CONTRIBUTIONS OF HIGH RESOLUTION X-RAY COMPUTED TOMOGRAPHY TO GOLD ORE GENESIS AND PROCESSING

High resolution X-ray computed tomography (HRXCT) is the industrial equivalent of medical CAT scanning and provides a mechanism for non-destructive, in-situ studies of the three-dimensional distribution of mineral and fluid inclusions within rocks. HRXCT produces two-dimensional images ("slices") that reveal the interior of an object as if it had been sliced open along the image plane for viewing. A HRXCT image is generated by differences in X-ray absorption that arise principally from differences in density within the object. By acquiring a contiguous set of slices, a density map for all or part of a sample volume can be obtained, allowing three-dimensional inspection and measurement of features of interest. Because the HRXCT technique differentiates mineral grains largely based on their contrasting densities, these studies are particularly effective in the study of gold grains in contrast with typical rock-forming minerals and other metallic minerals. Comparative studies of synthetic cores that incase gold grains of known dimensions demonstrate that HRXCT volume determinations are accurate on grains with minimum dimensions of roughly 1/1000 of the imaging field diameter.

HRXCT is an effective technique for megascopic studies of gold distribution in fracture-controlled ores. HRXCT volumetric data for many hundreds of grains in high-grade Cripple Creek ore samples indicate a substantial percentage of the gold is contained in a small percentage of grains, e.g. on average 50% of the gold in the samples is derived from the largest 9% of grains. Conversely, the smallest grains that comprise half of the population of grains only account for 15% of the total gold concentration. Microfocal imaging of ≤1-cm diameter cores of typical stockwork ore from the Grasberg porphyry deposit has identified gold grains as small as 6.5 µm. All of the gold grains observed occur in contact with copper sulfides, providing support for models of gold exsolution from high temperature sulfides. These investigations suggest considerable potential for HRXCT studies to contribute to the understanding of hydrothermal ore-forming systems and to practical applications such as ore dressing to maximize precious metal recovery.