Geology, geochemistry, mineralization of the Yellowbottom-Boulder Creek area, Linn County, Oregon.

Abstract

The Yellowbottom-Boulder Creek area is immediately west of the Quartzville mining district in the Western Cascades of Oregon. Volcanic rocks comprise a series of basaltic to rhyodacitic flows and volcaniclastic deposits which are believed to correlate with the Sardine Formation (mid-Miocene). These volcanic rocks dip to the southeast (5 degrees to 25 degrees ), presumably into the hinge zone of the Sardine Syncline located immediately to the east. Intrusive rocks consist of small stocks of intermediate composition (quartz diorite, quartz monzodiorite, quartz monzonite, granodiorite, and tonalite), and dikes and plugs of basaltic to rhyolitic composition. The largest intrusion is a quartz monzodiorite stock that covers approximately 1 km (super 2) , for which the name Yellowbottom Stock is proposed here. Pliocene flows of diktytaxitic basalt and porphyritic andesite of High Cascade origin are preserved on the crests of Galena Ridge and Packers Divide. A Quaternary flow of trachybasalt partly fills the valley of Canal Creek and is associated with two cinder cones: they presumably represent the last pulse of volcanic activity in the Quartzville area. Major-oxide concentrations in the volcanic and plutonic rocks display systematic trends (decreasing Al (sub 2) O (sub 3) , TiO (sub 2) , FeO (sub T) , MgO, CaO, and P (sub 2) O (sub 5) , and increasing BaO and K (sub 2) O, with increasing SiO (sub 2) content) on Harker variation diagrams. These trends may suggest differentiation from a single batch of magma. The chemistry of plutonic rocks from the study area, and from the Western Cascades in general (calc-alkaline, low K (sub 2) O content), is similar to that of some island-arc terranes, including the Southern California batholith, Caribbean, southwestern Pacific, and Pacific Northwest areas, and largely suggests that the magma was derived from the mantle with little if any crustal contamination. Hydrothermal alteration has affected all rocks of Miocene age. Propylitic alteration is widespread in areal extent and is thought to have been produced by the interaction of magmatic fluids with the host rocks. In contrast, argillic-phyllic alteration is structurally controlled, and may have resulted from hydrolysis reactions between the hosts and meteoric waters that invaded the structures subsequent to propylization. Zones of brecciated and silicified rock range from linear to cylindrical in shape, and are commonly mineralized.

8 samples from this reference