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A Reinterpretation of Lake Sediment Geochemistry in the NEA/IAEA Athabasca Test Area

Michel Mellinger1

Mellinger, M. {1989): A re-interpretation of lake sediment geochemistry in the NEA/IAEA Athabasca test area; in Summary of In­vestigations 1989, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report 89-4.

The NEA/IAEA Athabasca Test Area, located at the east­ern edge of the Athabasca Basin of northern Sas­katchewan, was chosen in 1979/80 for the purpose of testing a variety of exploration techniques for Athabasca Basin unconformity-type uranium deposits (Cameron, 1983),-0rganic-rich lake sediments and lake .waters were collected in the test area in 1977 and 1979. In addition to chemical data, field observations coded in the 1977 and 1979 data files included: sample colour, lake area and depth, surrounding relief, potential contamination resulting from exploration activity, and underlying bedrock. The chemical and underlying bedrock data were interpreted by Coker and Dunn (1981, 1983), but other field data were not considered. In addition to detecting samples with anomalously high element con­centrations and relating them to known uranium deposits, these authors noted spatial trends in the data which may reflect known northeasterly basement lithological trends and glacial geology. They also ex­amined geochemical associations by plotting a map of samples with concentrations higher than the 90th per­centile value for any element.

In the present project the field and geochemical data were examined using multivariate data analysis. This paper briefly summarizes the results obtained. A more comprehensive report is available (Mellinger, 1989) and a map showing the location of newly identified uranium anomalies can be obtained from the Saskatchewan Re­search Council.

1. Field and Analytical Data

The following data were extracted from the 403 samples collected in 1977 and 1979: sample number, UTM coor­dinates, field variables (underlying bedrock, lake area, lake depth, relief) and lake sediment chemistry (U, Zn, Cu, Pb, Ni, Co, Ag, Mn, As, Mo, Fe, LOI). To this was added: drainage basin, drainage order, presence of known mineralization in the vicinity, and Quaternary geol­ogy.

Underlying bedrock geology, coded in the initial data file, was modified to comply with the four basement lithologies described by Sibbald (1980). Lake area was kept in its original form, but lake depth, initially coded in whole metres (range: 1 m to 18 m), was recoded after examination of a percentile cumulative plot, into the fol­lowing depth categories: 1 m, 2 m, 3 m, 4-5 m, 6-7 m, 8-to-12 m, and 15-to-18 m.

(1) 5askatchewan Research Council, 5askatoon, 5askalchewan

Saskatchewan Geological Survey

2. Correspondence Analysis of the Field Variables Field observations are qualitative variables, either nominal (underlying bedrock, drainage basin, Quater­nary geology) or ordinal (lake area and .depth, relief). The data, from the table of 403 sample records with 28 completely disjunctive variables, were each submitted to correspondence analysis (Mellinger 1987). From this analysis clear field variable patterns can be identified that show non-linear relationships.

Lake morphology parameters show consistent relation­ships. For example, lake depth increases with increas­ing lake area and surrounding relief. The distribution of lakes with given characteristics is not homogeneous, however, with respect to drainage basin, Quaternary geology, or underlying bedrock. Thus even if lake mor­phology affects the chemical characteristics of their sedi­ments to a significant degree, it can still be expected that lake sediment chemistry will display regional pat­terns which also result from the influence of one or more of drainage basin, Quaternary geology, and under­lying bedrock.

3. Lake Sediment Chemistry Correspondence analysis of the geochemical variables produced the following results:

1) The dominant pattern is a negative correlation be­tween Mn and a group of elements comprising LOI (essentially organic content of the sediments) and the metals Zn, Cu, Ni, and Ag (factor 1).

2) Zn, Cu, Ni, and Ag are positively correlated themsel­ves, meaning that these metals are generally as­sociated with organic matter in the lake sediments.

3) A clear Fe-As-(Mo) trend also emerges, with minor residual Zn versus LOI negative correlation and Co contribution (factors 2 and 3).

4) A uranium, Ni, and Cu association (factor 4) is recog­nized which extracts a residual U trend ("residual" be­cause the 23 samples with U 20 ppm were not in­cluded in these calculations).

Applying a threshold to coordinates along this fourth fac­tor permits the recognition of an additional 22 samples which are anomalous in uranium, bringing the number of uranium anomalies to a total of 45 samples.

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4. New Uranium Anomalies

When sample coordinates along factor F4 and initial U concentrations are plotted together (Figure 1), two sub­populations of samples appear. One indicates a clear in­verse correlation between increasing U(ppm) and the value of the F4 coordinate. These samples are anomalous on either axis, generally 20 ppm U and 250 F4 coordinate, and the subpopulation is univariate anomalous. The other subpopulation closer to the F4 axis also defines an inverse correlation between U(ppm) and the value of the F4 coordinate; however, the com­ponent samples cannot all be identified as anomalous on the basis of U(ppm) alone, in particular for U 10 ppm. Their anomalous character is illustrated by an F4 coordinate of -250. These samples form a multivariate anomalous subpopulation which requires extraction of other multivariate patterns (along factors F1 to F30 before it can be recognized. The subpopulation repre­sents the new set of lake sediment U anomalies in the survey area.

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5. Field Variables and the Chemical Space When variables representing field observations are projected into the chemical factor space, it is clear ~hat environment has a significant influence on lake sedi­ment chemistry. Lake morphology affects the type of sediment encountered and therefore also the sediment chemical signature. Organic-rich sediments in contrast to those rich in Mn-oxides occur in relatively small, shal­low lakes surrounded by gentle relief. Some drainage basins show an affinity for particular chemical trends. The underlying bedrock appear to influence sediment type; for example, Mudjatik basement underlies lake sediments of both medium and high organic content, whereas Athabasca sandstone underlies mostly organic­rich lake sediments.

The Quaternary geology displays one chemical relation­ship that is particularly interesting: glacio-fluvial sedi~ ments are associated with the Fe-As-(Mo) trend, which explains why no particular drainage basin relates to the

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U (ppm) Figure 1 - Comparison of factor coordinates along F4 (U anom:3/ous tr_end) and analyzed U(ppm). Anomalous factor coordinates are towards negative values, with -250 as anomaly threshold (stippled /me).

96 Summary of Investigations 1989

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anomalies in the chemical space. The Fe-As-(Mo) anomalies may result from sub-glacial rivers transporting sediments into the survey area.

6. Conclusions Re-interpretation of the data has demonstrated that:

1) field variables can greatly influence the lake sedi­ment chemistry,

2) descriptive multivariate data analysis (in particular correspondence analysis) extracts clear information from complex data, both qualitative (field observa­tions) and quantitative (chemistry),

3) new uranium anomalies can be apparent which might not be identified simply on the basis of U con-·centration, and ·

4) the majority of Fe-As-(Mo) anomalies are related to glacio-fluvial sediments extending mostly from Dawn Lake to the southwest. This trend was noticed by Coker and Dunn (1983), but not identified as being related to glacio-fluvial features probably due to the lack of field data.

Saskatchewan Geological Survey

7. References Cameron, E.M. (1983) : Uranium exploration in Athabasca

Basin, Saskatchewan Canada; E.M. Cameron (ed.), Geel. Surv. Can., Pap. 82-11, 31 Op.

Coker, W.B. and Dunn, C.E. (1981): Lake water and sediment geochemistry, NEA-IAEA Athabasca Basin - Wollaston Lake, Test Area (64L, 741), Saskatchewan, Canada; Geel. Surv. Can., Open File 779.

---~ (1983): Lake water and lake sediment geochemistry, NEA/IAEA Athabasca Test Area; in Uranium Exploration in Athabasca Basin, Saskatchewan, Canada; E.M. Cameron (ed), Geel. Surv. Can., Pap. 82-11, p117-125.

Mellinger, M. (1987): Correspondence analysis: The method and its application; Chemometrics and Intelligent Laboratory Systems, v2, p61-77.

Qn press): Multivariate patterns of field information __ a_n....,d-g-eochemistry in a regional lake sediment survey; the

NEA/IAEA Athabasca Test Area revisited; in Proceedings of the Colloquium "Statistical Applications in the Earth Sciences," Geel. Surv. Can.

Sibbald, T.1.1. (1980): NEA/IAEA Test Area: Sub-Athabasca Group, basement geology; in Summary of Investigations 1980, Sask. Geel. Surv. Misc. Rep. 80-4, p57-58.

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