The Quartzburg District is seven miles north of Prairie City ineast-central Grant County, Oregon. The District is in a pre-Tertiarywindow of volcanic, plutonic, and sedimentary rocks ranging fromPermo-Triassic to Eocene-Oligocene in age. The Permo-TriassicDixie Butte volcanics consist of thick basalts and andesites with minorvolcanic breccias and intercalated argillite, chert, and conglomerate.This unit was moderately to intensely fractured by closely spacedfaults prior to the emplacement of plutons of Early or Middle Triassicage. These plutons include spinel peridotite, gabbro, and granodiorite.Serpentinite is found in and adjacent to the ultramafic plutons. Asecond episode of intrusion is represented by the Late Jurassic DixieCreek granodiorite. The Dixie Creek granodiorite is a concentricallyzoned pluton, which changes in composition gradationally from a thin(less than 30 meters in width) marginal diorite, through granodiorite,to a core of quartz monzonite. The granodiorite portions of the plutonfed north-striking dacite porphyry dikes. Magmatic segregations ofchalcopyrite and pyrite are present in the diorite phase. A potassiumargonage determination on biotite from this pluton yielded a 145 m. y.date (Thayer and Brown, 1964). Sedimentary rocks of mid-Cretaceousage are present in the northern and southwestern parts of the District.They include chert pebble conglomerate, sandstone (lithic arenite),and a single exposure of mudstone. Dikes of basaltic and andesiticbreccia, rhyolite, and basalt of the Eocene-Oligocene Clarno Formationintruded the older pre-Tertiary terrain and fed flows that coveredlarge portions of the Quartzburg District. Deposits of air-fall tuffbreccia and minor siltstone and calcareous mudstone are intercalatedwith the flow rocks. During mid-Miocene time, the rocks of theDistrict were covered by flows of the Strawberry volcanics and theColumbia River Group. Uplift of the Blue Mountains Anticline andsubsequent erosion has exposed the pre-Tertiary rocks. Erosion ofthe mineral-bearing veins has produced rich placer deposits in DixieCreek and the John Day River. Explosive eruption of Mt. Mazamaduring Recent time left deposits of volcanic ash along hill slopes andin topographic depressions.Structural evidence suggests a post Eocene-Oligocene age formineralization. The Clarno Formation is crosscut by west-northwestto northeast-trending faults. These are in turn crosscut by northeastto east-trending faults which, along with the permeable contact zonesof the dacite porphyry dikes, guided the mineralizing fluids from a source at depth. These fluids formed rich mesothermal gold-quartzveins and hypothermal copper-tourmaline massive sulfide veins andreplacement deposits.
The Quartzburg District is seven miles north of Prairie City ineast-central Grant County