Confusing Grade With RecoveryI have posted several times on the shortcomings of Davis Tube for "assays". "DT recoverable nickel" assaying was introduced by Cliff's, likely because it is the standard they use for magnetite iron ore and they are quite comfortable with it.
Both recovery and concentrate quality are important. Concentrate quality is the primary goal of magnetite productions, where recovery is far more important for FPX awaruite.
FPX initially embraced DT recoverable nickel, probably because it offered a shortcut around metallurgy. While this seemed to simplify things for FPX, it now leaves us in the position of having to fixed standard for ore grade. I say this because DT assaying results are dependent on several arbitrary test parametyers. Anong these are grind size; magnetic field strength; water flow rate; and tube inclination angle.
Baptiste is an extensive, consistent, homogenious, ore body derived from peridotite. So, the question needs to be asked, "If there is indeed variation in grade across the ore body, what is the cause?" In other words if there is less nickel present in some locations, where did the nickel go? It should also be pointed out that these peridotites are also the source of lateritic nickel deposits. The only difference is that the laterites are weathered at surface in warm, wet climates and end up as oxide ore bodies. Baptiste was "serpentinized" or weathered if you will, at depth and temperature, in the absence of both sulfur and oxygen. Lets just call these "Natural Hydrometallurgy"
I am going to go out on a bit of a limb here and suggest that there is no significant variation of ore grade at Baptiste. Working from the average .22% Total Nickel head grade and the statement that approximately 75% of this nickel occurs in awauite alloy, my estimate is the the entire ore body grades an average of .165% nickel-in-alloy (awaruite).
This leaves all of the reported DT assaying as no more than a proxy for grain size. It is not really measuring "grade" at all, except where the grain size it large enough that the DT captures all of the awaruite. And surprise, surprise, .165% nickel is about the global upper limit of all of this DT "Assaying".