Natural Resources

Open File 23I/0101 - A High-Density Lake-Sediment and Water Survey in the Knox Lake Region, Western Labrador (NTS Map Areas 23I/06, 07, 10, 11, 12, 13, 14 and 15)

S.D. Amor

St. John's, Newfoundland, May, 2013

Abstract

A detailed, helicopter-supported lake-sediment and water survey was conducted over approximately 4800 km2 of the Knox Lake region of western Labrador, on NTS map areas 23I/06, 23I/07, 23I/10, 23I/11, 23I/12, 23I/13, 23I/14 and 23I/15.

The sampled area is underlain by rocks of the Southeast Churchill Province (SECP), comprising mid-Paleoproterozoic granite, granodiorite and paragneiss, intruded by Early Mesoproterozoic granite and syenite in the east, Neoarchean granitic gneiss intruded by mid-Paleoproterozoic granitic rocks in the centre, and Neoarchean metatonalite and tonalite gneiss in the west. In the extreme northwest, sampling extended over rocks of the Labrador Trough, which comprise a diverse package of mainly supracrustal rocks of mid-Paleoproterozoic age.

A total of 1017 sites were sampled. The presence of larger lakes, notably the Smallwood Reservoir, reduced the nominal sampling density of one sample per 4 km2 to one sample per 4.7 km2. Water and sediment samples were collected at 985 sites, and water samples only at 32 sites. Field duplicate samples were collected at one site in 20.

Analysis of the sediment samples was carried out for 27 elements by Instrumental Neutron Activation Analysis (INAA), for 31 elements by Inductively-Coupled Plasma Emission Spectroscopy (ICP-ES), for Ag by Atomic-Absorption Spectroscopy (AAS), and Loss-on-Ignition (LOI) by gravimetry. The water samples were analyzed for 24 elements by ICP-ES, U by ICP-mass spectrometry, fluoride ion by ion-specific electrode, conductivity by Corning conductivity sensor, and pH by Corning combination pH electrode.

The concentration levels of many elements, in both sediments and waters, are conspicuously higher over rocks of the Labrador Trough. Certain other geological features, notably the eastward transition from Neoarchean to mid-Paleoproterozoic rocks, have a diagnostic signature in a number of geochemical variables.

Anomalous concentrations of rare-earth (REE) and associated elements are present in the east of the sampled area, near the contact with an early Mesoproterozoic granite-syenite stock over which REE enrichment had been noted previously. Anomalies characterized by Ti-Hf-Ba-Nb-Sc-Ta, As-Au-Ag-Sb, and Cu-Ni-Fe-Zn-Cd are present elsewhere; the last-named is underlain by Labrador Trough rocks and also associated with enriched sulphur, and low pH in the lake waters.

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