. Overall, alkaline rocks at Lac Lacroix show many similarities with the Saint-Honoré and Dolodau carbonatites (Bédard and Chown, 1992), where a syenite border facies surrounds a central carbonatite zone.

An unusual metallogenic setting

Carbonatite-syenite alkaline complexes offer an interesting metallogenic setting. On the one hand, these intrusions may host magmatic (primary) mineralization in high-technology metals such as niobium, tantalum and other rare earth elements, as well as certain industrial minerals such as apatite and nepheline. Niobium deposits at the Niobec mine and at Oka are two examples. On the other hand, a close relationship has been established in the Abitibi between certain gold deposits and carbonatites. In Québec, the Simard showing (Dolodau carbonatite), the Lac Shortt mine (2.7 Mt @ 4.6 g/t Au) and the Douay West deposit (0.57 Mt @ 5.7 g/t Au) are good examples of this type of mineralization. In Ontario, an association between alkaline rocks and gold mineralization is recognized in the Kirkland Lake district (Ploeger and Crocket, 1980) and at Springpole Lake (Red Lake area; Barron et al. 1989).

The area warrants additional work aimed at better defining the geological context and the economic potential of this discovery.

References

BANDYAYERA D., RHÉAUME, P., DOYON, J., SHARMA K.N.M., 2003, Géologie de la région du lac Hébert (SNRC 32G03), RG 2003-07, Ministère des Ressources Naturelles, de la Faune et des Parcs.

BARRON, K. M., DUKE, N. A., HODDER, R.W., 1989, Petrology of the Springpole lake Alkalic volcanic complex, In Geoscience research grant program summary of research 1988-1989, Ontario Geological Survey Miscellaneous Paper 143, pp. 133 – 146.

BÉDARD, L.P., CHOWN, E.H.,1992, The Dolodau dykes, Canada : an example of an archean carbonatite, Mineralogy and Petrology, volume 46, pp. 109-121.

PLOEGER, F.R., CROCKET, J.H., 1980, Relationship of gold to syenitic intrusive rocks in Kirkland Lake In R.W. Hodder and W. Petruk, Geology of Canadian gold deposits, Canadian Institute of Mining and Metallurgy, Special volume 24, pp. 69 – 72.