On-the-Rocks Field Trip, June 15, 2002

Title: On-the-Rocks Field Trip: Stratigraphy and Mineralization of the Leadville Limestone, Central Colorado

Trip Leader: Scott Hazlitt, Leadville Mining and Milling Company

Date: Saturday, June 15, 2002

Publication: The Outcrop, May 2002, p. 17-18

What could be better than getting out of town and into the mountains in June, driving scenic U.S. 24 along the Eagle River Canyon from Minturn to Leadville, and pounding a few rocks along the way. In addition to the thrilling views along the canyon, stops in the Leadville District will offer spectacular views of the Arkansas Valley, Mt. Massive, Mt. Elbert, and the valley glacier terminal moraines that form Twin and Turquoise Lakes.

World-class massive sulfide replacement bodies (mantos), primarily hosted by the Paleozoic Leadville Limestone (dolomite), were mined from the late 1880s until 1984 at Gilman and from the late 1870s until 1999 in the Leadville District. During Mississippian time, the Leadville Limestone was deposited in a shallow subtidal to intertidal near-shore marine environment (Wallace, 1986). A karst surface was developed on the Leadville Limestone during subareal exposure and erosion during Late Mississippian and Early Pennsylvanian time. The first stop on the field trip at Minturn will provide an opportunity to explore a superb exposure of the karst surface as well as the Castle Butte and Red Cliff members of the Leadville Limestone.

A Leadville Limestone outcrop, adjacent to the No. 1 Manto, is exposed at the next stop near the Gilman town site. This dolostone breccia may represent paleo-cave systems, thought to be an important ore control at Gilman (Beaty, 1988). The mantos and chimney deposits at Gilman were mined until 1984 and produced about 11.7 million metric tons of ore with an average grade of 8.5% Zn, 1.5% Pb, 0.7% Cu, 228 gpt Ag, and 1.7 gpt Au.

At Tennessee Pass, we will discuss the 7 km belt of small stratabound gold deposits hosted in Leadville Limestone north and south of the pass, as well as the placers of Tennessee Creek. Ore was first discovered in the district in 1877 and small-scale production continued until the 1930s. It is possible that the gold mineralization of the Tennessee Pass District “formed from up-dip fluid flow on the western periphery of a major intrusive center; the high-temperature, central portion of this area may contain a large group of manto-type ore bodies” (Beaty, 1988).

The first stop in the Leadville District will be at the Penn Group, on the north side of the Breece Hill stock. Strongly altered Leadville Limestone and magnetite skarn mineralization is exposed in the area and there are also exposures of the Breece Hill stock, Johnson Gulch Porphyry, Evens Gulch Porphyry, as well as post-mineral rhyolite (Hazlitt, 1984).

The concluding stop will be at the Hopemore Mine and will include a discussion of the quartz-gold-pyrite mineralization associated with the Breece Hill stock and the magnetite skarn-associated gold mineralization of the Hopemore Mine. Magnetite and pyrite can be collected from dumps in the area.

The trip will leave the Cold Spring RTD Park & Ride, 6th Avenue and Union Blvd, Saturday June 15, 2002 at 7:30 a.m., and will re-assemble in the rail yard at Minturn at 9:30 a.m. If you wish to meet the trip at the rail yard please let us know when you register. The trip will conclude in Leadville at about 5:00 p.m., returning to the Park & Ride by about 7:00 p.m. Bring water and a lunch. For field trip reservations contact Fred Mark, e-mail (preferred): fmark@csd.net, or 303-674-1682.

Make a weekend of it! Plan to stay at the historic Delaware Hotel in Leadville; RMAG field trip participants will receive a 10% discount on room rates, but you must call the hotel early: 1-800-748-2004.