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Bo Møller Stensgaard and Agnete Steenfelt

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Bo Møller Stensgaard and Agnete Steenfelt nl ree and G Exploration07 B Exploration in the new Millennium u r m e GeologicalGeological and StatisticalStatistical VValidationalidation of a GoldGold PredictionPrediction ModelModel u 5th Decennial International Conference a e u l o ro on Mineral Exploration f et M P inerals and based on low-density surface geochemistry and other geoscientific data, Nuuk region, West Greenland Toronto, Canada, Sept. 9 to 12, 2007 Bo Møller Stensgaard* and Agnete Steenfelt Geological Survey of Denmark and Greenland (GEUS), Geocenter Copenhagen, Denmark (*email: [email protected]) es es pl pl m m xa xa IntroductionIntroduction Gold Prediction Model Signatures of Gold Occurrences Geological Validation ExamplesE Inner Fiskefjord ExamplesE Southwest of Isua A prediction model for the favourability for gold is constructed by statistical analysis. Data signatures of gold occurrences and the background are established for each A statistical model is not valuable to the exploration geologist without a The Inner Fiskefjord area is predicted as being favourable for Isua gold occurrences. An area southwest of the well-known Isua greenstone belt, which host several Prediction models - are we able to predict group. Some of the most characteristic data signatures identified are given in the table geological validation that addresses questions like “Are the results veri- The favourable areas lie within greenstone belts and are proximal to the major Fiske- gold mineralized sites, is predicted as being favourable for both Isua and areas favourable for gold? fjord fault. No known gold occurrences have previously been reported from the area. Bjørneøen gold occurrences. No prior information on mineralization exists from All data are pixelated. Characteristic data signatures of pixels with known gold occur- below. fiable, geologically reasonable and explainable?” the area. rences are identified and extracted from various datasets. The ‘gold signatures’ are For this purpose, field visits were made to several of the areas with pre- Fieldwork: 14 days, 2 person, tent camp. Zodiac used for transport and field traverse different from the signatures of pixels without any known occurrences - the ‘back- by foot. Helicopter lift to northernmost area. The area is situated within two 100–300 m wide slivers of greenstone rocks in What are the ’fingerprints’ of gold The stream sediment geochemistry identified as characteristic for the gold occurrences: dicted favourability outside the known mineralized sites, and settings, ground’ signature. gneisses. It is also notable that the area is located 2–3 km away from the main ex- occurrences? The three groups have all high Ni/Mg ratios and high Cs as common features, but they lithology and available lithochemistry were studied in predicted areas. pression of the Ataneq-Ivinnguit fault zone, which may play a role for the gold min- A search is performed for pixels with similar signature as the established ‘gold signa- are different in their remaining signatures. Medium to high Rb, La, U and Th character- eralization at Storø. Outcrops at visited localities comprise mainly migmatised Given the spacing (5 to 6 km) of the stream sediment data, it is obvious gneisses, amphibolite and ultra-basic rocks. Rocks in the visited area are strongly ture’. The degree of similarities between the signature of a pixel and the ‘gold signa- ize the Storø and Bjørneøen groups while these elements are low for the Isua group. How do we validate our predictions? that the prediction is unable to characterize the small target of the gold sheared and folded. Garnetization and silicification occurs. ture’ determines the favourability of the pixel. The Bjørneøen and Isua groups have medium to high MgO, Fe O , Ni and Cr, whereas 2 3 mineralization itself but it may identify larger features, such as lithology, the Storø group is characterized by high Al O . The Isua group has low Na O compared Fieldwork: The ground follow-up in this area was carried out during a 1.5-hour re- Three groups of occurrences are established based on a requirement of mutual pre- 2 3 2 alteration and other characteristics for the mineralized environment. Methodology: to the other groups. Gold in the stream sediment geochemistry is only found to be in- connaissance stop. Signatures of diction capability of occurrences within each group. The groups, named after their key All known gold occurrences are located within mafic metavolcanic rocks gold is localities, are: Storø, Bjørneøen and Isua gold occurrence group. dicative for the Isua group. Trivial but important observations to keep in mind such as ??? ?????????? extracted and the fact that all predicted areas are situated within mapped supra- Southwest of Isua ??? ???? ??? ??? Maps of the favourability for gold are then made. high gold values in low-density stream sediment geochemistry are not often encoun- ??? ?????????? crustal rock units can be considered the initial verification of the results. Two empirical distribution functions tered. Arsenic is only found to be indicative for the Storø group. ??? ?????????? are calculated for different datasets - ??? ????? ???? Isua ? one for pixels with ’occurrences’ and DuniteDunite ??? ?? ???????? The stream sediment signatures for the three gold groups do not exhibit significant con- orizon one for pixels with 'no occurrences' uchsiitete horizoh Gold favourability map for the Isua gold occurrences group QuaQuarrtztz--fuchsf Occurrences s trasts from the background for the following components: CaO, K2O, P2O5, Sb, SiO2, AmphiboliteAmphibolite 1 1 e AmphiboliteAmphibolite 1 1 1 l Favourability map for p TiO2, V, Zr, and Zn. l 1 Isua gold occurrences Statistical Validation Outline of favourable areas: a ti Geological map of the gold favourable area 1 m Top Top Top n based on: e southwest of Isua 1 No occurrences a 0.5% 1% 2.5% ot d ’the background’ x • 10 gold occurences Statistical prediction models are of little use if their reliability or capability p e ExamplesE • 8 stream sediment d potentialm 1.5 km ol fir ial geochemistry datasets to predict is not tested. Users need to know how reliable the prediction is. GolG on ntialnt Au, Cs, MgO, Fe2O3, Data signatures indicative for Inner Fiskefjord Results confirmedc te No. of showings Southwest Na O, Ni+Cr, o d Data values of Isua 2 Results p e For this purpose, statistical cross-validation is carried out. One gold oc- • Large hydrothermal alteration m Ni/Mg-ratio gold occurrence groups against the old firmedir Inner • 2 aeromagnetic data- currence is selected from each group and its location assumed to be system recognized. • Mineralized sites discovered. GoldG o poten Fiskfjord sets background signature confc Quantified A search for VG-TMI, HG-TMI unknown. A gold potential map is constructed based on the remaining • Several new mineralized sites dis- • Strong shearing and alteration systems signatures similar Range indications (lowest, low, medium, high and highest) refer to characteristic data values rela- occurrences of the group. The pixel occupied by the selected occurrence covered - highest gold in rock observed. in different signatures... tive to the entire range of the specific dataset. A full coloured cell marks a data signature regarded samples being 377 ppb Au. datasets Examples of predicted as distinct for the group; a subdued coloured cell refers to a signature, which is moderately distinct; is then assessed in order to evaluate the ability of the remaining occur- • Data signatures confirmed by rock and areas without previously Favourability map known gold occur- blank cells are non-characteristic for the group. rences to predict the ’unknown’ occurrence. When the process has been • Rock units in the Fiskefjord are gen- sediment geochemistry. for ’gold signature’ erally enriched in gold and auferi- rences: Storø Bjørneøen Isua carried out for each occurrence in a group, a prediction rate curve is con- • Most significant for gold content is a soil • Inner Fiskefjord Data Background ous pathfinder elements. Localities visted during the 1½ hour reconnaissance stop. occurrences occurrences occurrences sediment sample running 158 ppb Au low high • Outer Fiskefjord structed. • Southwest of Isua Stream sediment geochemistry data • Gold content well-above back and 701 ppm As. Favourability The quality of the prediction can be judged from the slope of the predic- ground values encountered at sev- Low Low High Low for gold Au eral localities. • Geochemical results verify the favou- (as background) (as background) (> 60 ppb) (0 – 40 ppb) tion rate curve: the steeper the slope, the more reliable the prediction. Quartz-veined and rability for gold in the area - rock units High Low Low Low carbonate-altered amphibo- lite with disseminated Fe- • New sediment samples confirms are generally enriched in Cs, Rb, Sb and As (> 20 ppm) (as background) (as background) (< 4 ppm) sulphides. Rock samples gold potential - highest being a soil K2O. High High Highest Low from this locality yields 377 Cs ppb Au and 3 ppm As. sample with 144 ppb Au. (3.5 –5 ppm) (3.5 – 5 ppm) (3 – 6.5 ppm) (< 2.5 ppm) Prediction rate curves - how good is the prediction? Rb High High High Low s • Identified data signatures confirmed Craton Geology and Gold in the Nuuk region (40 – 80 ppm) (40 – 80 ppm) (30 – 70 ppm) (10 – 50 ppm) e1.0 Quartz-carbonate veined sequence of the strongly l Legend: by rock and sediment geochemistry. sheared garnet-fuchsite-bearing and altered am- High Medium Low Medium p phibolite. La (50 – 70 ppm) (35 – 55 ppm) (30 – 10 ppm) (15 – 50 ppm) Random area selection 52° W 50° W Storø gold prospect, West Greenland m ion StudyStudy aarearea The Nuuk region, West Greenland, forms the central part of the Archean North Atlantic craton; the Highest Medium Low Low a 0.8 ct All 52 showings - SSS0, AM0 ~13700~13700 kmkm2 Canadian counterpart is the Nain craton in Labrador. U x i d Inner Fiskefjord (> 80 ppm) (17 – 35 ppm) (as background) (< 15 ppm) ed te ExamplesE r a Isua group - SSS1 Highest High Low Low PredictionP liddated Isukasia The region represents an assemblage of several early to late Archean crustal terranes amalgamated Th valiva Isua group - SSS1, AM1 1600 m (12.5 – 20 ppm) (10 – 15 ppm) (as background) (< 12.5 ppm) 0.6 during thrusting at c.
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