principal veins are: the Swansea, Enterprise, Hiawatha, Jumbo and Eureka.1 The silver minerals are: argentite, polybasite and stephanite, all of which occur with galena and blende. Pyrargyrite and proustite and native silver are also found, but in small quantities. The three mining centers at Rico are situated on three mountains; namely, the Dolores, Telescope and Expectation. Among the first workings on Dolores Mountain were the Enterprise and the Rico-Aspen, and the C. H. C. on Telescope Mountain. The ore occurs in practically vertical fissures and contact-deposits, probably fissures, besides which there are numerous other veins. There are three main veins traversing Telescope Mountain; namely, the Logan, Leap Year and Bourbon, of which the first named is the most important. This vein carries galena, chalcopyrite, with values in gold, silver and lead. The ores change from sulphides to carbonates in passing from the fissures to the horizontal contact-deposits. Overflows from the fissures formed the contact-deposits which lie between a capping of iron above and limestone below. The contact-deposits occur in benches or terraces, which position has been assumed by portions having been broken off the main formation, and sliding downward. The horizontal portions contain ore, while those that are tilted are low-grade. The high-grade ores of the contacts on Telescope Mountain, occurring above the horizontal bedding-planes, carry silver and lead, while the fissures of the northwestern slope have walls somewhat broken, and yield low values in silver, gold and lead. The Sambo mine on the north portion of Expectation Mountain, is on a contact-vein between limestone and sandstone formations above, and a bed of limestone (" short lime ") below. The ore is galena and blende.2 Boulder County. Among the mines of this area the Golden Age is one of the most important, of which there are six claims: the Golden Age, Sentinal, Rambler, Boston, Terrible and Hurricane. The ore of the Golden Age lode is free-milling, occurring in a contact-vein; however, with depth, sulphide ores are encountered in the form of pyrite, galena, blende and chalcopyrite. The Sentinal is also a contact-vein in which are found free-gold and high-grade telluride ores. The Golden Age ore-body occurs largely in shoots on the contact of a quartz-porphyry dike and a country-rock of granite and 1 Eng. and Min. Jour., Vol. 54, p. 174. 2 Min. and Sci. Press, Vol. 81, gneiss. The vein bifurcates at a depth of 100 feet, the two branches being known as the hanging-wall and foot-wall streaks. The dike also becomes mineralized, forming a low-grade ore. Of the two branches the foot-wall bears the most sulphides. The gangue is a hard, vitreous, white quartz, the gold occurring near the surface in the free state, and is often found in nuggets of several ounces. In fact the mine is famous for its specimen ore. The richest portion of the vein is in the granite and schistose rocks and poorest in the porphyry. No telluride ores are found in this vein. The ores of this county may be grouped into two classes; freemilling ores at the surface, changing to sulphides in depth, and the telluride ores. With the oxidized ores are associated native copper and green carbonate. The telluride ores are: calaverite, sylvanite and petzite.1 Summit County. The country-rock of the ridge consists of clay, slate and porphyry, the porphyry occurring as dikes and sheet intrusions overlaying the slate. The slate is cut at right angles by a system of parallel veins, which have a content of oxides and sulphides of iron and copper, galena, gold and silver, with a gangue of calcite and quartz. Gold is found with all the minerals mentioned except quartz, which contains no gold when it predominates in the veins. The calcite bears gold in the form of threads and wires and occasionally as crystals. Thread and wire gold also occur in the iron and copper minerals often so dense as to resemble a bunch of moss. Nuggets are also found weighing from a fraction up to several ounces. The free-gold is found near the surface, but decreases with the other metallic minerals with depth, being replaced by calcite and barren quartz. These veins have been the source of the placer gold in the Georgia, Humbug and American gulches.2 Clear Creek County. The country consists largely of dark gneiss, composed of quartz, feldspar and black mica. This gneiss is well stratified, but considerably disturbed and often metamorphosed. Intersecting this formation are veins of porphyry, compactly crystalline and of light green and rose colors. The veins bearing the silver ores are found in the porphyry dikes. The ores are: argentiferous galena, gray copper, brittle silver (with ruby and glance), pyrite, chalcopyrite, blende and native silver, in wires and nodules. The gangues are: fine quartz crystals, barite and fluorspar. The porphyry dikes range in width from 10 to 100 feet, although the chim1 T. A. I. M. E., Vol. 19, p. 323. Eng. and Min. Jour., Vol. 51, p. 516. Ruby Mountain, Gray's Peak and Argentine Pass, near Montezuma, Colorado. (From Engineering and Mining Journal.) neys and shoots of ore seldom exceed 12 inches, and are often as narrow as one inch. Galena carries the highest values, being fully 50 per cent in ninetenths of the veins, and is very prominent in all. Pyrite is very common, while zinc and copper minerals are practically always present, arsenic and antimony only occasionally so. The Snow Drift vein is preeminently a silver sulphuret producer; its content having suffered decomposition, and in places the walls are difficult to follow. Between the walls pure silver glance occurs in some seams; in others are found galena and glance combined, while in still others the sulphuret; oxides of iron and lead, are present the sulphuret predominating in depth.1 At the Stanley mine, Idaho Springs, the country-rock is Archæan gneiss and schist cut up by masses of coarsely crystalline granite. The veins vary considerably in shape and character, having widths of from a few inches to 50 feet. The vein upon which this mine is located has a width of five to ten feet. The ores (1894) were chiefly pyrite, chalcopyrite, purple copper or bornite and peacock copper. The value of the free-gold content is small. Custer County. A sheet of andesite, commonly called the "Bassic" andesite, occurs at the Bassic mine, which is situated about five miles from Silver Cliff. There are two ore-bodies in this mine, one reaching to the surface, the other extending slightly above the 600-foot level. In horizontal section the bodies are elliptical, varying between 25 to 50 feet on the minor and 50 to 75 feet or more on the major axes. These ore-bodies, which may be considered as channels or pipes, stand practically vertical. The ore apparently occupies the duct of an extinct hot spring or geyser, and occurs as thin incrustations on boulders and pebbles which fill the inverted conical opening. The shape of the fragments is subangular and rounded, composed of volcanic rock, similar to the walls of the duct; granite is also present in small pieces. The voids between this mass of boulders are filled with a plastic, granulated mass of decomposed rock fragments. The minerals constituting the ore are: the sulphide, carbonate and hydrous silicate of zinc, sulphides of iron and lead, gray copper, tellurium and gold and silver. Quartz also occurs as a filling. The ore is very rich, being principally tellurides, no freegold having been found. The ore assays from $200 to $5000 per ton.2 1 Eng. and Min. Jour., Vol. 27, p. 73; Vol. 13, p. 260, and U. S. G. S., Bull. No. 285, p. 38, 1905. 2 Colliery Engineer, Vol. 12, p. 73, and Mines and Minerals, Vol. 23, p. 489. |