THE ORIGIN OF RARE MINERALS IN THE KIPAWA SYENITE COMPLEX, WESTERN QUEBEC*

KENNETH L. CURRIE and OTTO VAN BREEMEN
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A OE8

* Geological Survey of Canada contribution number 57638.


Abstract

The Kipawa Syenite Complex, in the Grenville Province of western Quebec, a concordant, folded sheet of mildly peralkaline kataphorite-aegirine syenite less than 200 m thick, can be traced for more than 50 km in the Grenville Province of western Quebec. Several lenses of biotite-aegirine nepheline syenite up to 200 long by 30 m thick occur within the complex. Along its lower margin, a region 1300 m long by 5 m thick consists of diopside and magnesiorichterite-rich schist with large amounts of eudialyte, agrellite and other rare minerals. This skarn-like lens is fringed on its upper side by alkali granite. The Kipawa Syenite Complex lies entirely within granitic gneiss derived from an Archean protolith and emplaced at 1247 47 Ma (Guo & Dickin 1994). The complex, emplaced about 1240 Ma ago, consists mainly of deformed alkaline igneous rocks (plutonic, volcanic, or both) as shown by its geochemical signature. Marble-bearing sedimentary rocks were thrust-imbricated with the complex during northwest-directed tectonic transport at amphibolite- facies (690 C, 9.5 kilobars) metamorphic conditions. Mechanical mixing during this process, followed by alkali metasomatism and local anatectic melting, produced skarn-like rocks (contents of CaO and MgO of 12 and 10 wt.%) containing the rare minerals. U-Pb dating of zircon from the rare mineral occurrence shows that rare mineral formation occurred at 994 2 Ma. Alkalis, Zr, REE, Be and other elements were redistributed on scales ranging from meters to kilometers by a combination of fluid flow and anatectic melting in the presence of F-rich brines.