The Kilmar Magnesite Deposits: Evaporitic Metasediments in the Grenville Supergroup, Morin Terrane, Quebec

Abstract

Mesoproterozoic magnesite deposits are found associated with dolomitic marble and intercalated with metasedimentary rocks of the Grenville Supergroup in the granulite facies Morin terrane (Grenville Province, Quebec). This study examines one of the remaining ore deposits exposed on the surface (at the Dobbie mine), and presents stable isotope and mineralogical data for a marine evaporitic origin. The magnesite ore zone has δ18O(Mag) = 25.5 ± 0.4‰ (VSMOW) and δ13C(Mag) = 1.7 ± 0.2‰ (VPDB; n = 7), while surrounding dolomitic marble has δ18O(Dol) = 24.2 ± 0.6‰ and δ13C(Dol) = −0.2 ± 0.7‰ (n = 11). These values are at the high end of the range for other Morin terrane marbles, and this and sharp transitions in stable isotope ratios between lithologies argue for preservation of evaporitic enrichment in δ18O and δ13C. Boron isotope ratios (δ11B = 15.5‰ to 22.7‰) are also consistent with a marine evaporite origin. Identifying evaporitic protoliths in metasedimentary rocks is important for determining pre-metamorphic depositional environments, and in this case links the sedimentary setting of the Morin terrane to the Adirondack Lowlands (New York, NY, USA). The identification of the Kilmar magnesite deposits as evaporitic also has implications for the formation of sedimentary exhalative base metal deposits in the Grenville Supergroup.

Editorial for Special Issue “Minerals of the Southern Grenville Province”

“…Peck and Eppich [7] address the origin of the Kilmar magnesite deposits in the granulite-facies Morin Terrane of Quebec. Traditionally interpreted to have originated by hydrothermal replacement of host lithologies [8,9], they are found associated with dolomitic marbles and intercalated metasedimentary rocks of the Grenville Supergroup.…”