Geology and hydrothermal mineralization in the vicinity of Rocky Top, Marion County, Oregon.

Abstract

The Rocky Top area is located within the Western Cascades subprovince of Oregon, approximately 65 km east-southeast of Salem. Late Oligocene to late Miocene age volcanic rocks exposed within the area form an impressive 3000 meter thick stack of calc-alkaline volcanic rocks which locally records subsequent events of tectonic deformation, magmatic intrusion, and hydrothermal mineralization. Pliocene to Pleistocene volcanic rocks in the Rocky Top area are unaltered, chemically distinct, and found as intracanyon flows into the older rocks. Plutonic rocks of late Miocene age have been hydrothermally mineralized and are exposed as northwest-trending dikes and small stocks. Their spatial distribution as well as mineralogical, textural, and chemical features indicate that they are related to the nearby Detroit Stock. Early formed quartz diorites at Sardine Creek and Rocky Top are exposed as dikes with sharp to slightly brecciated contacts, and were emplaced along pre-existing northwest-trending structures. Later hornblende granodiorites, with contacts defined by well-developed intrusive breccias, are exposed as irregularly-shaped northwest-elongate dikes and small stocks. Stratigraphic reconstruction from Sardine Creek to Rocky Top suggests that the later hornblende granodiortes were emplaced at a minimum depth of roughly 1 km, with the earlier quartz diorites intruding to shallower levels. Propylitic alteration is widespread throughout the Rocky Top area and intensifies with proximity to northwest-trending structures. Potassic alteration is limited to within the Detroit Stock, where several samples contain incipient veinlets and diffuse replacement zones of hydrothermal biotite. Late-stage sericitic (sericite-quartz) and argillic (clay-quartz+ or -barite) alteration is characterized by the replacement of groundmass and phenocrysts by sericite or clay minerals, quartz, and pyrite, along with a loss of primary textures, which accompanies mild to strong bleaching of the wall rocks. Late-stage alteration is structurally controlled, and overprints earlier propylitic and potassic alteration. Zones of hydrothemal metallization are narrow, weakly developed, and lack evidence of past exploration activity. Sulfide minerals occur as open-space fillings and as disseminations in the volcanic and plutonic rocks. The principal sulfide is pyrite, although sphalerite, chalcopyrite, and galena are locally abundant in small veins and disseminations associated with sericitic alteration. Sulfur isotopic compositions of these minerals range from +2.8 per mil to -3.3 per mil and average about -0.5 per mil. This relatively narrow range of delta (super 34) S values, near 0 per mil, is suggestive of a magmatic origin of sulfur and is consistent with data obtained elsewhere from the Western Cascades. Isotopic temperature estimates from coexisting sphalerite and galena indicate sulfide deposition occurred at 200-220 degrees C. More than 80 rock-chip samples from the Rocky Top, Sardine Creek, and Detroit Stock areas have been analyzed for Cu, Pb, Zn, and other trace metals. Concentrations of these metals in samples from the Rocky Top area range up to 16 ppm Ag, 16 ppb Au, 830 ppm Cu, 75 ppm Mo, 1330 ppm Pb, and 3570 ppm Zn. Threshold values dividing background and mineralized samples were determined to be 60 ppm Cu, 30 ppm Pb, and 100 ppm Zn. The relative proportions of these metals in mineralized samples depict a progressive change with increasing horizontal and vertical distance from more Cu (Zn) rich at the Detroit Stock, through Zn (Cu) at Sardine Peak, to Pb (Zn) at Rocky Top. Investigation of the interrelationships between mineralization and associated plutonic rocks combined with volcanic stratigraphy, structure, and topography suggest that Rocky Top may be one of the youngest and highest level hydrothermal systems recognized in the Western Cascades. Although plutonism and hydrothermal mineralization in this area has many features in common with nearby m

22 samples from this reference