AGE OF THE BIRD RIVER SILL, SOUTHEASTERN MANITOBA, CANADA, WITH IMPLICATIONS FOR THE SECULAR VARIATION OF LAYERED INTRUSION-HOSTED STRATIFORM CHROMITE MINERALIZATION

Abstract

The Archean Bird River sill, a 20-km-long and up to 800-m-thick mafic-ultramafic layered intrusion in the Bird River greenstone belt of southeastern Manitoba, Canada, contains significant resources of chromium and nickel-copper, and locally anomalous concentrations of platinum group elements. Stratiform chromitite, displaying both remarkable lateral continuity and local irregularities due to synmagmatic disruption, occurs in up to six main intervals over a thickness of ~60 m in the lower ultramafic part of the intrusion. Ni-Cu sulfide mineralization is hosted in ultramafic rocks at the base of, or just below, the sill (e.g., past-producing Maskwa-Dumbarton mines). In this study, we report a U-Pb age (chemical abrasion-thermal ionization mass spectrometry) for a leucogabbro from the Chrome property of 2743.0 ± 0.5 Ma (n = 8 single grains of zircon), which is interpreted as the age of crystallization of the sill and defines the timing of the associated Cr-Ni-Cu ± platinum group element (PGE) mineralization. Other Neoarchean mafic-ultramafic intrusions containing economic concentrations of chromite and/or Ni-Cu-(PGEs) occur in the northwestern part of the Superior province in the Canadian Shield (e.g., McFaulds Lake, Big Trout Lake, Puddy Lake, Shebandowan) and share broadly similar geologic relationships. The age of intrusion-hosted stratiform chromite mineralization worldwide, excluding ophiolite-hosted chromitites, ranges from Archean to Paleoproterozoic, which is comparable to that for komatiite-related Ni deposits. This temporal restriction is consistent with both deposit types requiring the involvement of high-MgO, Cr-rich parent magmas produced during large degrees of mantle melting early in Earth history.