Description of the Tenmile District Quadrangle

DESCRIPTION OF THE TENMILE DISTRICT QUADRANGLE. GEOGRAPHIC RELATIONS. stratified limestones around Leadville had directed under the patches of Cambrian quartzite that still black color near the top, and lighter colored near the attention of prospectors to this, then some rest upon the crystalline complex in the vicinity the base. It is in general heavily bedded and of The area shown on the Tenmile district special what novel, class of ore deposits. A very con of the fault, structure planes parallel with the granular structure. It contains characteristic con map is included between the meridians 106° 8' siderable number of more or less oxidized bodies bedding planes of the superincumbent beds occur cretions of black chert, is often fossiliferous near and 106° 16' of longitude west from Greenwich of pyrite, blende, and galena were opened along for a certain distance in the crystalline schists, the top, and passes upward into alternations of and the parallels 39° 22' 30" and 39° 30' 30" of the eastern slopes of the Tenmile Valley, but the apparently even beyond the limits of secular dis shale and sandstone. The average thickness of north latitude. It covers about 55 square miles, most important and the richest ore bodies were integration. the Leadville limestone is about 200 feet; it is and adjoins on the north the western portion of found in the Robinson mine, which is said to have sometimes considerably less, but rarely greater. the area represented on the map of the Mosquito produced, in all, ore to the value of over $6,000,- SEDIMENTARY ROCKS. Weber formation. This formation constitutes Range, Sheet VI of the Leadville atlas, Monograph 000. During the decade from 1880 to 1890, after Of the Paleozoic sedimentary series, there are the most siliceous member of the Carboniferous XII. This special district lies immediately north the richer oxidized ores had been for the most exposed at the surface in the Tenmile district system, and corresponds in a general way to the of the Continental divide, which here runs east part exhausted, many of the mines were closed outcrops of a portion only of the Carboniferous Weber quartzite of the Wasatch Mountain sec and west, connecting the Mosquito Range with down, the unaltered pyritous ores being too poor formation and a small patch of Cambrian is left tion and to the Lower Aubrey of the Colorado the northern part of the Sawatch Mountains, and for profitable exploitation. Since 1890 there has resting on the Archean, east of the Mosquito Canyon section. It includes a lower calcareo- is bounded on the east by the steep western slope been a partial revival of mining activity, as ore fault. Inasmuch, however, as the whole lower argillaceous member, designated in the Leadville of the Mosquito Range. Immediately north of dressing and concentration plants have been built Paleozoic series is found at short distances beyond report the Weber shales, the main siliceous forma it is the little-known mountain group called the to enrich the ore as it comes from the mine, and the limits of the quadrangle to the east and tion being there called the Weber grits. Gore Mountains, which is a northern extension, the demand for pyritous ores by the smelters has south in the Mosquito Range, and westward along The Weber shales constitute a transition series set off en echelon somewhat to the westward, of greatly increased. the valley of Eagle River a brief description of between the massive limestones below and the the Mosquito Range. The severity of the climate and the consequent the more common characteristics of the respective coarse sandstones of the Weber grits Weber shales. The principal drainage is northward through disproportionate length of the winter season has formations will be given here. above. They consist in the Mosquito Tenmile River, which flows around the northern always been an obstacle to the ready development Range of argillaceous and calcareous shales, alter end of the Mosquito Range into Blue of the very considerable mineral resources of the CAMBRIAN SYSTEM. nating with quartzitic sandstones. The former Drainage. River, the main south fork of Grand district. So/watch quartzite. This formation is made up are generally very carbonaceous and often con River in Middle Park. The southwestern portion of a series of remarkably pure, white, evenly tain seams of impure anthracite, up to several feet GENERAL GEOLOGY. of the area is drained by the east or main fork of bedded quartzites, with a fine-grained Sawatch in thickness, but of no commercial value. The Eagle River, which flows westward across the The Mosquito Range, which adjoins this area conglomerate of small rounded pebbles <>uartzite- calcareous members sometimes develop consider northern flanks of the Sawatch Range and joins on the south, is, as has been shown in the report of quartz and chalcedony at the base. In the able beds of impure limestones, generally fossil Grand River just before it enters its great canyon on the region around Leadville, a recent upper part the beds contain more impurities and iferous, which are distinguished from those of on the southern edge of the White River Plateau. uplift on the flanks of the older Sawatch pass into reddish and greenish argillaceous and the lower formation by fossils that are exclu Situated as it is near the crest of the Rocky Range. This uplift has been produced Range' calcareous shales, in which have been found fossils sively of Coal Measure aspect. The thickness Mountains this district is one of the most elevated as the result of lateral compression acting upon belonging to a Dikellocephalus fauna of Upper of this series, which is very variable, is assumed mining regions in the West, being on EIevated a series of Paleozoic and Mesozoic sediments Cambrian age. in this region to be about 300 feet. It occurs in the average nearly a thousand feet 8ituation- together with interbedded or intrusive sheets of The only observed exposure of .these rocks in the valley of Eagle River, just west of the limits higher than the neighboring Leadville district. igneous rock; the whole have been pushed up the present area is a small remnant of the basal of the quadrangle. Its larger valleys are between 10,000 and 11,000 into a series of corrugations or folds, and finally quartzite and conglomerate, with an included To the Weber grits belong the lowest beds feet above sea-level, the bottoms of the small side broken by great faults. In the vicinity of Lead sheet of porphyry, on the summit of Little Bart- exposed in the Tenmile district west of the valleys often reaching 12,000 feet, while its cul ville and southward the displacement has been lett Mountain. The thickness of the Cambrian Mosquito fault. Their average thickness here, as minating peak, Jacque Mountain, in the center of distributed along a number of more or less par averages from 160 to 200 feet. in the Mosquito Range, is about 2500 feet. They the area, has an altitude of 13,235 feet. The only allel fault planes, on which the upthrow is almost consist mainly of coarse sandstones or grits, often higher point within this area is Fletcher Moun invariably to the east. To the north of Leadville SILUEIAN SYSTEM. very micaceous, with a subordinate development tain (13,917 feet), a projecting portion of the this movement, which averages about 5000 feet, Yule limestone. This formation is represented of shales and a few thin and non-persistent beds main crest of the Mosquito Range, the slopes of has been concentrated on a single plane the by a series of light drab-colored, generally rather of dolomitic limestone. The sandstones are gen which rise at an angle of 35° from the glacial great Mosquito fault, which has a direction nearly thin-bedded limestones, which are often magnesian erally light gray in color, but near the base of the amphitheaters and valleys at its base. due north along the steep western face of the and always more or less siliceous, sometimes pass series are sometimes quite dark from the presence In contrast with these steep and rugged slopes, Mosquito Range as far as the northern edge of ing into calcareous sandstones. They are inclined of finely divided carbon, probably in the form of which form only a narrow strip along the eastern the area included in the present map, and then to be crystalline and in places form fine marbles. anthracite or graphite. Their prominent constitu edge of the area mapped, the rest of the region is bends sharply westward, following a northwest The few fossils that have been found in these ents are quartz and feldspar, evidently characterized by smooth and rounded topographic course along the southwest base of the Gore beds indicate a probable lower Silurian or Ordo- derived from the Archean; pink ortho- acte?sof thlr" 1 x- l, 5 A x Weber grits. forms, the surface being made up of gently dip Mountains, nearly to Grand River. vician age. Their thickness averages from 120 clase is sometimes so abundant as to ping beds largely covered by glacial gravels and From the uplifted area immediately east of this to 160 feet. impart a reddish tinge to the rocks. The abun soil. The soil supports in places a considerable fault the overlying sedimentary formations have, Above them is a series of siliceous beds, gener dant mica is mostly muscovite, biotite being pres growth of coniferous trees of hardy alpine varie for the most part, been removed by erosion, and ally quartzites, which are known as the Parting ent in subordinate quantity. The muscovite is ties, which sometimes, especially in the north the underlying complex of crystalline pre-Cam- quartzites, from their position between Partjng probably of secondary origin, for it is present in western portion, form a forest covering suf brian rocks is now exposed.

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