Multi-scale radiometric mapping of potassium alteration: An example from Hemlo, Ontario

A common feature of hydrothermal ore deposits, whether epithermal gold, porphyry copper etc., is the style of potassic alteration that has resulted from the interaction between hydrothermal solutions and the surrounding wall rocks. The geometry and mineralogy of the alteration sequence is dependant upon the specific setting and chemistry of the deposit type. Using standard radiometric procedures at a number of varying survey scales may allow for ore body delineation based on the relative relationships of K, U and Th. The level (or scale) employed in a survey may be indirectly linked to the development stage of each deposit in terms of exploration versus mining. An airborne radiometric, survey generally flown at an elevation of 100m+, with a line and sample spacing of 100m to 800m X 25m provides a methodology for initially locating a region of potassic enrichment. The association of this feature with other geophysical signatures (VLF anomaly, reduced magnetic field, lineament intersection) would warrant a follow-up ground survey. The smaller field of view, together with the tighter sample spacing of the ground survey may provide a technique for differentiating individual lithological boundaries. When applied in an underground setting tighter sample control may permit mapping the limits of an alteration halo especially in areas where the lithological contacts have been obscured or are difficult to locate, due to intense metasomatism. Finally, high resolution borehole spectral surveys may permit even further refinement in the ability to delineate the lithological and in some cases the mineralogical responses as associated with the K, U and Th signal. Results from the Hemlo gold camp provides examples at each of these scales of the differentiation between lithologic and alteration signatures. An airborne radiometric survey shows the gold deposit is associated with a structurally controlled potassium high, in-situ ground measurements delineate the extent of the ore-zone, while borehole surveys have documented the spatial extent of the ore horizon in the mine.