XRF will pick up gold, silver and zinc just fine, but it will...

XRF will pick up gold, silver and zinc just fine, but it will only pick up what you're pointing the gun at. When you have a whole bunch of fractured rock chips coming up out of your RC hole, it is impossible to get a representative sample result with the XRF without firing it at pretty much every piece of dust in your sample bag, so it is utterly meaningless, and would be grossly misleading to report. With an assay, you'll take the whole sample, and then the lab will take care to take a representative portion to assay.

XRF is useful in the field for testing grab samples (although again, you tend to point it at the most interesting looking rocks, which can be somewhat biased, or if you have some drill core.

Yes, sulphides could be barren pyrite, that's always a risk, but if you're in an area with known copper occurrences (i.e. a historic mine) and there are green rocks at the top of the hole, then its at least encouraging that the sulphides might have something other than just iron and sulphur...