Gold distrubution in glacial sediments and soils at Spyder Lake, Hope Bay greenstone belt, NWT and the effects of a permafrost environment
Creators: name: Laurus, Kathryn Anne
Publisher: name: University of British Columbia
Year: 2009

description: There is a high potential for economic gold deposits in the Slave Structural Province, NWT. Gold exploration, however, is hampered by a complex cover of Quaternary sediments and limited bedrock outcrops. Furthermore, there is a lack of information on the glacial dispersion of gold and the effects, if any, of postglacial redistribution of gold under prevailing periglacial conditions. Therefore, the partitioning of gold among size and density fractions was determined for the various surficial materials in the vicinity of, and down-ice from, known gold mineralization in the southeastern portion of the Hope Bay Greenstone Belt, 650 km northeast of Yellowknife, NWT. Terrain mapping by Ryder (1992) identified several types of surficial materials consisting of weathered rock, till, marine sediments and mixtures of the different types. Soil and humus samples were collected from thirty-seven pits along five lines on the Wally Grid. The lowermost horizons from each pit, and A and B horizons from selected soil pits, were wet sieved into four size fractions between 2000 um and 212 um; the resulting -212 um fraction was analyzed for gold by fire assay-atomic absorption spectroscopy (FA-AAS). Samples from one grid line were further sieved to obtain the -53 um fraction which was analyzed for gold by FA-AAS. LFH horizons and bog samples were milled to -1000 um and organic-rich soils (Ah and O lenses) were dry sieved to -70 um, and were analyzed for gold by neutron activation analysis. Regional till samples down-ice from known mineralization were wet sieved into six size fractions. The -53 um fraction and light and heavy mineral separates of the - 212+106 um and -106+53 um fractions were analyzed for gold by FA-AAS. The -53 um fraction was analyzed for gold by aqua regia column-inductively coupled plasma spectrometry (AR column-ICP). Two distinct types of geochemical anomalies were identified in the study area: (i) an older gold anomaly in regional till, and (ii) a younger gold anomaly in locally-derived materials. Gold results of heavy mineral concentrates (HMC) of the -106+53 um fraction and the silt-clay (-53 um) fraction of regional till identified an anomalous gold zone that is detected for at least 2 km down-ice. Although the contrast and gold concentrations in the silt-clay are lower, the -53 um fraction of regional till contains over 60% of the gold with most of the balance residing in the -106+53 um HMC fraction. For the younger anomaly, gold concentrations in soil profiles decrease from the upper A and B horizons to lower C horizons. Exceptions occur, however, for horizons directly overlying mineralized bedrock; in these cases, gold values increase with depth. In cross section, anomalous gold zones have a 'mushroom-shaped' vertical distribution pattern in which the anomalous zone defined by the near-surface horizons is 10 to 50 m broader than the anomaly in the underlying horizons. Gold values from weathered rock are typically 10x to 100x greater than from other surficial materials as they contain a high proportion of weathered, local bedrock fragments. Results show there is no systematic variation of gold values across the surface of individual frost boils developed in these weathered rock materials. For reconnaissance scale exploration, a 500 g sample of -2 mm field material should be collected from the central portion of regional till frost boils. A 50 g subsample of the -53 um fraction should be analyzed for gold by a reliable analytical method having a low detection limit for gold. Considering the width and length of the gold dispersal train, a rectangular sampling grid should be utilized whereby grid lines oriented perpendicular to ice flow direction are spaced 500 m apart and samples collected every 40 m. As a complement to gold results from the -53 um fraction, a large, 2 to 5 kg field sample could be used to analyze the -106+53 um HMC fraction for gold. Costs would increase but would be worthwhile to provide an extra degree of assurance in identifying gold anomalies. For property scale exploration, a representative 30 g subsample of the -53 urn fraction could be analyzed by FA-AAS. However, although the -53 um fraction contains over 75% of the gold for the local materials, sample representativity is only slightly reduced if a 30 g subsample of the -212 um fraction is analyzed for gold. In most cases, a 500 g field sample of -2 mm material should initially be collected at 10 to 20 m spacings along grid lines 100 m apart. Infill sampling lines should be spaced at 50 m. It appears that marine sediments blanket geochemical anomalies but additional research is needed to address this problem, and to suggest a possible geochemical method that could be used in these areas.