Introduction
Abundant Canadian porphyry copper case histories originate from the prolific copper belts in British Columbia, including the famous Quesnel (green, right) and Stikine terranes. These formed during a narrow time interval (212-195 million years ago) at the Jurassic-Triassic boundary, along a migrating, pre-accretionary magmatic arc1, producing many porphyry deposits along parallel alkalic and calcalkalic belts. Dozens of examples have been documented, where potassic alteration associated with the mineralizing hydrothermal system measurably changes the normal radioactive element signatures of the host rocks. Gold-rich, Cu-Au alkalic porphyries are of particular economic interest.
Fundamental Method
The cartoon below illustrates the basic spectrometric principles. In all cases, gamma rays from potassium, uranium and thorium within the earth’s surface material (rocks, overburden) travel upward into handheld/airborne detectors where they are measured.
On the geological block diagram, volcanosedimentary host rocks (green) are intruded by barren (purple) and mineralizing porphyry (pink) plutonic rocks. The latter drives hydrothermal system(s) which alter host volcanic and plutonic rocks, in this case adding K (orange and yellow respectively), but NOT significantly changing the various thorium signatures of each protolith. We can therefore use gamma ray spectrometric determinations of K, eU and eTh to map the various rock types AND to detect the alteration, in most cases.
Data from a ground spectrometry traverse (black line, positioned across the front of the diagram) are shown as profiles of K (light blue), eTh/K ratio (red) and eU/eTh ratio (yellow). At the top of the diagram, corresponding airborne K map patterns are shown.
K appears to be highest over the altered rocks, but is also high over the barren intrusion. This presents a problem: how can we separate the two, so we do not waste time and money chasing “barren” K anomalies? The answer frequently is provided through use of the RATIO of K to Th (eTh/K), to highlight unusual K concentrations RELATIVE to the thorium. Individually, both of these elements vary depending on rock type, from low values in mafic rocks to higher values in felsic rocks, for example. But the RATIO between them stays more or less constant. Therefore, any addition or removal of K in either situation will be indicated by the ratio eTh/K. Because the barren felsic intrusion contains relatively more thorium (not shown here) than the volcanic rocks or porphyry, it’s eTh/K ratio is not as low. Thus the ground traverse shows eTh/K lows over the K “of interest” in the core of the alteration and along the fault-controlled mineralization to the right, but not over the K “of no interest” within the barren pluton. Several factors influence the degree to which this approach works, and caution must be used where the radioactive elements are in very low concentrations, but when used in concert with magnetic, geochemical and known geological information, low eTh/K ratios determined from airborne, ground or borehole spectrometry have been shown to provide valuable exploration guidance where potassic alteration (enrichment or depletion) has occurred.
- Jim Logan, Mitch Mihalynuk, Tom Ullrich, Richard, Friedman, 2006. Geology and Mineralization of the Ironmask Batholith. GeoFile 2006-5. Poster.