Depends if it is partial leach or 4-acid digest. It also depends on the region because it is all relative, you need to look at background levels for that particular area, and take away any possible sources of contamination. 100ppb is high though, 100ppb = 0.1ppm or 0.1g/t Au. Some mines in AUS mine gold at 0.8g/t Au (some even lower now that gold is $1600!). So your only looking at a 8x dilution factor for potentially underlying ore. It is generally more encouraging to have a sizable geochem anomaly with other geological and geophysical supportive evidence than to have a spot geochem high though, although it depends on the situation and each target needs to be evaluated individually.
