Petrologic and Geochemical Characterization of a Carbonate-Bearing, Amphibole-Phyric Intrusion at Hjalmar Lake, South Rae Craton, Northwest Territories: Preliminary ResultsSoapbox Wednesday, November 21, 2018 - 13:14 to 13:20 Theatre 2
The Nonacho lake area consists of Archean to Paleoproterozoic granites and gneisses unconformably overlain by the ca. 1.91-1.82 Ga alluvial-fluvial sequence of the Nonacho Group. The area was the site of extensive mineral exploration in the 1950-80s focused on unconformity associated uranium ± rare earth elements (REE) and polymetallic (Cu-Au-Ag) showings. Although previous research in the area has been largely directed towards the stratigraphy of the Nonacho Group, the geology of the basement rocks and mineralization potential of the area remain relatively understudied.
Reconnaissance mapping of the Nonacho Lake area in 2018 included an investigation of the Nonacho Group and basement rocks at Hjalmar Lake. The basal breccia and conglomerate member of the Hjalmar Formation, including a distinct cream-white, fine-grained, fluorite-bearing rhyolite, is well-exposed in unconformable contact with granitic basement. Mapping of the basement revealed an approximately 500 m by 1500 m, carbonate-bearing, amphibole-phyric intrusive body and associated dykes. The rock contains very coarse (up to 3 cm diameter), dark green to black, euhedral to subhedral amphibole crystals in a groundmass that varies from vitreous fine-grained greyish green (clinopyroxene) to leucocratic pink (feldspar) to white (carbonate) crystalline material. This body intrudes strongly foliated hornblende diorite and pink, highly altered, granite with amphibolite xenoliths. The intrusion is crosscut by a feldspar megacrystic granite and younger pegmatitic phases. Fluorite-calcite-sulphide bearing veins were observed in fine-grained mafic basement rocks ~100 m from the intrusion. The mineral assemblage in these veins is similar to that found in other veins in the region including those associated with the Crest (Cu-Ag-F) showing ~35 km to the northwest.
The spatial association between the carbonate-bearing intrusive rocks, the fluorite-bearing rhyolite, and fluorite-calcite-sulphide veins in the basement rocks raises questions regarding the intrusion's nature and relationship to hydrothermal mineralization in the region. The presence of interstitial carbonate in a mafic intrusive body suggests this intrusion may have a carbonatitic affinity. Carbonatites have a high potential for hosting multi-commodity mineralization and they are the main global sources of niobium and REE, critical metals used as alloys and catalysts in key economic sectors such as manufacturing.
Ongoing work includes: (1) petrological and geochemical analysis of amphibole and carbonate using microscopy, SEM, and electron microprobe to determine the origin of the minerals; and (2) U-Pb geochronology of potential U-Th-Pb-bearing accessory phases to determine the age of crystallization and provide insights on the tectonic environment during magmatism.