High sensitivity, quantitative airborne gamma-ray spectrometry has been applied extensively since the mid-1970s in support of geological mapping and mineral exploration.

The method depends upon the fact that absolute and relative concentrations of the radioelements K, U and Th vary measurably and significantly with lithology. Surveys undertaken in Greenland, North and South America, Africa, Australia and Europe show that the method is applicable to surface mapping in all types of environment. The method is very effective at subdividing acid igneous and metamorphic rocks in hitherto poorly mapped Shield areas wherever they are not masked by impermeable transported cover. It highlights those rock types characterized by unusual amounts or proportions of radioelements such as peralkaline, carbonatite, and ultrabasic complexes. During the last decade, applications have been extended into less radioactive environments, such as sedimentary basins, volcano-sedimentary terrane, and heavily glaciated or tropical weathered areas, where subtle contrasts offer reliable mapping guides.

In 1967, airborne gamma-ray spectrometry was a qualitative prospecting technique, that was beginning to be used for uranium exploration. By 1977, improved instrumentation and well-established procedures allowed reproducible quantitative data to be obtained and the method had become a primary tool for uranium exploration. Since then, the technique has been broadly applied to geological mapping and general exploration in a wide range of environments.

Geological Map of Canada	Radioactivity Map of Canada (Map 1600a)
	(Darnley et al, 1986)

It is perhaps ironic that, for decades, the worldwide success of gamma-ray detection methods in the search for economic uranium deposits, impeded general acceptance of the technique as a valuable exploration tool for many other commodities. However, with ongoing development of many case histories where uranium was not the target, came general acceptance by mappers and explorationists, that many other applications were possible by using the three radioelements as pathfinder elements, directly or indirectly associated with the elements of interest.

Airborne gamma-ray spectrometry surveys can be of direct assistance to exploration for many commodities, most obviously for U and Th, but commonly also for Sn, W, REE, Nb and Zr. Less often, but of importance in specific circumstances, radiometric anomalies can point to Au, Ag, Hg, Co, Ni, Bi, Cu, Mo, Pb, and Zn mineralization, either because one or more of the radioelements is an associated trace constituent or because the mineralizing process has changed the radioelement ratios in the surrounding environment.

A few of many GSC application examples are described below, grouped by generalized deposit types.

The role of gamma-ray spectrometry in the search for, and delineation of, these base metal deposits, depends on the type of VHMS mineralizing system and associated hydrothermal alteration chemistry.

	For example, in our case histories from Pilley's Island (1989, GSC Open File 1993) and Tulks volcanic belt (1992, GSC Open File 2481), Newfoundland, potassic alteration in the form of sericite and/or K-feldspar occurs proximal to mineralization, and is easily detected in the air and on the ground using a gamma-ray spectrometer.
Geology and images of K, eU, eTh, eTh/K and magnetic total field for the Extech 1 study area An 8-bit image representation of the Snow Lake data set also appeared on the CDROM prepared by the GSC's NATMAP group for the Shield Margin Project.	In other cases, however, such as the Cu-Zn deposits in the Snow Lake area, Manitoba (1991, GSC Open File 2300), (Shives, 1996), direct detection of the alteration associated with mineralization is not obvious, because the three radioelements are in low abundance in the tholeiitic host rocks and not significantly introduced or remobilized during hydrothermal alteration. However, the spectrometry here does provide important indirect exploration guides, such as identification of subvolcanic intrusions and, phases within them, related genetically and spatially to the ores. The spectrometry also reflects subtle lithogeochemical variations between deposits in the area, and between various volcanic cycles, in effect serving as a chemostratigraphic mapping aid.

In Canada, the search for low-grade bulk tonnage Cu-Mo deposits has been episodic. In the late 1980's emphasis shifted towards the gold-bearing alkalic porphyry systems. Our case histories from the Canadian Cordillera in British Columbia and Yukon Territory document the powerful exploration vectoring provided by gamma-ray spectrometry, through detection of intense potassium enrichment in the core and periphery of many of these deposits.

Airborne gamma-ray spectrometry results over the Casino deposit area, YT (1994, GSC Open File 2816) and Mount Milligan deposit area within the Quesnel Trough, BC (1992, GSC Open File 2535).

Stacked profile sections of 3 flight lines flown over the Casino Cu-Au-Mo porphyry deposit (1994, GSC Open File 2816) with corresponding colour contour maps of K, eTh and Magnetic Total Field. Of several relatively high anomalies, the Casino deposit signature can be distinguished by its unique eTh/K ratio low value.

In 1996, a large multisensor helicopter-borne survey, combining physical data from aeromagnetic and electromagnetic (EM) sensors with the chemical information derived from gamma-ray spectrometry, was completed over a large portion of the Selwyn Basin in southern BC (1996, GSC Open File 2628).

Results over the famous Sullivan Mine and the surrounding Sullivan-North Star Corridor, suggest that the radiometric data do offer exploration guidance when used in combination with the mag and EM. The mineralization lies within subtle eTh/K ratio lows, related to potassium alteration in the form of muscovite.

There are numerous varieties of skarn-hosted mineralization, many of which contain elevated levels of uranium in association with other economic minerals. In the Sandybeach Lake area, NWT, a gold-bearing, polymetallic skarn was discovered through GSC ground follow-up to airborne uranium anomalies, coincident with aeromagnetic highs. Grab samples collected here contained 91 ppm Au (nearly 3 ounces per ton) along with pyrrhotite, arsenopyrite, scheelite, molybdenite, telluride, chalcopyrite, cobaltite, ilmenite, and minor uraninite. The occurrence has been pursued by industry and a significant discovery is anticipated!

These are many application examples that can be listed under this very broad group. Many are based on using uranium and thorium variations to map mineralizing intrusive phases or alteration that may be late or post-magmatic, such as the Sn, W deposits common in Carboniferous peraluminous granites in Nova Scotia, e.g. Davis Lake Pluton (1988, GSC Open File 1784). In other specialized intrusions, U may provide a pathfinder for molybdenum or other gold-enriched zones. Unusual intrusions such as carbonatites often contain elevated U and/or Th in association with various rare or strategic metals, including Be, Nb, Ta, Ce, La, Y, Zr, Mo, P and others, e.g. Thor Lake, NWT (1990, GSC Open File 2252) and Allan Lake, Ontario (1986, GSC Map 36231(01 & 02)G).

In northern Canada, polymetallic mineralization discovered using gamma-ray surveys (1995, GSC Open File 2806) (Gandhi et al, 1996) has resulted in exploration and development expenditures of tens of millions of dollars and has produced significant additions to Canada's mineral inventory. These represent new Canadian deposit types and contain gold, cobalt, copper, bismuth and tungsten in over 150 million tonnes of ore, for a total resource of over US$7 billion. See below for more information on this important discovery.

Airborne survey results over one of these, at Lou Lake, shows the well defined potassium high (A), eTh/Th ratio low (B), eU/eTh high (C) and total field magnetic high (D), that surround several known, small, scattered mineral occurrences, placing them within the context of a large, potassium and iron enriched, polymetallic hydrothermal system.

Radiometric data can also distinguish between different types of surficial deposits (i.e. clay, sand and gravel), because of differences in the chemical and physical properties of the materials. For example, in the Snow Lake area of Manitoba (1991, GSC Open File 2300), both clay and sand deposits produce well-defined potassium anomalies, but only the clay has an associated uranium anomaly, due to the increased uranium content that is not present in sand.

Airborne radioactivity map of the Virden area, Manitoba, showing variations in the total radioactivity (Exposure Rate) that correspond to different soil types. Note the higher radioactivity in the northwest corner suggesting additional clay deposits in this area.

The discovery of several mineral deposits in northern Canada is a direct result of airborne gamma-ray spectrometry surveys carried out under the GSC's NATGAM program and from metallogenic work of various groups within Mineral Resources Division.

Three of these mineral deposits in the Southern Great Bear Magmatic Zone of the NW Shield resulted in exploration and development expenditures of tens of millions of dollars and have produced significant additions to Canada's mineral inventory.

Southeast of Great Bear Lake, Fe oxide breccia polymetallic mineralization at Lou Lake (Nico) and at Sue Dianne represent new Canadian deposit types and contain Au, Co, Cu, Bi, W in over 150 million tonnes of ore, for a total resource of over US$7 billion. Along the east arm of Great Slave Lake, the Thor Lake deposit contains several rare and strategic metals, including a combined Be, Ta, Ni resource exceeding US$5 billion. Mining feasibility studies are underway at all three deposits.

Featured review paper

This fully-illustrated paper provides further details on the Pilley's Island (Newfoundland), Lou Lake (NWT) and Casino (BC) deposits.

Early (1969) experiments in airborne gamma-ray spectrometry indicated that the information would be specialized and not readily applicable to geological mapping. Since then, surveys totalling many millions of square kilometres, carried out on all continents except Antarctica, have demonstrated that, in many situations, the airborne gamma-ray spectrometry technique is probably more useful than any other single airborne geophysical or remote sensing technique in providing information directly interpretable in terms of surface geology (Darnley & Ford, 1987).

Although airborne gamma-ray spectrometry is a technique dependent on physical phenomena, the results obtained are, for geological and exploration purposes, best considered in geochemical terms. Thus the technique provides a fast method of undertaking a ground-level geochemical survey from the air. As with any other type of surficial geochemical survey, the method indicates the radioelement composition of whatever material forms the surface. The relationship of the chemistry of this surficial material to the composition of bedrock must be inferred from consideration of complementary evidence, provided by geological maps, air photos, satellite imagery or ground inspection.

The usefulness of airborne gamma-ray spectrometry as a geological mapping and exploration tool hinges on two factors:

1. the extent to which the distribution of the radioactive elements relates to differences in the lithology of common rocks and the extent to which these are recognizably modified by mineralizing processes

2. the extent to which the radioelement content of bedrock is reflected in the composition of surficial materials that can be spatially related to their bedrock source

Unless there are detectable compositional differences between lithologies, and these differences are retained in surficial materials, the method can not be effective for lithological mapping. Wherever surface material consists of impermeable transported alluvium, lacustrine or marine silts or clays, clay till or aeolian sand, the method can only indicate the geochemistry of these materials and not that of the underlying bedrock.

The usefulness of airborne gamma-ray spectrometry as an aid to geological mapping is determined by the extent to which the variables add significant information to the features normally distinguished on a geological map. As with any geophysical survey, differences between AGRS data and the mapped geology are always much more interesting than the similarities. AGRS is a powerful aid to geological mapping in regions where the geology is complex, or access is difficult, however it can only provide information about underlying formations if their composition is reflected in surface material. It is a particularly useful method for subdividing and exploring those extensive areas of unmapped granites and gneisses that exist in many parts of the world.

Ground gamma-ray spectrometry allows a geologist to obtain higher resolution data to better resolve an anomaly observed on maps of airborne survey data. It provides the geologist with an opportunity to obtain higher-resolution data to define the anomaly in greater detail. In this photo, the geologist is obtaining more detailed information by taking spectrometry readings on both fresh and altered zones within a single outcrop.

In tropical regions, bedrock radioelement distribution is significantly modified by intense weathering. A systematic increase in thorium concentration and corresponding decrease in potassium (upsection) was observed at this highly weathered site in the Amazon Basin, Brazil.

The following maps are representative samples of geological mapping using airborne gamma-ray spectrometry data. The maps represent data sets that can be ordered in a variety of formats, both analogue and digital (point & grid data).

Northeast Alberta (1994, GSC Open File 2807)

Magnetic Total Field & Exposure Rate		Geology Map & Ternary Radioelement Map

Southeast Manitoba (1993, GSC Open File 2725)

Geology, Magnetic Total Field & Ternary Radioelement Map

Coldwell complex, Northern Ontario (1992, GSC Open File 2516)

In these experimental images, the intensity of the radiometric ternary image (red=eU, green=eTh, blue=K) has been modulated by airborne side-looking Synthetic Aperture Radar (SAR) data to provide a pseudo 3-D view of the ground surface. The water mask was obtained from Landsat Thematic Mapper (TM) data.

Newfoundland: Ternary radioelement map (GSC Paper 87-14, 1987)

Ternary radioelement map of the south coast of Newfoundland with a geological overlay (Broome et al, 1987). Note that this product is a map/paper combination that contains a detailed discussion of the ternary mapping technique.

Ternary radioelement & geology maps, Nova Scotia (1991, GSC Open File 2375)

This radiometric map of Nova Scotia is comprised of the data from a dozen airborne gamma-ray spectrometry surveys carried out over several years. Careful calibration of the spectrometer systems used in all of these GSC surveys allows them to be joined seamlessly, with litle or no levelling required.