Siobhan McGoldrick

1^st Class Honours
University Medal in Earth Sciences
NSERC CGS-M Scholarship to pursue graduate studies at the University of Victoria
Michael J. Keen Memorial Award - 2011

Neoarchean (ca. 2680 Ma) transitional to calc-alkaline volcanic rocks in the Tumpline
Lake subarea of the Cameron River - Beaulieu River volcanic belt in the southern Slave Province are being assessed to determine their potential as a prospective volcanogenic-hosted massive sulphide (VMS) environment. Bedrock mapping at Sharrie Lake, approximately 70 km ENE of Yellowknife, was completed at 1:7500 in 2012 and 2013. Samples of mafic and intermediate pillow lava flows from this strongly bimodal suite were collected for further study. In addition to some true pillow basalts and andesites, many flows have basalt-like textures yet have the weathered appearance and silica values of a dacite or rhyolite. The VMS-potential of these pillow lava flows will be assessed by characterizing the rock geochemistry and alteration characteristics, and constraining the timing of alteration and deformation relative to metamorphism.

Geochemical discrimination diagrams demonstrate that primary lithology varies from
basalt to andesite, and several flows can be identified based on their distinct geochemical
signatures. Lava flows have variable trace element patterns although all show arc-like
characteristics. The secondary mineralogy is dominated by carbonate, quartz, epidote, chlorite, and minor sericite. In thin section, disseminated matrix carbonate is common and ovoid aggregates of quartz and carbonate, interpreted as deformed amygdules, display textures indicating pre-kinematic origin and recrystallization during regional metamorphism.
Lithogeochemical characterization of least altered - most altered sample pairs indicates that alteration is pre-metamorphic and possibly syn-volcanic. The principal components gained or lost in the metasomatic alteration reactions vary between flows of different primary lithology. The relative timing of some secondary minerals is ambiguous owing to the effects of heterogeneous strain. Sericite and some chlorite are generally present replacing earlier phases such as plagioclase, amphibole, and biotite, and both are interpreted to be late. If the secondary mineral assemblage represents the alteration assemblage, the Sharrie Lake samples have undergone carbonate propylitic alteration related to diagenetic (seafloor) alteration processes. It is more likely, however, that the secondary assemblage represents the early seafloor alteration mineralogy overprinted by the later amphibolite facies metamorphic assemblage. Peak lower amphibolite facies metamorphism (~500 °C and 4.0 ± 2 kb) was syn- to post-kinematic based on syn- to post-foliation hornblende growth and post- foliation garnet growth.

Although the alteration mineralogy and geochemistry of the samples in this study do not
appear to indicate a strong hydrothermal alteration signature, other characteristics of the
Tumpline Lake subarea, such as the geochemistry of felsic volcanic rocks and proximity to
known VMS-type showings and deposits, demonstrate that this belt warrants further study as a VMS-potential environment.

Keywords: geochemistry - alteration - greenstone belt - Archean - thermobarometry - Slave Province - Northwest Territories
Pages: 171
Supervisorr: Rebecca A. Jamieson, Luke Ootes (NTGO), and Valerie A. Jackson (NTGO)