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Walker Valley, Mount Vernon, Skagit Co., Washington, USA

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The Walker Valley collecting area is located about 15 km east of Mount Vernon, Washington. The collecting site became popular among local mineral collectors in the late 1960’s after a period of quarrying for road fill that exposed geode and agate bearing outcrops (Mustoe, 1996a). “The outcrop appears to be a 100-m-wide mass of brecciated rhyolite bordered on either side by unaltered black basalt” (Mustoe, 1996a). In actuality, though, the rock is an andesite of “fairly uniform composition,” the lighter color of the brecciated rock is due to hydrothermal alteration, and an explosive hydrothermal event resulted in the brecciation of the deposit (Mustoe, 1996a). The early stages of mineralization at Walker Valley were probably the result of contact by hot hydrothermal solutions with the cooler wall rock, which caused minerals to precipitate (Mustoe, 1996a). A likely significant factor in the last stages of mineralization was when mineral-laden fluids encountered fractures, which caused precipitation due to lower pressures and boiling (Mustoe, 1996a).

A typical feature of Walker Valley geodes is a layer of the unusual mineral hisingerite, which forms a boundary between the andesite and the subsequent minerals (Mustoe, 1996a). The layer of hisingerite serves as an aid to collecting specimens at Walker Valley, because it is much softer and more brittle than the ultra-tough rock, forming a parting layer (Mustoe, 1996a).

Visits to the locality produced widely divergent site descriptions over time. Based on trips taken in the 1970’s, the site could be described as a steep series of benches and short drop-offs occupying a zone about 100 meters wide and trending up a steep hill over a distance of perhaps 300 meters. There was an ample amount of rock debris from earlier collectors, but sufficient exposed rock to make collecting possible, but only with some hard work. This was mainly because the rock itself was very hard, and although the hisingerite layer made collecting specimens possible, it was nonetheless very difficult to remove decent sized specimens intact. A trip made in 2000 revealed a stark contrast with the visits from over 20 years before. By 2000, the debris from past collecting efforts had completely covered the underlying outcrop, with no fresh rock exposed. In addition, it would be difficult to determine where to dig to remove overburden in order to get to what had been the most prolific areas. There was no reason to think in 2000 that the deposit could not still produce good specimens, but a substantial amount of “mucking out” would be required.

There are adjacent deposits, the Junior Agate Seam, and Fly-by-Nite are known for the production of “seam agate” (chalcedony).

References:

Cannon, B. (1975). Minerals of Washington. Mercer Island: Cordilleran.

Mustoe, G. E., D. G. Vandenburg, & L. M. Vandenburg (1996a). The minerals of Walker Valley, Skagit County, Washington. Washington Geology v. 24, n. 1 (March 1996), pp 22-29.

Mustoe, G. E. (1996b). Hisingerite—a rare iron mineral from Walker Valley, Skagit County, Washington. Washington Geology v. 24, n. 4 (December, 1996) pp 14-19.

Mineral List



12 entries listed. 9 valid minerals.

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