THE GEOLOGY OF JOSEPHINITE IN THE JOSEPHINE PERIDOTITE, SOUTHWEST OREGON
Josephinite, a mineral assemblage of native metal alloys, oxides, silicates and sulfides, is found in serpentinized portions of the Josephinie peridotite and as placer nuggets along Josephine Creek. Josephinite nuggets are several orders of magnitude larger than in-situ grains and contain abundant andradite garnet. These differences have led some investigators to suggest that nugget josephinite has originated through a different process than in-situ josephinite.
In the Josephine Mountain area, the peridotite is cut by a number of northeasterly trending shear zones. The Josephine Creek shear zone has been the locus for intrusions, enhanced serpentinization and repeated strike-slip deformation. In-situ metal alloys and josephinite are most abundant in highly serpentinized bedrock, and their distribution appears to be controlled by the degree of serpentinization of the parent rock.
Several mechanisms have been suggested for the genesis of josephinite. These models differ in their consideration of the metal alloys as primary or secondary components of the peridotite. A secondary origin is preferred through syn-serpentinization formation under locally reducing conditions. Experimental studies modeling desulfurization of Fe-Ni-Cu sulfides in a serpentinizing environment have shown this to be a feasible model.
Prior to the emplacement of the peridotite into the Klamath Mountain Arc, the bedrock of the Josephine Creek shear zone may have been enriched in metallic elements. Following obduction of the peridotite, serpentinization may have been locally enhanced by hydrothermal circulation surrounding dikes which were intruded into the shear zone. At this stage, under locally reducing conditions, josephinite was produced in unusual abundance.