Introduction

Declining ore reserves of producing gold mines and reduced exploration in the mining industry have resulted in the need for a long-term strategy for sustainable mining in the Yellowknife area. Terrain Sciences Division initiated regional surficial geology mapping through the EXTECH program to provide data of surficial materials, ice flow history and soil geochemistry. Reconnaissance surficial geology mapping was undertaken in the Yellowknife volcanic belt area: NTS 85J/7, 8, 9, 10, 11, 16; 85I/ 4, 5, 12, 13; 85O/1 and 85P/4, (Figs.1, 2). An environmental geoscience component was incorporated into the investigation within a 50 km radius of Yellowknife in order to provide regional data to potential mining-related contaminant studies being carried out in the City of Yellowknife.

Methods

In order to provide regional coverage in this orientation study, the area was surveyed by road access, fixed wing and helicopter-assisted traversing, and interpretation of 1:60 000 scale airphotos. Samples collected include: 58 two kg soil samples for trace element and grain size analyses, 60 litter fall and humus samples of 25 g to 100 g for trace element geochemistry determinations, and 50 pebbles (2 to 6 cm in diameter) from 47 sites for provenance and glacial transport investigations. In addition, 12 ten kg soil samples were collected to document the range and background concentrations of kimberlite indicator minerals, and 25 samples of 20 kg as part of a detailed dispersal train study in the Drybones Bay area, southeast of Yellowknife. Glacial striae were measured at 66 locations.

Glacial History

The Yellowknife region was ice covered to about 11 000 BP and became ice-free by about 10 000 BP. Ice flow indicators relate to ice movement towards the southwest, as evidenced by striae in conjunction with the subparallel orientation of eskers (Fig. 3 and 4). These are interpreted as representing the last phase of ice flow which occurred prior to and during deglaciation. Glacial Lake McConnell, a large ice marginal lake, formed along the western margin of the retreating ice, up to an elevation of 280 m (Photo 1). Great Slave Lake reached its present level of 157 m a.s.l. by about 8 500 BP. Above the limit of inundation, there is evidence of widespread washing of till and bedrock by meltwater which likely occurred during the easterly retreat of the ice front from the study area.

Surficial Sediments

Till is the most common surficial sediment, and consists of a loosely compact, stony, matrix-supported diamicton, with the matrix ranging from coarse to fine sand with minor amounts of silt. It is generally <2 m thick, and forms a discontinuous veneer. Locally, much of the fine grained sediments in the matrix have been removed by meltwater and wave action, resulting in lag deposits with variable textural characteristics (Photo 2).

Glaciofluvial sediments are relatively uncommon, and consist of fine sand to cobbles in the form of eskers, kames and subaqueous outwash (Photo 3). Eskers have a linear to slightly sinuous form, and generally trend parallel to the glacial flow direction defined by striae: southwest.

Glaciolacustrine sediments consist of coarse to fine sand, silt and clay deposits estimated to be up to 20 m thick. They contain variable amounts of pebbles, cobbles and boulders, occurring preferentially in topographic lows, up to 275-280 m asl, as 70 km or more inland. Organic deposits may overlie glaciolacustrine sediments (Photo 4).

Implications for Drift Prospecting

The similarity between washed till and some glaciolacustrine sediments makes surficial geology mapping particularly difficult and poses a problem for drift prospecting. Both can be represented by a wave-washed stony diamicton, which may or may not retain a geochemical signature indicative of underlying bedrock.

Preliminary studies suggest a relatively uniform, regional southwestward ice flow across the area, permitting dispersal trains to be traced up ice to a source. Minor changes in ice flow direction can be expected on a local scale, requiring more detailed investigations (Photos 5, 6).

Glacial dispersal patterns of pebbles in till are not immediately apparent because of bedrock lithology constraints (Figs. 2, 5). However, distributions strongly reflect the predominant southwestward flow, the predominance of metasedimentary clasts in till, and maximum transport distances of 150 km.

Acknowledgments

The authors would like to thank Indian and Northern Affairs canada for logistical support, as well as Great Slave Helicopters, David Smith for samples and data from Drybones Bay, the City of Yellowknife for access to certain sample sites, and R. Knight for drafting and review.

Author: Daniel Kerr