Platinum-group elements in ordovician magmatic Ni-Cu sulfide prospects in northeast Scotland

Previous work on early Paleozoic mafic-ultramafic intrusions in northeast Scotland identified two targets for Ni-Cu-PGE exploration. The first prospect is at Arthrath, near the town of Ellon, and the second, in the southeastern corner of the Knock intrusion, near the town of Huntly, around the farms of Littlemill and Auchencrieve. Both prospects occur within a group of mafic-ultramafic rocks known as the Younger Basic intrusions, emplaced broadly synchronously with the later stages of Ordovician amphibolite-facies metamorphism of metasedimentary units of the Dalradian Supergroup. We have re-examined the available samples from the Ni-Cu exploration program in terms of their PGE potential, placing particular emphasis on the less-well-known Arthrath prospect. PGE abundances of up to 418 ppb Pt (Littlemill) and 458 ppb Pd (Arthrath) associated with zones of Cu-Ni-Fe sulfide mineralization have been identified. At Arthrath, the Pd is hosted primarily in merenskyite enclosed in base-metal sulfide; the occurrences of platinum-group minerals define a bimodal distribution. At Littlemill, the occurrences of platinum-group minerals involve mobilization and redeposition of precious metals in sheared rocks enclosed within the base-metal sulfide zones. When the mineral compositions of all the rocks associated with the zones of Cu-Ni-PGE mineralization are considered relative to the known differentiation series present within these intrusions, a model involving the mixing and mingling of relatively late, primitive magma and pre-existing differentiates is favored for both prospects. In particular, a cumulate orthopyroxenite unit at Arthrath closely associated with a zone of Cu-Ni-PGE mineralization is directly interpreted as the result of the influx into a magma chamber of a late, primitive magma. Previous work had suggested crustal contamination as the mechanism of sulfide immiscibility. Although evidence of crustal contamination is present, it is not favored as the direct mechanism for the formation of the sulfide-rich zones that comprise the principal targets of Ni-Cu-PGE exploration.