GEOLOGY OF TABLE MOUNTAIN,
OREGON NEPHELINE DEPOSIT

Regionally, the Table Mountain area is surrounded by sandstone and siltstone sedimentary beds of the Tyee Formation (middle Eocene). The plutonic sill that underlies Table Mountain, which consists a peralkaline nepheline syenite composition*, locally intruded these sediments.

The sill exhibits a tabular shape that extends over 1.5 mi**, ranging in thickness from 250 to 400ft and gently dipping 6degrees in a S65degreesW direction. A few remnants of sediments overlay the nepheline syenite. A north-south fault intersects the sill body with a transverse movement.

There is very little overburden covering the nepheline syenite making it amenable to open pit mining.

Most of the area is heavily wooded with commercial conifers.

Additional dike intrusions of nepheline syenite, shonkinite*** , and camptonite **** compositions crop out in the area. These intrusions are thought to be the arteries of the sill at Table Mountain.

A nepheline syenite dike at Indian Creek, Mapleton quadrangle (approximately 18 miles south of Table Mountain), has a potassium-argon age of 33.6 m.y. (determined by R.W. Kisler, U.S. Geol. Survey).

The fresh nepheline syenite presents a somewhat homogeneous appearance but locally it ranges in color from gray to light gray to greenish. It is a fine-grained holocrystalline rock (phonolite-like) with a trachytic texture characteristic of plutonic flowage.

The rock is extremely tenacious and resists breakage when struck with a sledgehammer, making fresh rock chips difficult to obtain. On the surface, the nepheline syenite is weathered, exhibiting a pitted surface due to the leaching nepheline and analcime.

*Nepheline Syenite is a light-colored, medium to coarse-grained holocrystalline, silica-deficient, feldspathic, plutonic igneous rock largely made up of nepheline, sodium feldspar (albite) and alkali feldspar (orthoclase, microcline), but no quartz.
**Stan Mertzman, Professor of Geosciences, Department of Earth and Environment, Franklin & Marshall College,Lancaster, PA 17604-3003
***Shonkinite is an uncommon type of intrusive igneous rock found in Montana, Ontario, Canada, and the southeast Indonesian island Timor. The dominant materials that comprise the dark-colored rock are orthoclase feldspar and augite. Other minerals include olivine, biotite, and nepheline, with little plagioclase feldspar and no quartz. Shonkinite is closely related to several other relatively rare types of rock, such as theralite, essexite, and teschenite.
****Camptonite, sannaite, and monchiquite, are types of the rock in which the predominant mafic minerals are amphibole, augite, olivine, and biotite. Camptonite is distinguished form these closely related rocks, however, by the fact that it contains a greater amount of feldspar than feldspathoid minerals and more plagioclase than orthoclase feldspar.