Lithogeochemistry and geothermobarometry of mineralized cordierite- orthoamphibole and related rocks at Atik Lake, Manitoba, Némiscau and Montauban, Québec

• Main Author: Bernier, Louis, 1961-
• Other Authors: MacLean, W. H. (advisor)
• Format: Thesis
• Language: English
• Published: McGill University 1992
• Subjects: Geology; Geochemistry; Atik Lake; Nemiscau
• Online Access: http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=39515

Cordierite-orthoamphibole rocks associated with contrasting polymetallic mineralization occur in a wide range of metamorphic conditions in the Atik Lake, Manitoba, and Nemiscau and Montauban, Quebec, metamorphic terranes. Lithogeochemistry and geothermobarometry investigations using whole rock major, trace and rare earth elements and mineral chemistries have established the chemical and physical changes that occurred during hydrothermal alteration and subsequent regional metamorphism. The Log(Ratio) method, utilizing ratio profiles of the immobile elements $ rm Al sb2O sb3$, TiO$ sb2$, Zr, Nb, Y V and Cr, was designed to identify the protoliths of the metamorphosed altered rocks. Detailed geothermobarometry and textural evidence indicate that cordierite-orthoamphibole rocks in the three terranes formed at peak P-T conditions of ${ sim}2.5$ kbar and ${ sim}55 sp circ$C at Atik Lake (andalusite zone), ${ sim}4.5$ kbar ${ sim}650 sp circ$C at Nemiscau (sillimanite zone), and ${ sim}6.5$ kbar and ${ sim}620 sp circ$C at Montauban (kyanite-sillimanite zone). Cordierite-orthoamphibole rocks in the terranes exhibit a range of bulk chemical compositions with the most Fe-rich occurring in the Atik Lake belt, and the most Mg-rich in the Montauban mine sequence. Mineral assemblages in these rocks were in equilibrium with contrasting sulfide-oxide $ pm$ arsenide fO$ sb2$-fS$ sb2$ buffers: pyrrhotite-ilmenite $ pm$ arsenopyrite $ pm$ lollingite at Atik Lake, pyrrhotite-pyrite-magnetite at Nemiscau, and pyrrhotite-rutile-sphalerite at Montauban. The partitioning of Fe-Mg between cordierite and anthophyllite was studied for use as a geobarometer. The partitioning coefficient ($ rm K sbsp{D}{Fe-Mg}$) is slightly temperature dependant and strongly pressure dependent. This new geobarometer was empirically calibrated using mineral chemistry from the three terranes and from the literature. High $ rm X sb{H sb2O}$ of the metamorphic fluid is required so that the X$ sb{ rm Fe}$ values of the anthophyllite will be in equilibrium under peak P-T conditions with the appropriate sulfide-oxide $ pm$ arsenide assemblage. Metamorphic dehydration of chlorite-rich hydrothermally altered rocks in the three terranes is compatible with the generation of such water-rich fluids during the crystallization of the cordierite-orthoamphibole rocks.