RUTILE-BEARING ILMENITE DEPOSITS ASSOCIATED WITH THE PROTEROZOIC SAINT-URBAIN AND LAC ALLARD ANORTHOSITE MASSIFS, GRENVILLE PROVINCE, QUEBEC

Abstract

Deposits of concentrated Fe–Ti oxide minerals are a characteristic component of many Proterozoic anorthosite massifs. Ferrian ilmenite (ilmenite with exsolution lamellae of hematite) is the principal ore mineral of titanium in the largest deposits, Lac Tio – Lac Allard, Quebec, and Tellnes, Norway. Rutile, a rare oxide mineral in these deposits, occurs as discrete grains (<5.5 mm in length) with ferrian ilmenite, and as lenses (<200 μm across) included in the ferrian ilmenite, in both the Saint-Urbain and Big Island deposits, Quebec, which intrude the 1.05 Ga Saint-Urbain anorthosite and 1.06 Ga Lac Allard anorthosite suite, respectively, of the Grenville Province. At Saint-Urbain and Big Island, both rutile and sapphirine (Mg–Fe–Al silicate; X Mg in the range 78–84) occur with ferrian ilmenite ( X hem between 11 and 30), plagioclase of intermediate composition (An39–51), high-Al orthopyroxene (5.2–9.1 wt% Al2O3; X Mg between 70 and 75), and hercynite ( X Mg in the range 61–70), with trace amounts of apatite, corundum and sulfide minerals. Textural and mass-balance constraints suggest that sapphirine formed as a result of subsolidus reactions ( e.g., hercynite + orthopyroxene + rutile ± corundum → sapphirine + ilmenite) that took place during slow regional cooling (3–4°C/million years) after crystallization. On the basis of mineral textures and a combination of bulk-rock and mineral geochemical variations, both ilmenite and the larger discrete grains of rutile in these deposits are interpreted as magmatic phases that segregated and accumulated by gravitational settling from Fe–Ti-enriched residual magmas (ferrobasaltic, jotunitic) following crystallization of the host anorthosites. The presence of the minor lenses of rutile 10 to 200 μm thick within ilmenite, typically surrounded by hematite, is related to late oxidation. The ore-forming magmas evolved under conditions essentially closed to oxygen, with preferential incorporation of Fe3+ into the early-crystallized ilmenite, which was stabilized by cocrystallization of hercynite rather than magnetite. As a result, the residual magmas were characterized by progressively increasing Fe2+/Fe3+ with a corresponding decrease in f (O2), and they ultimately reached saturation in rutile.