re: Ann: EVG Updates Results from San Gerardo... An SG of 2.5 is a good thumbsuck. An SG of 1.8 is OK for laterites. Oxide material in this type of rock won't go down too far. The gold deposits nearby have refractory ore at shallow depths so the depth of oxidisation may not be too deep. Thus an SG of 2.5 is near enough and it makes the calculations easier.