Secondary Environment Geochemical Maps: Favorable Zones and Exploration Targets

Daniel Lamothe and Patrice Roy
Géologie Québec

A new mineral potential assessment has been carried out based on secondary environment geochemical data from MRNF regional surveys stored in the SIGEOM database.

After analysis and spatial integration of the data, a first group of maps was presented at Québec Exploration 2009 (November 23 to 26, 2009, in Québec). This initial assessment identified 2,054 favorable zones for 11 mineral elements (Table 1), which were posted on the MRNF website. Of these zones, 1,373 are unstaked targets corresponding to secondary environment geochemical anomalies.

A second group of anomalies, enhanced to eliminate background geochemical “noise” from certain metals of interest, will be presented at PDAC 2010 (March 7 to 10, 2010, in Toronto). The targets will be unveiled simultaneously on the MRNF website in Google Earth format and in GESTIM.

Data Processing Method for First Group of Geochemical Maps and Targets

The first group of secondary environment geochemical maps was produced for 11 metallic substances (Ag, As, Au, Co, Cu, Li, Mo, Ni, U, Y, and Zn). In addition to data from previous years, the database includes over 50,000 ICP-MS results from new surveys and re-analyses conducted by MRNF in 2008–2009. These maps are available in ArcGIS, ASCII, or PDF. To make them easier to read, they have also been incorporated into the Google Earth module.

The maps were produced using the ModelBuilder module in ArcGIS, which made it possible to automate the spatial analysis and integration steps. The procedure extracts samples from different survey types (lakes, streams, till, or soil) separately and interpolates the raw values and their percentile equivalents for selected elements by the natural neighbor method. A distance buffer, based on the element’s estimated mobility and type of sample is then applied. The interpolated raw values are recalculated into natural log values. The resulting image of Québec is sequentially divided into 25 portions corresponding to distinct geological environments. Québec is first divided into five geological provinces (Appalachians, St. Lawrence Lowlands, Churchill, Grenville, and Superior). Then Churchill is subdivided into four sub-provinces, Superior into 13 sub-provinces, and Grenville into six lithological assemblages. Thus, the populations of values from each portion are broken down into 25 classes using the natural breaks method and then recombined at the end of the process. Ultimately, three maps are generated for each element: a map of raw values, a second map showing six percentile classes (0, 50, 75, 95, 97.5, and 100), and a third map of the 25 classes divided by natural breaks.

For all elements except gold and lithium, an analysis of the spatial association between classes of natural breaks and mineralized deposits containing these elements established clear spatial associations of known deposits of these substances with higher value natural break classes. These well defined classes were used to create secondary environment favorability zones in Shapefile that can be viewed in GESTIM. The unstaked portions of these zones, as of October 31, 2009, represent targets, which are also available in Shapefile format in GM 64290 (Table 1). All of these zones are available in Google Earth format.

Data Processing Method for Second Group of Leveled Maps and Targets

Work at CONSOREM on lake sediments shows that in contrast to gold, arsenic, and uranium, base metals (Ni, Cu, Pb, Zn) require signal enhancement (Trépanier, 2009). The second data processing phase involves a database of 5,948 isolated targets delineated by the enhancement method developed by Sylvain Trépanier at CONSOREM, which uses the residual values from a multiple spatial regression (except for pure zinc). The data is processed using a database of 90,844 lake sediment samples leveled by the author for 18 elements (Al, Ba, Ca, Ce, Co, Cr, Cu, Fe, K, La, Mg, Mn, Ni, P, Ti, U, V, and Zn). The leveling is based on a group of 43,336 samples re-analyzed for 53 elements by ICP-MS in 2008–2009. The target database is made up of the following element groups: (see Table 2):

1. One group of five elements associated with pure deposits (Cu, La, Ni, U, Zn)
2. One group of two elements (Cu, Zn) associated with volcanogenic massive sulphide (VMS) deposits
3. One group of two elements (Ni, Cu) associated with magmatic Ni-Cu deposits
4. One group of three elements (Cu, La, U) associated with iron oxide deposits enriched with Cu-U-Rare earth elements

The predictability of the proposed target groups was tested in each case by the weights-of-evidence method (W of E), which determines the effectiveness of the approach used.

Table 2: Lake Sediment Targets by Multiple Spatial Regression

Publications

The first group of geochemical maps and favorable zones was published in GM 64290 and is available in the MRNF’s e-SIGEOM-Examine databank:

http://sigeom.mrnf.gouv.qc.ca/signet/classes/I1102_index?l=f&entt=LG

The favorable zones are also available in Google Earth format on our website:

http://www.mrnf.gouv.qc.ca/publications/mines/zones-favorables.zip

Remember, a sneak preview of the second group of targets will be posted in Google Earth format on our website and in GESTIM on March 7, 2010, as part of PDAC 2010. The maps leveled for the 18 elements listed above as well as the targets will be available in digital format and PDF in the document EP 2010-01, slated for publication in April 2010.

References

Trépanier, S. 2009. Rehaussement des anomalies géochimiques dans les levés régionaux de sédiments de lac : exemples du Québec et du Labrador, Résumé des conférences et des photoprésentations, Ministère des Ressources naturelles et de la Faune, Québec, DV 2009-05, 74 pages. 

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