The silicate saturation of olivine is 0.44-0.46% depending on various things including forsterite/fayalite content, oxygen fugacity of the melt, etc. However, if there are any magmatic sulphides at all, >95% of the Ni, and 98%+ Cu and PGE will go into the sulphide phase. Of course, in cumulate rocks the presence of sulphides does not automatically mean that the sulphides are in equilibrium with the silicate cumulate, as they can be a late-stage remobilised melt of silicate.
Serpentinites can contain up to 50% volatiles in a fully hydrated state, meaning the original nickel content of 0.46% can be as low as 0.23-0.3% in olivine adcumulate rocks on a hydrous basis, so this can be a trap for young players.
At Sayabouly, therefore, 0.56% to 0.79% Ni is therefore more than the silicate saturation capacity of an olivine adcumulate. Considering this is a serpentinised mesocumulate to adcumulate, therefore, the anhydrous nickel content is definitely indicative of a nickel sulphide phase.Given as much as 0.23-0.46% (hydrous or anhydrous) could be bound up in silicate, this leaves 0.25-0.5% available as nickel sulphide.
The particular nickel sulphide mineralogy could also throw you a curve ball, as nickel tenor in the sulphide will determine the volume percent of sulphide within the rock. If the intrusion has exsolved millerite, 0.2% Ni in millerite is as little as 0.3% volume. I have personally seen 0.8% Ni in millerite in core, and it's so insignificant you can log it as 'trace' and get surprised by the assays.
Assuming the best case scenario of pentlandite and very little silicate nickel, 0.75% Ni in sulphide would be ~2% pentlandite.
Further, in lower saprolitic to saprock, the pentlandite (brassy) could oxidise to violarite,which is a black-purple mineral. Black-purple violarite within black serpentinite is devilishly difficult to estimate. I have also, again, personally been surprised by a violarite intersection of 3% Ni which was very difficult to spot when logging core at production rates.
So, yeah, EZ PZ guys.
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