U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY GEOLOGY OF THE ENGINEER PASS TO CINNAMON PASS LOOP, WEST OF LAKE CITY, SAN JUAN MOUNTAINS, COLORADO by Anna B. Wilson 1 and Marc D. Woodard 1 Open-File Report OF 99-133 also available online at http://geology.cr.usgs.gov/pub/ open-file-reports/ofr-99-0133/ 1999 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey (USGS) editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1 Denver, Colorado 2 If you had visited this area between 30 and 35 million years ago, you would have been here when dozens of volcanoes, similar to Mt. St. Helens in southwestern Washington, were erupting lavas and relatively small amounts of ash. About 30 million years ago, the source and style of volcanism changed to voluminous eruptions of hot, gas-charged ash from large circular depressions many kilometers wide called calderas or cauldrons. The ash formed a thick blanket over the previous landscape and formed the volcanic rocks that are so prevalent in this area. The earliest of these caldrons to form in the Lake City- Silverton area were the San Juan and Uncompahgre calderas, which erupted simultaneously about 29 million years ago. Later caldera eruptions formed within the sites of these earlier calderas and include the 27 million year-old Silverton caldera and the 23 million year-old Lake City caldera. About 10,000 years ago, at the end of the last ice age, water from rivers and melting glaciers cut down through these rocks, forming the landscape and rugged peaks you see today. The route from Lake City to Engineer and Cinnamon Passes begins in the Uncompahgre caldera, then traverses the rim that separates the Uncompahgre and Lake City calderas from the San Juan and Silverton calderas, and returns along the south flank of the Lake City caldera. Gold was probably first discovered in the Lake City area in 1842 (or ‘48) although no one seems to know exactly where. At that time, all the land in the San Juan Mountains belonged to the Ute tribe. In 1874, a treaty ceded a strip of land 60 miles by 75 miles to the U.S. Government opening the way for mineral development and settlement of the region. Lake City to Henson Along Henson Creek, from Lake City to Henson and beyond, the canyon and cliff walls are made up of 27 million year-old volcanic rocks of the Uncompahgre caldera. If you look closely at them, you may see layers that indicate separate eruptions, fragments of other types of rocks that indicate the violent nature of some eruptions, and many other features. You may also notice different colors, such as rusty-looking iron stains, in the rocks. Most of these are due to mineral-rich waters circulating in the rocks causing a breakdown of the original minerals into alteration products. In a few areas, you will also see slopes of loose rocks, or talus. These rocks have broken off from their 3 original locations and slid down slope due to gravity after the last ice age. Some of the old mines along this part of Henson Creek include (from east to west) the Mount Morris, Cleveland, Fanny Fern, Joker, Pelican, Grand LaPlata, Risorgimento, Ute-Hidden Treasure, and California. Henson (Note that this is private property. Please respect all “no trespassing” signs.) The Ute and Ulay veins were discovered in 1871, but it wasn’t until after the treaty was signed in 1874, that the claims could be developed. Together, these veins comprise the Ute-Ulay mine which produced mostly silver and lead ore from veins within the 27 million year-old rocks. The main vein can be traced on the surface for 2700 ft. It is about 4 feet wide, on average, but can vary from nearly nothing to 20 ft. Together, the mines near Henson produced about $13 million (at the time of mining) in ore. For most of the mines’ productive lives, gold was valued at $20.67 per ounce. In addition to other metals, about 630,000 oz of gold was produced. Henson townsite was platted in 1880. The mill site was located in the valley to the east of the dam. The dam provided electric power for the mill and townsite. Ore was hauled to the mill by tram. You may see remnants of old trams and their cables and towers on your journey today. Henson to Capitol City Approaching Capitol City , if you look ahead or slightly right, you may notice a rusty stain on the lower slope of Sunshine Mountain. This is mineral alteration and is often an indicator that hot water circulating through the rocks may have deposited metals. And, indeed, there were productive mines at Capitol City including the Capitol City, Yellow Medicine, Excelsior, and less productive mines, such as the Czar , Gallic-Vulcan, Silver Chord, Chord Extension, and Woodstock. Most of these mines contained lead and silver ore. Other minerals such as native silver and native copper, were reported locally. The Gallic-Vulcan mine also has rare telluride minerals. These mines produced between $18,000 and $41,000 before 1909 (Irving and Bancroft, 1911; value at the time of mining). Capitol City was a 200-acre townsite named Galena City in 1877. Soon thereafter, it was renamed either because its inhabitants optimistically expected it to 2 J. Coleman-Fike, BLM, written commun., 3/21/01. 4 surpass Denver in size and importance or for a local peak that resembled the dome on a captiol building 2 . Capitol City to Engineer Pass Beaver ponds, some with well-maintained lodges, are prevalent along Henson Creek. The dams create wetlands and wildlife habitat. Avalanche chutes are conspicuous on the south side of Henson Creek. Snow avalanches can roar downslope at 40 to 80 miles per hour and clear anything in their path. Notice several chutes cross the creek and start up the opposite side of the valley. At the base of some of the chutes are triangular clearings called fans. A large red headframe of the Empire Chief mill stands by mine workings on the right (north) side of the road. In 1929 a large avalanche from the mountain south of the road killed 4 people and demolished the circa 1904 mill 2 . The current mill was reconstructed later that year. Abundant pyrite (iron sulfide, also called fool’s gold) can be found in the waste piles. Under certain conditions, pyrite can contribute to acid mine drainage. A turnoff to the site of the Rose Cabin is on the left. Built by Croydon Rose as a tavern and dormitory, it was one of the first structures in the area 2 . At one time it was famous for its bar and entertainment. If you chose to see the old site, it is recommended that you retrace your tracks back to the main road after your visit as the continuation of the road is very rough. At the junction of Henson Creek and Palmetto Gulch, is a new log home with a suspension bridge on the north of the Creek. Here, the road leaves Henson Creek to follow Palmetto Gulch for the final ascent to Engineer Pass. Only about 1/4 mi farther, on the south side of Palmetto Gulch, a tiny old prospector’s cabin is preserved. Within a mile, on the south side of the road at the first switchback, is the old Palmetto silver mine. The mine produced silver ore prior to 1891. Most of the richest silver ore was near the surface. Ore from the 400-500 ft shaft was lower grade. Negligent visitors have misused this site for human waste and trash. Please be considerate and respectful of the environment. At this elevation, waste does not break down for many years. About 1/2 mi farther west, and only a few hundred feet east of Engineer Pass, on the north side of the road is the Frank Hough mine. It consisted of a shaft with