The paragenesis of some gold and copper ores of southwestern Oregon

Abstract

Copper ores in Southwestern Oregon are associated with greenstones and serpentines, while gold ores are associated with Tertiary lavas and nearly all of the pre-upper Cretaceous rocks. The ores consist of hypogene minerals deposited by mineralizing solutions ascending along fractures, faults and shear zones. The mineralizing solutions are believed to be related to quartz-diorite and related rocks. The ores represent deposition in the hypo-, meso- and epithermal zones. Generally deposition occurred in three stages separated by minor fracturing. Gold and copper minerals were deposited in the intermediate stage. Calcite and hydrous gangue minerals--chlorite, sericite and prehnite--were well developed along fractures in the sulphides during the late stage. Secondarily enriched ores are comparatively rare. Oxidation zones are commonly 25 to 30 feet deep rarely exceeding 100 feet. Unusual minerals identified included cassiterite, native bismuth, and tellurium.