New field, geochemical, and petrographic evidence from the Bon Accord nickel body: Contamination of a komatiite by deep mantle or meteorite source?

Abstract

The Bon Accord nickel body has been known since the 1920s to contain rocks with up to 50 wt% NiO. Numerous nickel-rich minerals have been described from this deposit. However, none of these minerals contains significant Cu or S, making the deposit chemically distinct from all other known Ni deposits. The origin of the Bon Accord nickel body is highly contentious, with previous studies suggesting three major possible origins: (1) a hydrothermal origin; (2) an Fe-Ni meteorite that fell into and was altered by an active ultramafic lava flow; or (3) a deep mantle plume that contained a fragment of nickel-rich material. Here, we present new field, petrograph ic, and geochemical data in an attempt to constrain the origin of this enigmatic body. Based on our fieldwork, there are at least two distinct Ni-rich bodies. Based on the trace-element chemistry, the protolith of the body was a komatiite, likely belonging to the Weltevreden Formation. Because the Ni end member of olivine (liebenbergite) is present in the form of euhedral crystals, this mineral most likely crystallized from a Ni-rich melt. The redistribution of the nickel appears to be due to hydrothermal activity that occurred during the intrusion of the Stentor pluton. Consistent with previous studies, we find that the komatiitic affinity of the host rocks, the stratigraphic controls on the deposit, and the regional distribution of Ni-rich material are inconsistent with a meteorite origin; instead, a komatiite plume sampling a Ni-rich portion of the mantle is currently the best explanation for the origin of the Ni-rich material.