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UBC Theses and Dissertations

A genetic model for alpine-type chromite deposits in the Josephine peridotite, Northwest California and Southwest Oregon Grond, Helen C.


The Josephine Peridotite of northwest California and southwest Oregon is host to numerous alpine-type chromite deposits. The deposits were classified into two groups according to textural varieties, host rock associations, degree of deformation and ratio of dunite to chromite. These include: class I (cumulate chromite) and class II (deformed chromite). Class I deposits are characterized by relatively undeformed cumulate textures, dunite host rocks and high dunite/chromite ratios ( 95:5). Class II deposits generally occur as massive to disseminated, highly deformed pods and lenses. They occur within shear zones in harzburgites and have a very low dunite/chromite ratio ( 10:90). Field evidence from shear zones within class I type deposits indicates that the deformation of cumulate chromite may result in the formation of class II deposits. This evidence includes the occurrence of small massive pods and blocks of deformed cumulate chromite which appear to be derived from the surrounding cumulate chromite but have many similarities with class II deposits. A genetic model is proposed in which chromite originally formed by cumulate processes (class I deposits) has undergone intensive solid state deformation in the upper mantle. This resulted in metamorphic differentiation between chromite and olivine, producing the massive concentrations of chromite (class II deposits).

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