DESCRIPTION:
Mineral Resources in the Mount St. Helens Area

Mining claims for copper, gold, and silver were staked in the St. Helens mining district north of the volcano as early as 1892. ... Mining fever broke out about 1900, and hundreds of claims were staked in the Spirit Lake area as prospectors sought high-grade vein deposits. About 14 tons of copper ore from the Sweden Mine were hauled to a Tacoma smelter in 1905 and used to cast the bronze statue of Sacajawea for the Lewis and Clark Exposition held in Portland, Oregon, which commemorated the 100th anniversary of their expedition. ... Other mines in the area included the Margaret (Earl) group. Although thousands of prospect pits and more than 11,000 feet of underground workings were dug, the veins proved difficult to work and contained only modest amounts of gold and silver. By 1929, most of the mines had been abandoned, although exploratory work continued sporadically until the eruption of 1980.

The metal deposits of the southern Cascades are in stockworks or breccia zones and narrow fissure veins, containing only moderate amounts of copper and small amounts of lead, zinc, molybdenum, gold, and silver. Predominant ore minerals are pyrite, magnetite, chalcopyrite, bornite, galena, and sphalerite. At some occurrences these minerals are accompanied by free gold, especially in the oxidized parts of the veins. Common secondary copper minerals associated with chalcopyrite and bornite consist of azurite, malachite, and chrysocolla. Tourmaline is a common accessory mineral at several copper mineralized breccia deposits.

The metal deposits of the southern Cascades are confined to certain areas ... In the St. Helens district the deposits occur chiefly in the southeastern part of the Mount Margaret stock, and along the south-central section of the Goat Mountain stock. ...

The St. Helens and Washougal mining districts, the southernmost mining districts in Washington, are on the western slopes of the Cascade Mountains. ... The St. Helens district is chiefly in north-western Skamania County, but includes part of north-eastern Cowlitz County and part of south-central Lewis County. Mountainous country prevails ... Hillsides are steep and the summits of most mountains are rounded. However, several of the highest mountains contain alpine-type summits characterized by talus-covered slopes and craggy summits. ... In the St. Helens district, elevations range from a low of 600 feet on the Cowlitz River to a high of 5,858 feet on the summit of Mount Margaret. ...

Although deposits of gold, silver, copper, lead, and zinc are present in the St. Helens district, the deposits consist chiefly of narrow quartz-sulfide veins that are not sufficiently mineralized to be of economic value. To date (1977), out of 30 vein-type deposits that have been explored, only the Norway-Sweden vein appears to contain copper-bearing ore shoots with tonnages and grades required for a profitable venture on a small scale. Exploration and development work at the Sweden mine has established inferred ore reserves of about 20,000 tons of 2.5 percent copper. However, parts of the vein contain as much as 10 percent copper over 5-foot vein widths. ...

The rocks of the St. Helens mining district consist chiefly of thick sequences of andesitic flow rocks, breccias, tuffs, and volcanic sandstones and siltstones, which range in age from Eocene to early Miocene. Open north-to northwest-trending folds predominate and dips seldom exceed 30 degrees. On the eastern edge of the district, massive andesite flows predominate and appear to be related to the Fifes Peak Formation (Oligocene or Miocene) of Fiske, Hobson, and Waters (1963). In the central part of the district, the rocks consist chiefly of andesite, volcanic breccia, volcanic siltstone and sandstone of the Ohanapecosh Formation (upper Eocene). Along the western edge of the district basaltic lavas and associated pyroclastic rocks of the Goble Volcanics (upper Eocene) predominate.

On the northeastern edge of the district, rocks of the Fifes Peak Formation have been intruded by small stocks, dikes, and sills of andesitic porphyry and dacite. Near the central part of the district several bodies, from less than a quarter square mile in area to as much as 18 square miles, intrude volcanic rocks of the Ohanapecosh Formation. The largest of these bodies crops out in the Goat Mountain-Strawberry Mountain area, 6 miles north of Spirit Lake. The stock consists of medium-grained hornblende granodiorite, which is, in part, quartz monzonitic and dacitic. ...

Immediately north of the north end of Spirit Lake and east of Mount Margaret, andesites of the Ohanapecosh Formation have been intruded by a stock 2.5 square miles in area. The stock is predominantly granodiorite, but contains minor diorite and quartz diorite. ...

In addition to the above-mentioned stocks, smaller bodies of granodioritic rock crop out between Spirit Lake and Strawberry Mountain. ...

Pyrite is a common associate of most volcanic and plutonic rocks of the St. Helens district, whereas other ore minerals appear to be related to intrusive granodioritic rocks. ... Areas of significant mineralization are the upper Green River area and the area between Mount Margaret and Bismark Mountain. In both areas, fractures in granodioritic intrusive rocks and the intruded volcanic rocks, near the intrusive contact, are mineralized. The fractures range from less than 1 inch to as much as 4 feet in thickness. Underground mining operations show some fractures to be in excess of 2,000 feet in length, whereas surface expressions suggest lengths in excess of 4,000 feet for some fractures. The depths to which the mineralized fractures extend are unknown; however, mine workings on several persistent fractures indicates depths in excess of 300 feet. The fractures commonly occur in parallel sets, and intersecting sets produce stockworks or breccia zones that are loci for mineral deposition. In the Ryan Lake area, most fractures exhibit northwest and east strikes, whereas in the Spirit Lake area northerly strike predominate. Most fractures are steeply dipping, with dips that average 70 degrees. Dips of mineralized fractures of less than 40 degrees are uncommon.

Vein material of the fractures consists of quartz, calcite, gouge, and wall rock fragments. Disseminated cubic pyrite grains can be found in most vein material, as well as in the altered wall rocks. In some veins the pyrite is accompanied by chalcopyrite, sphalerite, galena, pyrrhotite, arsenopyrite, and gold, in order of decreasing abundance. These ore minerals commonly occur as small lenses, thin stringers, or a disseminations and appear to favor the silicified parts of the veins. In most veins the ore shoots are small and discontinuous. Only at the Sweden and Norway mines were the ore shoots somewhat persistent along the strike and dip of the veins. ...

The proximity of the mineralized veins to the granodiorite stocks, as well as silicic, pyritic, and sericitic alteration of the vein's wall rocks suggest a hydrothermal origin for the veins. As such, metalliferous hydrothermal solutions were deposited along fracture zones as a late product of cooling plutons, and are probably middle to late Miocene in age.

Mining History Around Mount St. Helens -- Excerpt from: Moen, 1977

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