BLAST ROCK MOVEMENT AND ITS IMPACT ON ORE GRADE CONTROL AT THE COEUR ROCHESTER MINE

摘要

Most western United States open pit gold mine ore bodies are disseminated deposits. The mineralization is so irregular that it is difficult to visually identify the ore/waste boundary. Blast hole samples are used to interpolate digging polygons based on the pre-blast position of the bench. Dilution caused by blasting is of major concern at these mines, since blast movement can mix and dilute the ore and move it beyond flagged digging polygons established on the blasted rock according to their pre-blast location. Typically, mines try to reach a compromise between blast fragmentation and movement.A recent survey of blasting practices in 27 Nevada gold mines, conducted by the Mackay Mineral Research Institute (MMRI), has shown that many mines choose to use a small powder factor to control rock movement. This often results in poor fragmentation and digging efficiency. In mines where geology and/or equipment requirements dictate a high powder factor to achieve optimum fragmentation and productivity, susceptibility to excessive blast movement increases significantly. To date only a few operating mines are actively monitoring rock movement induced by blasting.This paper presents the results of a research project into blast-induced rock movement at the Coeur Rochester Mine. A total of six blasts were monitored using the Quarryman laser profiler to locate internal and surface markers. It was found that on average 17.7-28.5 feet horizontal movement occurred with a powder factor of 0.57-0.76 Lb heavy ANFO/Ton. This rock movement results in dilution from 3 to 47 percent depending on the amount of rock movement and the size of the digging polygon. Methods to reduce blast-induced dilution are discussed, and it is concluded that the dilution induced by the rock movement at a particular mine can be minimized based on a systematic study of rock movement caused by blasting.