Changes in grade, volume and contained gold during the mining life-cycle of gold placer deposits

CIM Bulletin, Vol. 80, No. 903, 1987

J.D. BLISS, G.J. ORRIS and W.D. MENZIE, U.S. Geological Survey, Menlo Park, CA

Analysis of gold placer data from gold placers throughout the world suggests that gold grades and volumes cannot be used to distinguish between most types of gold placers. Only the alluvial plain and fan placers are significantly different among the types of gold placers considered. Gold grades and volumes change when working gold placers go from small-volume methods to large-volume methods. The small-volume methods are either of the surface or subsurface (drift mining) type. The odds that a gold placer will be dominantly worked using small-volume methods at the surface are about 5:3. Once small-volume mining has occurred, the odds against subsequent large-volume mining are about 4:1. If a placer deposit is suitable for large-volume mining, and the amount of gold produced from small-volume mining was reported, an estimate of the remaining gold (log,gleg) can be made using the following equation:log10(GoldLVM) = 1-76 + 0.46*log10(GoldSvM) The relationship shown by this equation suggests that very large gold placers have over half their total gold content in the rich gravels appropriate for small-volume mining. These relationships in conjunction with other data and computer simulation can be used to estimate placer gold resources for an area targeted for mineral resource assessment.

Keywords: Gold placer deposits, Simulation, Gold recovery, Grade-volume models, Grade estimation.