Day 3A Road Log: From Ouray to Rico Hot Springs of the San Juan Region
— J. Hrncir, B. Holt, D.A. Gonzales, A. Aslan, K.E. Karlstrom, L. Crossey, and M. Heizler

Several geologic topics are addressed in this road log that complement other aspects of the field conference. Short summaries are as follows.

Glacial history of the western San Juan Mountains: In the Late Tertiary, rivers drained the San Juan Mountains into low relief regions to the north (Uncompahgre River and Lake Fork of the Gunnison River) and west (Dolores and San Miguel Rivers). During the Pleistocene, the Uncompahgre and San Miguel River valleys seen on this trip were heavily glaciated during at least three major glacial episodes. From youngest to oldest, Atwood and Mather (1932) mapped “Wisconsin,” “Durango,” and “Cerro” tills in the San Juan Mountains. Later workers recognized Pinedale, Bull Lake, and pre-Bull Lake glaciations in the San Juan Mountains. At its peak extent, the Uncompahgre glacier originally occupied approximately 100 mi2 and reached 22 mi (35 km) northward to a series of end moraines at Dallas. End moraines are also present at Ridgway. Immediately above Ouray, the ice lobes merging from the drainages of Canyon, Red Mountain, Bear, Uncompahgre and Mineral Creeks may have reached a maximum thickness of 3000 ft (915 m) while in the U-shaped valley below Ouray, ice was present to at a depth of at least 1200 ft (365 m). The Uncompahgre River valley from Ouray to Ridgway is almost a mirror image of the lower Animas River valley north of Durango. At Ouray, the U-shaped bedrock valley is incised into Paleozoic sedimentary units and becomes progressively filled with alluvium and lacustrine sediments downstream to form the wide flat valley floor seen between Portland and Ridgway. The final river valley visited on this trip, the Dolores River, escaped major glaciation.

Ore deposits of the Ouray Region: The Uncompahgre mining district is approximately 15 mi2 in area and is centered along the east side of the Uncompahgre River canyon just north of Ouray. The principal ore deposits of the district are associated with intrusions of Late Cretaceous porphyritic granodiorite and are zoned around a heavily altered intrusive stock known as The Blowout that has been periodically explored for concealed porphyry Cu-Mo mineralization. All productive gold deposits generally occur within one mile of this body and lead-silver deposits extend northward as far as four miles to Cutler Creek. The intensity of mineralization decreases noticeably on the west side of the valley and few productive deposits were located there. Both base metal vein and replacement ores were deposited dominantly in the Mesozoic sedimentary units (to a lesser extent in the uppermost Paleozoic) and are in some cases truncated by the Tertiary unconformity at the base of the Oligocene volcanics (see comprehensive review in Burbank and Luedke, 2008).

Hot Springs and geothermal setting of the western San Juan Mountains: The western San Juan Mountains are home to several CO2-rich hot springs such as those at Ouray, Orvis, Placerville, Rico, and Dunton. These springs are predominately sourced in Precambrian basement rock and travel to the surface via deep-penetrating fault networks. Average spring temperatures in the region are around 40°C. These springs are located above low mantle velocity domain, and recent work on volatile chemistry of these springs has revealed that they express a signature of regional mantle degassing. This remarkable mantle-to-surface (groundwater) connection is the main topic of this optional fieldtrip.