The weighting of assays and the importance of both grade and specific gravity

CIM Bulletin, Vol. 86, No. 967, 1993

Peter A. Bevan, Consulting Geologist, Mississauga, Ontario

In massive and semi-massive deposits, it is essential to take samples continuously for specific gravity determination as well as for assays of the various metals of interest. Failure to use specific gravity in mixed deposits of high density contrast will result in incorrect determinations of the average grade. Graphs and linear equations of specific gravity against the grade of one metal are not considered accurate enough in a multimineral deposit, but appear to be very satisfactory in a theoretical one-sulphide/gangue mix. Specific gravities based on base-metal values only in massive sulphide deposits will result in underestimating the effect of low grade areas on the average grade and also cause a reduction in the calculated tonnage.

Keywords: Specific gravity, Grade determination, Ore estimation.