An Integrated Model for Ilmenite, Al-Spinel, and Corundum Exsolutions in Titanomagnetite from Oxide-Rich Layers of the Lac Doré Complex (Québec, Canada)

Abstract

The titanomagnetite of the Lac Doré Complex, an Archean layered intrusion that is located in the Abitibi greenstone belt in Québec (Canada), contains a wide variety of exsolution textures, which are the remnants of a complex cooling history. In the present study, we reconstitute the decomposition stages of the original solid solution in order to explain the formation of ilmenite, Al-spinel (hercynite and gahnite), and corundum exsolutions in magnetite. This was conducted through a detailed mineralogical and textural examination and in situ determination of mineral chemistry. Our investigation reveals two discrete types of ilmenite exsolutions, which are ascribed, respectively, to the oxidation of ulvöspinel at temperatures above and below the magnetite-ulvöspinel solvus. Exsolutions of Al-spinel result from either a decrease in the solubility of the (FeZn)Al2O4 components upon cooling, or local excesses of Al and Zn due to the removal of ulvöspinel during the early oxidation. The origin of corundum is ascribed to the oxidation of pre-existing hercynite exsolutions. The trace element composition of the titanomagnetite indicates stratigraphic reversals in Cr, Mg, Co, Ti, and Si and important changes in redox conditions. We interpret this as a direct consequence of a major event of magma chamber replenishment, which strongly influenced the distribution of exsolutions.

Origin of the Vanadiferous Serpentine–Magnetite Rocks of the Mt. Sorcerer Area, Lac Doré Layered Intrusion, Chibougamau, Québec

“…The LDC magnetites also have compositions that are comparable to those of magnetites from other layered complexes (e.g., V), except for the distinctively lower Zr, Hf, Al, and Zn; and higher Si and Y concentrations ( Figure 12). Compared to titanomagnetite from the S limb layered zone ( Figure 12 of Arguin and collaborators [3]), the studied oxides contain more Ca and Y and 10 times more Si, as well as equivalent concentrations of P, Cu, Mn, and Co. The studied titanomagnetites also contain less Zr, Hf, Al, slightly less V, Nb, and Ta, and 10 times less Ti, Zn, and Zr.…”

“…A recent study focused on the S limb of the LDC also minimizes the importance of contamination. This study concluded that the crystallization and distribution of titanomagnetite was controlled by successive injections and mixing of magmas, crystal settling and sorting, and expulsion of interstitial melt during compaction [3].…”

“…The ~1.5-3.3 km thick layered zone is characterized by a centimeter-to meter-thick modal layering [1] described as a rhythmical succession of magnetitite magnetite-bearing anorthosite and gabbro [3]. In the S limb, the layered zone is divided into the magnetite-, chlorite-, and amphiboledominated P1, P2, and P3 members and the albite-, epidote-, actinolite-, and chlorite-dominated A1 and A2 members ( Figure 2).…”

“…In the S limb, the layered zone is divided into the magnetite-, chlorite-, and amphibole-dominated P1, P2, and P3 members and the albite-, epidote-, actinolite-, and chlorite-dominated A1 and A2 members ( Figure 2). In the S limb layered zone, V, Cr and Ni decrease upward, and Ti increases upward [1,3,16,17]. In the NW limb, the anorthosite members are missing and the magnetite-, chlorite-, amphibole-, albite-, and epidote-bearing units are divided into the NP1, NP2, and NP3 members [18].…”

“…As many similar Fe-enriched rocks observed in layered intrusions around the world, the layered zone of the LDC is potentially economic for V, Ti, and Fe. The layered zone is the focus of most studies carried on the LDC [3][4][5] but many questions remain on its origin and magmatic evolution. Addressing these issues may have consequences for our general comprehension of Fe-and V-enriched magmas.…”