Slater Mine, Eminence, Shannon County, Missouri, USAi

Regional Level Types
Slater Mine	Mine (Abandoned)
Eminence	Village
Shannon County	County
Missouri	State
USA	Country

The Slater mine is about 2 miles east of Eminence, at a point where one of the common porphyry ridges of the district projects through the Gasconade or Cambro-Ordovician dolomites. The particular knoll, on the east versant of which the copper occurs, is one of a chain extending for several miles in a northwest-southeast course. The porphyry is the usual red type so common in southeastern Missouri. It is of pre-Cambrian age. Separated from it by a marked unconformity are the gently dipping granular dolomites of the Gasconade formation. The cherty dolomites and sandstones of the Roubidoux formation occur in the higher hills surrounding the exposure.

The contact between the porphyry and the limestone is in an irregular plane, dipping at a low angle to the east. It is occupied by a well-developed conglomerate in which there are bowlders and pebbles of porphyry in a matrix of coarse crystalline dolomite. This conglomerate is not now exposed in its full thickness, but old workings are said to have shown a total of 12 feet. At the particular point at which the copper is found the surface of the porphyry dips down, forming a shallow basin or cove, in which the conglomerate was laid down.

Copper in the form of chalcopyrite (CuFeS2) and the green carbonate, malachite (CuOH2C03), occurs in the matrix of this conglomerate. The chalcopyrite forms small, irregular crystals and grains varying in diameter from 1 millimeter to a half inch. They occur with rhombohedral crystals of dolomite of nearly uniform size and grains of quartz in a paste of greenish clay. The carbonate is found in similar disseminated grains, apparently the altered remnants of chalcopyrite, and also in flecks and thin seams penetrating and replacing the porphyry fragments. At the surface, where the rock has been exposed, the cracks and crevices contain a black, sooty powder, apparently mainly manganese oxide, bearing copper probably in the form of the oxide or of chalcocite (Cu2S). An assay of this material in the Survey laboratory, by George Steiger, gave 6.44 percent copper.

The largest amount of work done on the property has evidently been in the stripping and mining of this conglomerate over some 3 acres. To the east, where the rock dips under cover, it has not been followed. Along the strike less prominent copper showings are said to occur at intervals for nearly 2 miles.

Copper also occurs in the porphyry, and several shafts have been sunk, following small veins into this rock. One of these shafts is reported to be 160 feet deep. This was filled with water at the time the property was examined. The rock on the dump showed thin seams in the porphyry, and in these small crystals of quartz, dolomite, and chalcopyrite occur. In no specimen seen was the amount of the latter sufficient to make the rock workable. In other pits not so deep similar crevices in the porphyry were seen filled with the green carbonate. This occurred at one place as much as 4 inches thick and practically free from admixture with other minerals. All the crevices examined were small, an inch or two in width at most, and all have a general northwest-southeast course, parallel to the general trend of the porphyry hill. They are not larger, more numerous, or, except for their content of copper, more impressive than at many other points in the porphyry of the region. Several of the earlier observers, have mentioned "true fissure veins" as occurring in the porphyry, but so far as observations of the present writers go these minor crevices are the only foundation for such statements.

The prospective value of the property is so intimately connected with the probable genesis of the ores that it is necessary to consider at least the possible character of the latter.

The chalcopyrite is not apparently present in quantities of economic importance; the question of the origin of the sulphides is therefore relatively unimportant. It is believed that they were derived from the porphyry by leaching either (1) of minutely disseminated quantities of copper, known to be present in the rock, or (2) of small veins locally present, of which the ores now exposed are representatives. This is believed to have occurred at the time the conglomerate was formed, though it is quite possible that the process continued in operation through later geologic ages.

The carbonates, oxides, and possible chalcocite present are alteration products from the sulphides, and the topographic and structural features of the deposits indicate clearly that this alteration has been accompanied by at least some concentration. The open, porous conglomerate affords abundant opportunity for the circulation of waters, while the decomposition of the lime-soda feldspars of the porphyry fragments would serve to precipitate as carbonate any copper sulphate present. It is notable that the decomposed porphyry pebbles and the carbonate are closely associated. In any such process the carbonates would be carried down, more or less, and precipitated in the upper fissured part of the porphyry below, and this is believed to have been the origin of the carbonate found in that situation.

It follows from the above considerations that future prospecting should be directed toward finding extensions of the conglomerate and points of local enrichment rather than toward following the small veins into the underlying porphyry. The cove in which the deposits occur opens out to the east, in which direction also the rocks dip. A series of drill holes parallel to the outcropping edge of the conglomerate should determine the depth and extent of mineralization very cheaply.