Table of Contents
The results of these investigations show that bournonite (PbCu-SbS3) is present in the ores in greater or less quantity. This fact has never before been reported, as far as can be learned. The failure to recognize this mineral is due to its similarity to tetrahedrite, with which it has probably been confused during a long period of years.
Jamesonite (Pb2Sb2S5) has been reported from the region in a rather doubtful manner, but the writer has seen it in notable amounts from several mines, and can offer the first analysis of this mineral from the district.
Calamine (H2Zn2SiO5) was found at one mine and has never been recognized and reported from the district.
Geography and Geology
Park City, Utah, is located at an elevation of about 7,000 ft. above sea level on the eastern slope of the Wasatch mountains. It is situated approximately 25 miles southeast of Salt Lake City. The geology and ore deposits have been described in detail by Bout well and Woolsey.
The mines visited are situated south-southeast from the town. The Silver King Coalition mine is located in Woodside gulch about 1 mile southeast of Park City, at an altitude of about 8,100 ft. The other mines visited were the Daly West and Quincy, which are in Empire canyon about 2 miles southeast of the town, at elevations of 8,300 to 8,500 ft. respectively. The Daly Judge tunnel goes into the mountain from Empire canyon about 1 mile south of the city, at an altitude of 7,700 ft.
About two-thirds of the area of the district is covered by sedimentary rocks, and the remainder are of igneous origin. According to Boutwell, the sedimentaries are divided as follows:
The sediments were intruded from Cretaceous to Eocene by diorite, diorite porphyry, and finally andesite. It was these igneous rocks, especially the earlier ones, which are thought to have caused the formation of the ores of the district. Both previously and subsequently to the deposition of the ore, more or less-faulting has taken place.
Occurrence of Ores and the Common Ore Minerals
The ores of the region are found both in fissure veins, and as replacement deposits in limestones. The two types are often associated, as in the Silver King Coalition and Daly West mines. The fissures have a general northeast-southwest trend. Most of the replacement or Bedded deposits are found in the limestones of the Park City formation, although a few occur in the limestones of the Thaynes formation.
The commonest sulphide minerals, according to previous, writers, are galena, tetrahedrite, sphalerite, and pyrite. To this rather short list bournonite can be added. At one time the common occurrence of tetrahedrite in the region was doubted by the writer but now it is known that both tetrahedrite and bournonite are fairly common ore minerals. It is certain that in the past more or less bournonite has been called tetrahedrite, but which of the two minerals predominates is still a matter of conjecture. The chief oxidation products are cerussite and anglesite, with small amounts of malachite and azurite. According to Boutwell, considerable amounts of bindheimite (Pb3Sb2O8+aq) are found. This would seem to indicate that bournonite might be a more prominent constituent of the ores than tetrahedrite, because the former is a lead-antimony sulphide, whereas the latter is a copper-antimony compound, and would not oxidize directly to bindheimite without interaction of other substances.
One interesting feature which the writer has never before seen under natural conditions was observed on “coarse cleavable” galena from the Silver King Coalition mine. It was a polysynthetic twinning in two directions perpendicular to each other, although one set of striations was much better developed than the other (see Fig. 1). The twinning lines make angles of 45° with the cleavage cracks after the cube. In other words, the twinning plane is parallel to the diagonals through a cube face or to the secondary planes of symmetry, and consequently must be the face of a rhombic dodecahedron. For galena, this face is a gliding plane, and the phenomenon may be produced artificially by placing cubes of galena in a vise and applying pressure. A similar behavior of calcite is perhaps more widely known, where a proper application of pressure will produce artificial twin crystals parallel to the face of the negative rhombohedron ½R. To the writer the presence of such striations on galena, indicates pressure after the deposition of the mineral, and the lines were undoubtedly formed by the pressures caused by the postmineral faulting
of the district. Faults of this character have been recognized in such mines as the Silver King and Daly West.
Occurrence of the Bournonite
Bournonite is a very rare mineral, and has been reported previously from but three or four localities in the United States. The writer first saw a crystal of the mineral from Park City in the magnificent collection of the late Albert F. Holden, of Cleveland. A. T. Dalley, of the Silver King Coalition Mines Co., gave the writer a crystal which is now in the mineral collection of Case School of Applied Science at Cleveland. At the time, Mr. Dalley had in his possession another bournonite crystal, which he later donated to the Geological Department of the University of Utah, at Salt Lake City. It was also learned that the crystal in the Holden collection was originally obtained from Mr. Dalley. The three crystals came from the 1,300-ft. level of the Silver King Coalition Mine and as far as the writer knows there are no others like them from the Park City district in existence. Mr. Dalley did not see the crystals in the mine and therefore does not know the exact conditions under which they were found. He has since seen other “smaller crystals that are in quartzite associated with other sulphides such as pyrite, galenite, sphalerite, and jamesonite. ” Because of the rarity and size, the dimensions and weights of these crystals are given, as follows:
Other specimens from the Silver King mine were given to the writer by the Superintendent, George D. Blood. In these the mineral was massive and was supposed to be tetrahedrite. However, most of the specimens contain lead in varying amounts up to 15 per cent., and are probably bournonite, as they resemble that mineral in color and luster. It is nevertheless possible that the massive mineral might be a plumbiferous tetrahedrite, which is a very rare variety, but has been found at several other places. Bournonite and tetrahedrite, when massive, resemble each other very closely. As far as the writer can see, the bournonite appears to possess a more brilliant metallic luster. The color is blackish load-gray and the streak is black. In tetrahedrite from Park City, the luster is duller; the color has none of the lead-gray appearance, but is more like iron-black; and the streak is distinctly reddish brown instead of black. Possibly more than one variety of tetrahedrite occurs in the district. Chemically it is also difficult to distinguish the two minerals, especially if the tetrahedrite is plumbiferous, which the writer believes to be the case at Park City, because both minerals might contain lead, copper, antimony, and sulphur. In general, however, lead should predominate in the bournonite, and copper in the tetrahedrite. A quantitative analysis is necessary to determine the two minerals from a chemical point of view.
Mr. Talbot, the Superintendent of the Daly West mine, gave the writer several specimens of so-called “tetrahedrite” from the lower levels (1,000-1,200 ft.). One of these showed crystals on which we could not recognize any of the forms of tetrahedrite, but which seemed to possess an orthorhombic habit. These crystals were quite rough and much distorted, and seemed to disappear into a massive mineral, which had the same color and luster as the crystals, and which was in our estimation certainly the same mineral as the crystals. Toward the center of the specimen pyrite in pentagonal dodecahedrons was found, and this was followed by coarse cleavable galena. Portions of some crystals and the adjacent massive mineral were broken off as carefully as possible, and analyzed by Dr. W. R, Veazey, of the Chemical Department of Case School of Applied Science, with the following results:
An inspection of the results obtained. shows conclusively that the mineral is bournonite, and that the ratio obtained from our analysis is very close to the theoretical composition required by the formula PbCuSbS3. The specific gravity, 5.829, also conforms well to the average density obtained from specimens of the mineral from other localities. In all specimens seen by the writer the mineral was associated with pyrite and coarse cleavable galena, and was evidently’formed later than either.
Occurrence of the Jamesonite
Jamesonite is another rare mineral, and has been found at but two or three localities in the United States. In the very able monograph on the Park City district, Boutwell says: “Three specimens of a crystalline gray metallic mineral were found on the California dump.
The material was too meager to permit thorough chemical determinations, but Dr. Hillebrand obtained qualitative tests for sulphur, antimony and a salt of lead, and accordingly is inclined to regard the mineral as jamesonite or warrenite. ” Under “Bournonite” the present writer has mentioned a personal communication from A. T. Dalley which shows at the Silver King Coalition mine an association of jamesonite with bournonite, galena, pyrite, and sphalerite. In the same letter Mr. Dalley says: “The best jamesonite (fibrous) occurs at the Daly Judge mine with (most commonly) pyrite, occasionally pyrite and galenite. ” In the Case collection are notable amounts of a mineral from the Silver King Coalition mine. It occurs in capillary crystals which are woven together into a felt-like mass. The mineral has a dark lead-gray color, a brilliant metallic luster, and a black streak. In one specimen the mineral rests upon a perfect striated cube of pyrite. In three other specimens the jamesonite rests upon coarse cleavable galena, which in turn incloses pyrite. It would therefore seem that the jamesonite was the latest mineral to form. The felt like mass was found to contain many small crystals of pyrite, which were carefully isolated so that an analysis of the capillary crystals could be made. The results obtained by Dr. W. R. Veazey. were as follows:
Our analysis shows that the mineral investigated conforms in general to the theoretical composition of jamesonite. It was found on comparing 38 different analyses that there is a wide variation in the constituents between certain limits. The lead in our analysis is higher than the average, although one analysis of material from Russia contained 63.61 per cent. One feature of the Park City mineral, which is not common in most jamesonite, is that part of the antimony has been replaced by arsenic. It would therefore seem that a small amount of dufrenoysite (Pb2As2S5), which is generally regarded as isomorphous with jamesonite, must be present in the Silver King mineral. In order to compare this analysis with most jamesonite analyses, the arsenic was converted into antimony (4.14 As = 6.63 Sb), after which the combining weights and molecular ratios were obtained as follows:
It will be seen that the ratio of the recalculated analysis conforms quite closely to the theoretical formula Pb2(Sb,As)2S5. If the original analysis be compared with the theoretical composition of dufrenoysite on one side, and that of jamesonite on the other, it will be found that, in general, the Park City jamesonite stands between the two, and is therefore probably an isomorphous growth of the two minerals.
The only discrepancy is in the sulphur, which is a trifle low. It is also to be remembered that our analysis reached a total of 99.66 per cent, and was not calculated on the basis of 100 per cent.
Occurrence of the Calamine
Calamine (H2Zn2SiO5) has never been reported before from the Park City region, as far as can be ascertained. The mineral was found on the dump of the Quincy mine as a drusy coating on cavities and pores. Several specimens were collected. The calamine was always associated with a brownish manganese oxide resembling wad. It was evidently entirely manganous oxide, as the usual manganese reactions were not observed until the mineral was oxidized with niter. The calamine crystals were grouped in the characteristic sheaf-like masses, which were in some places assembled into botryoidal and mamillary imitative shapes. The crystals were all thin tabular after the brachypinacoid b(010, ∞ PS ∞), which was striated vertically. The prism m (110, ∞ P) was present as small planes, as were also a macro- and brachydome [evidently t(301, 3P ∞), and i(031, 3P ∞)]. The basal pinacoid c(001, 0P) was not observed, and if present was very small. Although the writer was perfectly convinced as to the identity of the mineral, a qualitative chemical test was made for verification. The mineral was dissolved in hydrochloric acid and evaporated to dryness. It was taken up in hydrochloric acid and water, and boiled, after which the silica was filtered off. Nitric acid was added to oxidize any iron, and ammonium chloride was added to keep up any zinc. Ammonia was added to precipitate iron and manganese, and the precipitate was filtered. Hydrogen sulphide was passed through the filtrate, and a white precipitate of zinc sulphide was obtained, which fully corroborated the mineralogical determination.
The writer is unable to say whether the calamine exists in large amounts, but the discovery and associations here remind one very decidedly of the occurrence at Leadville, Colo., where large amounts of calamine and smithsonite have been recently found after having escaped recognition for many years. It is, of course, an oxidation product of sphalerite, and was probably associated with the usual minerals found in the upper levels of the Quincy mine, such as cerussite, anglesite, malachite, and azurite.
Paragenesis and Conclusion
The common associated minerals, as well as the probable order of deposition, have been mentioned in the detailed discussion of the occurrence of the various minerals. The writer is, moreover, impressed with the fact that there is evidently constantly present in the ores of the Park City district, a lead-copper-sulphur-antimony series of minerals represented by galena, bournonite, tetrahedrite, and jamesonite. One gets the general impression that Park City is a camp mainly of argentiferous galena. The fact that an antimonial lead series of minerals is present is shown by the constant oxidation to bindheimite (Pb3Sb2O8+aq), which is repeatedly mentioned by Boutwell. Study of the ores leads to the further conclusion that the sulph-antimonides bournonite, tetrahedrite, and jamesonite were all deposited later than the galena, and therefore that the lead-antimony solutions entered at a subsequent period. There are certain questions relative to depth which the writer cannot answer. The copper content of the Park City veins is evidently increasing with depth. Is this also true of the zinc? What relation have tetrahedrite and bournonite to each other? I have never found both minerals on the same specimen. What is the critical level of bournonite compared with tetrahedrite ? Was bournonite more common on the upper sulphide levels, and has it simply escaped recognition during a long period of years? Is it being replaced by tetrahedrite in the lower levels as the copper content of the veins increases with depth? Or on the other hand is the bournonite now being found for the first time in the lower levels? These questions cannot be answered with the material and data at command. The suggestions are given in the hope that some of the questions may be answered by members who are more familiar with the district.
