A series of tests were undertaken for the Aurora Partnership-Aurora Project to solve inter-laboratory variations in the analyses of blast hole cuttings. Initial concern was centered on the probability that free gold occurrence was resulting in erratic results for the mine’s cyanide shake assays and outside laboratory fire assays.

Test Procedure

A 50 kilogram bulk sample of the Aurora belt cut head ore at nominal 6.30 MM was submitted for extensive testing. The sample was split using a bench rotary splitter, developed by Homestake Mining Laboratory, to obtain two working samples. The splitter is comprised of an Eriez magnetic 15A 4″ x 20″ vibratory feeder discharging to a set of four 2 liter plastic receptacles rotating at 60 rpm’s. Opposite receptacles were combined to form two separate sub-samples. Each sub-sample was crushed to minus 3.35 MM using a Barber-Green 6 inch laboratory gyratory cone.

Screen analyses were conducted on 3 of the riffle split/disk pulverized samples and 3 of the rotary split/ring and puck pulverized samples to compare particle size distribution in the pulps for each method of pulverization. Selected pulp fractions of corresponding prep methods were composited for 3-hour hot cyanide leach assay to determine leach extractable gold for those fractions.

Discussion

Summarization of the data indicates that despite a wide fire assay range, from 0.118 to 0.139 ounces per ton gold, inter-laboratory analytical method correlation of results for this sample are a function of particle size distribution in the pulp rather than the presence of free gold in the sample.

Rotary disk pulverizer plates require careful alignment and gap adjustment during installation to prevent uneven wear. Careful attendance to the disk pulverizer plate gap setting will reduce cyanide leach problems associated with coarse pulps. Better yet, optimization of a timer controlled ring and puck pulverizer removes the coefficient of variation for constantly changing disc pulverizer plate setting.