Open File Report 1998-06

Author(s) Date 1997-12-31

A biogeochemical orientation study, using trembling aspen trees growing over and around the Mountain Lake Diatreme, will help to determine the usefulness of biogeochemistry studied for kimberlite exploration in the boreal forest regions of northern Alberta. Especially, since several kimberlites discovered in Alberta form topographic highs, which may locally be dominated by aspen vegetation. Furthermore, airborne magnetic survey highs, in association with topographic highs, have been deemed priority exploration targets and biogeochemistry may provide a quick and cost effective method for ground-testing these targets.

Bark, twig and stem samples from trembling aspen trees over the Mountain Lake Diatreme proved to be viable sampling media in their ability to up take elements indicative of ultramafic and kimberlitic sources and their analysis provided a distinctive biogeochemical signature that may be used elsewhere in Alberta. Bark, twig and stem aspen samples yielded a significant geochemical signature (up to 28 times background) directly over, or in the immediate vicinity of, the Mountain Lake Diatreme for trace elements including Co, Cr, Mg and Ni, along with incompatible elements such as La, Rb and Sm. Other notable elements that yield elevated concentrations directly over the pipe include Al, As, Au, Fe, Na, V and W. Elements that yielded an elevated concentration over the pipe, but only in selected sampling media, include: Hf, Sb and Zr from bark, Ce from twig, and Cs and Nd from stem samples. Elements that are depleted directly over the pipe include Ba, Ca, Cu, and Zn; K yields a slight depletion over the center of the pipe; P is concentrated over the eastern slope of the pipe and in the lowlands to the west of the pipe.

Analysis from a second sampling transect, located over a similar- but non-kimberlitic - topographic high 6.5 km east of Mountain Lake, did not yield any anomalous concentrations for elements indicative of kimberlites. The geochemical variance between the second sampling transect and the Mountain Lake Diatreme transect indicate that the positive geochemical pattern obtained over the diatreme is not related to the physiographic environment and that biogeochemical sampling can detect an ultramafic signature from associated soil and bedrock.

Stem samples, followed by bark and then twig samples, provide the best overall geochemical correlation with samples from the adjacent soil and bedrock, particularly for key kimberlite indicator elements such as Ce, Co, Cr, Cu, La, Ni, Rb, Sm and Sr.

The soil layer over the diatreme is enriched with varying combinations of soluble and immobile elements, which influence the surficial expression or vegetation characteristics, and may be of importance for defining kimberlitic surficial expressions in northern Alberta.

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Eccles, D.R. (1998): Biogeochemical orientation survey over the Mountain Lake diatreme, Alberta; Alberta Energy and Utilities Board, EUB/AGS Open File Report 1998-06, 65 p.