Ann: High Grade Copper and Gold Identified at Big One, page-57

Exactly - most pXRF cannot be used for gold, especially on un-homogenised samples, as the detection limits and the accuracy/margin for error for pXRF gold are +/- 5 ppm - thats the reason why gold exploration companies don't bother to collect or report pXRF gold. By the time the pXRF can detect it, you are already in ore grade material, and its still less precise and accurate than a traditional lab assay.

Copper on the other hand is reasonably reliable ( around +/- 5% accuracy) in the 1000ppm to 50,000ppm Cu range (which is 0.1% to 5% Cu), higher copper grades than that are indicative but tend to drop off in precision.

The X ray beam is only 5-10mm so the instrument is only ever seeing a small part of the sample, homogenised crushed and pulverised samples are better.