MINERALS OF THE LAST CHANCE MINE, SKAMANIA COUNTY, WASHINGTON
by Don Howard and Bill Tompkins

Men exploring the hills of western Washington just north of the Columbia River discovered evidence of copper mineralization over 100 years ago. This resulted in a cluster of mines along the Washougal River straddling the line between Clark and Skamania counties that have come to be known as the Washougal mining district. 1
The area is composed geologically of the Skamania Volcanics, andesite flows, tuffs, and volcanic breccias into which the Silver Star granodiorite has intruded in places. About a dozen specific copper-bearing seams and veins have been identified in this area, and a number of mines and placers have been established to explore these deposits. During this last fall, together with Martin Jones, we have been examining one of these mines, the Last Chance. This appears to be the southernmost of the mines in the district. It is located along the west fork of the Washougal River about a mile and a half from the Clark Co. line. The Last Chance mine is on deeded land in the center of S1/2 SE1/4 sec. 29, T. 3 N., R. 5 E., and at an elevation of 1,600 feet. From Washougal, State Highway 8B, County Road 11, and logging roads may be followed for 16 miles to the property. The last 2 miles of the road are not maintained, and are generally inaccessible to vehicle travel. The Last Chance mine was discovered and developed at the same time as the Skamania mine, which is 1.5 miles north of the Last Chance. The properties are often confused with each other because the mine workings are almost identical at both properties. Although Washougal Consolidated Copper Mining Co. undertook considerable development work at the Last Chance in the early 1900's, no shipments of ore were made. The mine has been idle for at least 20 years and all mining equipment has been removed. The Last Chance operation consists of a shaft and two horizontal tunnels. One tunnel, located on the west side of the river, is difficult to reach from the road, which is on the east side. That tunnel, which is reported to be 267 feet long, 1 was not explored. The tunnel and shaft are on the east side of the river just below the level of the road. A small dump, consisting mostly of fine-grain material, lies directly below the adit. Some green color is evident, but since the material has been undisturbed for many years, it does not appear very productive for collecting.
The tunnel is cut back into very solid rock and has not needed timbering except at the very mouth. There, the tunnel appears to be located under a small stream only a few yards above, and the first few tens of yards provide a shower bath, even when it is not raining outside. Water collects a foot or so deep in this area and must be waded, so appropriate raingear and boots are a must. The shaft, which is just to the north of the adit to this tunnel, has completely filled with water. It is reported to be 540 feet deep, and to have side shafts cut at five different levels. 1
The east tunnel runs some 936 feet into the hillside; it slopes up very gradually, just enough for the water dripping inside to drain out the mouth. Except for the first twenty to thirty yards, the floor is only shallowly covered in pooled water and it is possible to walk in most areas on solid footing on one side or the other of the old, iron covered boards that served as rails for ore cars. The ceiling is everywhere high enough to be able to walk upright without danger of hitting your head. The tunnel follows a pair of quartz veins, each several inches to a foot wide, that form an inverted V that met about ceiling level. The seams are clearly seen running along the center of the ceiling throughout the length of the tunnel, and the V structure can clearly be seen at the back end of the tunnel. Numerous short crosscuts show that a second similar quartz seam runs parallel a few yards to the south. Traces of these seams have been followed for nearly two miles. 2
The central region of the quartz seam in several places opens into gaps that are lined with quartz crystals, the terminations generally being half a centimeter or less. The seam originally consisted of both quartz and calcite, as witnessed by numerous regions of angular, rhombohedral cavities where the calcite has been completely dissolved away. These cavities indicate that the original calcite rhombs were up to a centimeter or two in diameter. Some of these cavities are still sharp, while others have been nearly filled with the growth of drusy quartz crystals. Many of the cavities show thin wall-like partitions where the quartz filled into cleavage cracks in the calcite. Currently, calcite is quite rare in the mine. Copper mineralization is mainly in the form of chrysocolla. Some radial, fibrous growths indicate that at least at one time, this was at least partly in the form of malachite, but currently all the green to blue material is the silicate chrysocolla. Bornite, pyrite, chalcocite, and chalcopyrite have been reported as forming blebs disseminated within the quartz vein. 1 Much of the chrysocolla is present in greater or lesser amounts on the contact on either side of the quartz vein; this is several inches thick in places and in the wet environment appears intensely green. Much of the color fades, however, upon drying. The chrysocolla is mostly botryoidal, and except for the fibrous places indicating malachite alteration, there is not much evidence of pseudomorphs of previous minerals. Surfaces examined at higher magnification look like masses of wet, matted fibers. The most interesting mineral in the mine forms as yellow to ocher coatings and stains on the quartz crystals in the gaps in the seam. This material was described as "descloizite (lead vanadate)" in the summary article. 1 In fact, it contains practically no zinc at all; zinc is quite rare in the minerals of the mine. The material is essentially end-member mottramite - Cu Pb (VO4) (OH) . While being a very interesting occurrence, the material makes particularly ugly specimens. The color is not at all attractive, and the crystalline structure of the crusts is only evident at a magnification above x200 (see micrograph #339). The material is quite plentiful, however, and can be easily collected. Other cavities in the quartz seam have been filled with chrysocolla (bright blue-green) or other silicate crusts fading to almost white. Many of these are coated in turn with small dark spots and patches of a manganese oxide that also contains a wide variety of impurities, such as iron, copper, lead, and even a trace of zinc. Some regions have been stained orange by iron oxide - a distinctly different color from the mottramite stains.
A few small cavities have been found to contain tiny rhombohedrons of calcite. These are generally colored blue-green by copper or yellow by mottramite. Carbonates are not at all common in the mine.
Close examination has turned up a few specimens that show needle-like or ribbon-like scattering of crystals in the voids where the calcite has dissolved, or mounted on the quartz crystals in the central openings. In few instances is there enough material to properly analyze. XRF done on the SEM shows a variety of compositions. At least four such unknowns have been identified:
1) Sprays of flattened needles, some terminated, which appear to be orthorhombic. Chemically they appear to be a calcium aluminum silicate with considerable iron content (perhaps 1/3 iron to calcium).
2) A rod-like crystal, squarish cross-section, that is dark green to black. Chemically, it appears to be a silicate with nearly equal iron and copper concentrations. There is also manganese (about 1/5 manganese to iron). This is mostly embedded in the quartz, with tiny quartz prisms growing on the surface, which is fuzzy like some of the chrysocolla surfaces.
3) Flat yellow blades, very thin with irregular ends, that occur in clusters. The chemistry is about equal parts of copper, iron and lead. There is some silicate present, but no other anion seems to be present (ie., no vanadium).
4) dark yellow lath-like crystals scattered individually around the inside of one of the rhombohedral holes. The blades look feathery, as if they have fractures at an angle, and the ends are sloping or forked where broken. The chemistry has not yet been determined. Because so many of the cavities show thin coatings over the crystals, it is difficult to be sure of any of the chemistry - is the material intrinsic to the crystal or merely a stain on the surface? Further examination will be needed to isolate enough of any one form to properly analyze and identify it.

References
1. Wayne Moen, St. Helens and Washougal Mining Districts of the Southern Cascades of Washington, State of Washington, Department of Natural Resources, Division of Geology and Earth Sciences, Information Circular 60 (1977).
2. R.M. Brerton, Plat of the Last Chance Mine, Washington Division of Geology and Earth Sciences, unpublished map (1930).

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