Geophysical Logging Applied to Ore Characterization in the Zinkgruvan Mine, Sweden

The Zinkgruvan mine is a sulphide ore deposit in central Sweden where mainly Zn, Pb and Ag are mined. In 1988 a new mining method was developed in the mine. The main difference between this method and ordinary sublevel stoping is that no personnel operate inside the stope. Activities such as drilling, charging and loading of ore are done from drifts out of the stope.

When selecting a method for ore characterization emphasis was put on, besides accuracy, low costs for purchase and production, short time between measurement and result, and user-friendliness. The method that proved to be the most suitable was geophysical logging.

Cross-plots of density and magnetic susceptibility from samples collected in the mine showed that ore could be distinguished from waste rock. Tests with surface logging equipment were made in the mine with positive results. This led to the purchase of a specially designed combination probe built by ABEM AB, which records the density, natural gamma radiation and susceptibility logs.

The density log outlines the ore zone, which consists of skarn, compact ore and a number of veins. Because of pyrrhotite in the skarn, the susceptibility log can distinguish between skarn and ore. Finally by combining the two logs with the natural gamma ray log, up to 8 different rock types can be located. The mine geologist interprets the logs using a computer program. Running the program requires no expertise in geophysics but some experience of the geology of the area. The program locates ore boundaries with high accuracy, well within the requirements of mining (about 10 cm), and gives a reasonable estimate of the Zn grade.