Ann: Lab Results Confirm Kimberlite Target Areas, page-177

Thanks 1S1 for that extra information, it ties in well with the theory. The older pipes were cooler and contained the big diamonds that erupted earlier as you say. Being older also means they were exposed to the elements longer resulting in allowing more time for them to become more deeply weathered (low-magnetic) as they released their diamonds. The erosion and subsequent sedimentation means they will also be more deeply buried (further reducing magnetic response to EM surveys). The pipes on the lease would have come from a geologically similar magma chamber so I don't think the magnetic variability is due to the resulting kimberlite composition, that should be fairly similar. I think all the pipes on the lease, regardless of age were highly magnetic when they first erupted (non-oxidised kimberlite). Weathering (oxidation) is the factor that reduces their magnetism as well as sediment overburden. Therefore, the older pipes have had more time to become more weathered (oxidised) and eroded (more overburden) resulting in low magnetic signatures compared with the younger highly magnetic (non-oxidised) pipes.