ACCURATE IN SITU THREE-DIMENSIONAL MEASUREMENT OF ECONOMIC TRACE PHASES IN GEOLOGICAL MATERIALS USING HIGH-RESOLUTION X-RAY COMPUTED TOMOGRAPHY

High-resolution X-ray computed tomography (HRXCT) is a powerful method for nondestructively imaging the interiors of geological specimens. Each HRXCT image corresponds to a virtual slice through the object being scanned in which gray levels reflect relative X-ray attenuation, which is a function of density and atomic number. By acquiring a contiguous series of slice images, data describing an entire sample volume can be obtained. The high density and atomic number of many economic trace phases results in their having very high attenuation coefficients, making even very small grains distinguishable and potentially measurable. However, the finite resolution of tomography can complicate such determinations due to blurring and partial volume effects, which cause the attenuation signal of a particular grain to extend beyond its true boundaries, making high-attenuation trace phases appear larger than they actually are. We present here a methodology that has been developed to allow accurate measurement of the volume, shape and orientation of such trace phases by determining the total attenuation signal associated with each grain. The method was tested by embedding native gold grains with known dimensions and volumes and various shapes into a simulated rock matrix of quartz and epoxy, and scanning the sample at successively decreasing levels of spatial resolution. Overall, results obtained from analysis of the scan data are in excellent agreement with physical measurements of mass and cross-sectional area. Volume determinations were accurate even on grains with minimum dimensions of roughly 1/1000 of the imaging field diameter. As resolution decreases, accuracy is in part linked to grain shape, which can also be evaluated from the HRXCT imagery, allowing a compensating factor to be formulated.