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