Rapid Pyroclastic Mapping In Base Metal Exploration

CIM Bulletin, Vol. 70, No. 779, 1977

J. S. Fox, Research Scientist, Geology Division, Saskatchewan Research Council, Saskatoon, Sask.

Pyroclastic rocks are useful guides to volcanic centers due to the tendency of their fragments to increase in size and volume toward the source vent. The felsic rocks of these centers can be the prime hosts of base metal deposits in the Precambrian Shield; their sub-volcanic equivalents are often the focus of porphyry copper mineralization in the Cordillera. Quantitative mapping of pyroclastic rocks has not yet developed into a routine prospecting tool in Canada, however, possibly due to the cumbersome nature of existing "manual techniques and their limitation to structurally simple terrains. A rapid pyroclastic mapping method has been developed whereby the field geologist acquires only a down-plunge outcrop photograph, a record of the fragmental deformation ellipsoid and a record of some descriptive features from each station. Mean fragment size and volume per cent are obtained later in the laboratory from a photo-enlargement by means of a digital analyzer. A computer program uses the strain parameters to 'undeform' the field measurements and obtains genetic information from, the descriptive data. Results can be superimposed on a geologic map, with values for each station being represented by a dot of a specified diameter corresponding to the size or volume percentile range to which the value belongs. Control traverses were run over the pyroclastic rocks surrounding the Anderson Lake mine and the Flin Flon South main shaft in northern Manitoba. In both locations, the mapping has delineated regular increasing trends in mean size and volume per cent toward the headframe. Three hundred pyroclastic data points have been obtained by means of this method in the course of a routine mapping program in the Amisk greenstone belt 25 miles west of Flin Flon. Three hidden erruptive centers, two of which underlie aero-magnetic highs, have been outlined.
Keywords: Base metals, Exploration, Pyroclastic rocks, Volcanic centers, Down-plunge photography, Deformation ellipsoids, Size percentiles, Anderson Lake mine, Plin Flon South main shaft, Amisk Lake area.