UNCONVENTIONAL RUTILE DEPOSITS IN THE QUEBEC APPALACHIANS: PRODUCT OF HYPOGENE ENRICHMENT DURING LOW-GRADE METAMORPHISM

Abstract

Unconventional rutile deposits occur in heavy mineral paleoplacers of the Pinnacle Formation in the Humber zone of the Quebec Appalachians. Detrital hemo-ilmenite represents 65 percent of the heavy mineral assemblage and is the main titanium mineral. Detrital rutile is also present in trace amounts. Postdepositional and metamorphic alteration processes increased the titanium content of the hemo-ilmenite grains by leaching elements, particularly Fe. New minerals created by these processes include pseudorutile, anatase (a TiO 2 polymorph), and neorutile. Early metamorphic magnetite enclosed detrital grains and preserved evidence of premetamorphic weathering or diagenetic processes by isolating the grains from metamorphic fluids, thus preventing further leaching. Detrital grains that were not protected by this magnetite were metamorphically leached to the purer TiO 2 mineral phase of anatase. Grains of anatase locally recrystallized to neorutile during a late orogenic back-thrusting event in the Silurian. This is supported by δ 18 O water in equilibrium with metamorphic magnetite and by the fact that neorutile is limited to the eastern part of a synformal keel near the back-thrust fault and is rarely observed farther west, where anatase is predominant. Rutile formed under greenschist metamorphic conditions from primary hemo-ilmenite detrital grains via intermediate pseudorutile and anatase phases. This is the first known report of titanium mineral enrichment occurring under low-pressure and low-temperature metamorphic conditions. Our results could have significant implications for titanium exploration because they demonstrate that paleoplacer source regions are not restricted to eclogite and granulite facies terrains.