“On-the-Rocks” Field Trip Geology Along the Mineral Belt Trail, Leadville, Colorado

Title: “On-the-Rocks” Field Trip Geology Along the Mineral Belt Trail, Leadville, Colorado

Leader: Charles G. Patterson, Ph.D.

Date: Saturday, August 4, 2001

Publication: The Outcrop, July 2001, p. 16-17

This scenic 12-mile loop traverses one of the most famous mining districts in the world. Outcrops are scarce, as the bedrock geology is mostly overlain by glacial moraine, but the overall story has been worked out by generations of geologists and miners.

Leadville sits near the northern terminus of the Rio Grande Rift, a N-S trending, mid- to late-Tertiary rift that ultimately extends all the way from Colorado, to New Mexico, Texas, and out to sea in the Gulf of Mexico. It is a giant horst-graben structure, with the upthrown block of the Mosquito Range to the east, and the Sawatch Range (and the Continental Divide) to the west. Step-down normal faults extend from the crest of the Mosquito Range to the valley floor. Near Malta, the valley fill is over 4000 feet thick, and is the lowest gravity low in the conterminous U.S., at -338 milligals. The Arkansas River heads on Fremont Pass and flows south along the alignment to Salida, where it turns east and heads out to the great plains through the scenic Royal Gorge.

Rocks exposed in the area range from Precambrian granites, gneiss and schist, to lower Paleozoic sediments including the Cambrian Sawatch quartzite, some Ordovician and Devonian carbonates, the famous Leadville Limestone (or dolomite) of Mississippian age, and the lower Pennsylvanian silts and conglomerates of the Molas, Belden and Minturn formations. These rocks have been intruded by a number of Tertiary intrusives. These bodies are mainly porphyritic sills, a stock, and smaller dikes, and range from quartz monzonite to quartz latite in composition. Some late rhyolitic explosion breccias are also found in the area. The section is repeated several times as one progresses down into the valley in each fault block. The skyline to the east shows a composite sill of intruding the Paleozoic sediments. To the west, the Sawatch Range is mainly Precambrian rocks in the core of the great Sawatch anticline, with some Tertiary intrusives seen near Turquoise Lake (named for turquoise deposits, not the color of the lake).

The world-class ore deposits range from vein and disseminated pyritic gold deposits on Breece and Iron Hill, to the large replacement bodies of lead-zinc-silver ores hosted by the Paleozoic carbonates on Fryer and Carbonate Hill.

Leadville has produced over 500 million dollars in silver, lead, zinc, copper, and gold. The ores are apparently zoned around a 34 ma stock under Breece Hill. Leadville was first known as a rich placer district, with California Gulch the center of the action. Over 10,000 miners lived in the gulch in 1859-64, and extracted more than 5 million dollars in placer gold from diggings in the area. First known as a gold district, the rich silver ores were not developed until 1875. Until then, as the story goes, the “black mud” (actually lead carbonates) that clogged the placer workings, was considered a nuisance.

The district has been mined steadily since the early days, with some activity still occurring, but at a much-diminished rate. The closing of the great molybdenum mine at Climax in the early 80’s brought an economic downturn to Leadville, from which it has yet to fully recover. ASARCO’s Black Cloud mine, which was the last of the big Leadville-style mines, closed in 1999. That closing was probably the end of the Leadville mining era. A few small operations persist, but nothing like the old days.

Nowadays, environmental issues dominate the scene at Leadville. Yard soils are pervasively contaminated by lead, zinc, and cadmium. Acidic drainage from tailings piles and from the YAK and Leadville drain tunnels also must be contained and neutralized. Around town, historic mine and mill tailings, and smelter waste piles are being removed and relocated. Some would say with great historic loss to Leadville. In California Gulch, the giant water treatment facility neutralizes acidic drainage from the YAK tunnel, and will probably operate into perpetuity, at taxpayers’ expense. Such is the legacy of western mining. It is ironic that the miners and smelters, who forged American history, were given government awards for productivity during WWII, and now are treated as environmental criminals by the same government.

The trip will meet at the Cold Springs Park ‘n Ride, at the southeast corner of 6th Ave. and Simms St, at 8:00 a.m. Trip participants are responsible for their own food and drink, as well as appropriate field gear and a bicycle with good low gears. The bike ride is twelve miles and is described as pretty hilly. The ride is also at elevations around 10,000 feet. Use good judgment concerning your capabilities. To register, call Denis Foley at 720-963-9426.