The gold-base metal deposits of the Greenhorn Mountain area of northeastern Grant County, Oregon are contained in Carboniferous argillites and greenstones, Jurassic (?) gabbro and ultramafics, late Jurassic or early Cretaceous biotite-quartz diorite, and early Tertiary (?) porphyritic and granophyric dikes. A fracture pattern consisting of west-northwest trending strike-slip faults and east-northeast striking reverse faults cuts all of the above mentioned rock with the exception of the dikes.All veins and lodes are associated with dikes that occupy the zones of high-angle reverse faulting. They are most common and attain their greatest development as "footwall" veins, and ore shoots occur along local flat-tenings of the dip of the fault surface. Additional ore shoots are associated with minor gash fractures that branch off the reverse faults.