USE OF A HAND-HELD GAMMA-RAY SPECTROMETER AS A MAPPING TOOL: EXAMPLES FROM THE CANADIAN SHIELD OF ONTARIO

Assay-mode measurements of K, U and Th content or rocks, obtained from a hand-held Exploranium™ GR-135G MiniSpec gamma-ray spectrometer are an effective field mapping tool. Initially used primarily to document radioactive mineral occurrences located in the Grenville and Southern provinces of the Canadian Shield in Ontario, since 2007, the spectrometer has been used to distinguish between different granitic suites of Proterozoic and Archean age and to map differences in clay content in low-grade metasedimentary rocks (see Table). It has also been effective in distinguishing between mafic extrusive and intrusive rocks of Archean and Paleoproterozoic age, including mafic dikes, even at low abundance levels (see Table). The spectrometer results have been verified by subsequent whole-rock geochemical analyses, as well as by detailed tests of reproducibility and accuracy at several test sites. The spectrometer has the advantage of providing real-time data during mapping, as well as providing data from a greater number of stations than would be possible solely on geochemical sampling.

Rock unit	K	U	Th	Total
Average results based on a 300 second counting time, excluding dead time. Results include dead-time correction.	wt. %	ppm	ppm	cpm
Non-radiogenic granites, Superior Province
Equigranular monzogranite	4.1	3.6	11.1	761
Radiogenic granites, Superior Province
Equigranular monzogranite	4.4	18.2	39.8	1362
Equigranular monzogranite with pegmatitic patches	2.9	32.7	20.3	1581
Metasedimentary Rocks, Southern Province
White metasandstone, upper Matinenda Fm	0.6	8	19	655
Green metasandstone, middle Matinenda Fm	3.2	6	14	762
Green metasandstone, lower Matinenda Formation	4.3	43	102	3235
Mafic Igneous Rocks. Southern and Superior provinces
Thessalon Fm mafic metavolcanic rocks (~2.47 Ga)	0.8	0.9	4.5	293
Thessalon Fm intermediate metavolcanic rocks (~2.47 Ga)	3.2	2.0	6.1	466
vesicular dikes (feeders to Thessalon Fm intermediate metavolcanic rocks) (~2.47 Ga)	2.8	2.7	7.0	547
Archean metagabbro (>2.65 Ga)	0	0	0	95
Matachewan dikes (2.47 Ga)	0.5	0.0	1.4	191
Nipissing metagabbro (2.22 Ga)	0.8	0.0	2.3	210