Geoscience BC Report 2011-3

The application of surface organic materials as sample media over deeply buried mineralization at the Kwanika Central Zone, North-Central British Columbia (NTS 93N)
by D.R. Heberlein (Heberlein Geoconsulting) and C.E. Dunn (Colin Dunn Consulting Inc.)

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Report Description
As a follow up to the 2009 Geoscience BC study by Heberlein and Samson (2010), the authors carried out an expanded geochemical orientation survey over the Kwanika Central Zone focusing on the geochemical responses in organic materials to the mineralization. Results of the original study showed that the most effective combination of sampling medium and chemical extraction for detection of the deeply buried Cu-Au porphyry mineralization was Ah horizon soil with a modified aqua regia (Ultratrace) digestion with ICP-MS determinations. This project further investigates the use of Ah horizon by comparing sampling results from an offset 100 by 100 metre grid with other organic materials, namely vegetation and charcoal.

Sampling was carried out during September 2010. Five different samples were collected at each site, including: two samples of Ah horizon, lodgepole pine bark, subalpine fir twigs and (where present) charcoal fragments from the LF and Ah horizons.

Analytical results for the Ah horizon soils were examined to determine whether levelling was necessary to remove the influence of various field parameters. Soil moisture content was found to have an important influence on metal concentrations in the sodium, pyrophosphate and Ultratrace extraction results on the Ah horizon samples. Data were leveled by converting to Log(10) Z-Scores using soil moisture as a classification variable. Sodium pyrophosphate results were also normalized to LOI to correct for variations in organic carbon content. For the vegetation samples, lithology was found to have an influence on metal concentrations in pine bark. These results were also corrected using the Log(10) Z-Score method.

The weakest extraction, distilled water, was found to be ineffective at detecting the mineralization. It did produce visually interesting patterns over the zone but hypergeometric probability analysis demonstrated that there is a reasonable chance of similar results being produced by random chance. Sodium pyrophosphate and Ultratrace aqua regia both performed well in defining statistically meaningful apical anomalies for As, Cu and W. Gold and Sb anomalies were also detected by the Ultratrace and sodium pyrophosphate methods respectively. Tungsten has by far the strongest response of all elements tested and appears to be the most effective pathfinder for the blind mineralization.

Charcoal results, while not definitive, do show that this material has promise as a sampling medium. Patterns produced by Ultratrace aqua regia digestion of this medium are comparable to those seen in the Ah soils. Unfortunately the relatively small number of samples over the mineralized zone and the highly variable nature of charcoal distribution across the study area resulted in a relatively poor precision for most elements that translates into noisy patterns. Nevertheless, this study has demonstrated that recognizable anomalies for ore elements can be obtained in this medium over the mineralization and with improved sampling techniques more reliable results could be achieved. More work is needed to refine charcoal as a sample medium.

The vegetation results show different patterns from the Ah soils and charcoal suggesting that the metals in the soil have not been derived exclusively through recycling of plant tissues. Furthermore, the occurrence of halo-like patterns with lows directly over the mineralization imply that different geochemical dispersion processes are responsible for the formation of soil and vegetation anomalies and this is probably because the roots of an individual large tree integrate the geochemical signature of several cubic metres of the substrate, including all soil horizons. As a sampling medium to detect deeply buried mineralization, fir needles show promise, particularly for pathfinder elements like Tl and As that form compelling halo patterns around the edges of the mineralized zone. Dispersion patterns for these elements significantly enlarge the size of the target making it easier to find. A combination of fir needles and Ultratrace analysis on Ah horizon soils would be an effective exploration approach in forest environments. Pine bark had less clear results but did produce base metal anomalies in close proximity to known faults of which one, the Pinchi Fault, is known to intersect mineralization at depth. This observation could be useful in interpretation of pine bark survey results.

Suggested Reference
Heberlein, D.R. and Dunn, C.E. (2011): The application of surface organic materials as sample media over deeply buried mineralization at the Kwanika Central Zone, North-Central British Columbia; Geoscience BC, Report 2011-3, 67 p.

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