Geochemistry of PGE enriched Proterozoic dolerites of the southwestern United States

Volumetrically significant Proterozoic dolerite sills and subordinate dykes intrude a variety of basement terranes across the southwestern United States. These dolerites are similar in age to the well mineralised Mid-Continent Rift intrusive complex near Lake Superior, yet there is no such mineralisation known in the Southwest. Both extensional and compressional regimes have been proposed for their emplacement, which overlaps temporally with the Grenvillian Orogeny. This study presents new geochemical data for Proterozoic dolerite sills from the Superior district in east central Arizona.

Immobile element data show two or three geochemically distinct groups, one of which is highly anomalous in PGEs (up to 0.013 Pd and 0.01 ppm Pt). Further investigation suggests the PGE enriched phase has a shallower, garnet bearing lower crustal influence, compared to the more mantle derived phases. This is supported by petrographic evidence of huttonite suggesting significant crustal contamination. Close spatial association micron scale grains of galena suggest the radioactive decay of Th. The apparent crustal contamination observed in the PGE enriched phase is at odds with conventional wisdom regarding the formation of orthomagmatic PGE deposits and remains unresolved.

Although this area of the crust has a complex geologic history, insights into the Proterozoic development remain enlightening. Further work is required to fully understand the processes at work and the significance of this finding. However, this discovery is likely to be of interest to explorers and may provide a competitive ‘first-mover’ advantage into an unrecognised Cu-PGE terrane.