The La Plata Mountains, located 10 miles northwest of Durango, Colorado, are an isolated branch of the main San Juan massif. The La Platas are separated from the San Juan Mountains by a terrane of late-Paleozoic and early-Mesozoic sedimentary rocks. The La Plata Mountains themselves consist of a highly dissected, elliptical dome of folded sedimentary rocks intruded by Laramide sills, dikes, stocks, and laccoliths.
The La Plata Mountains (and adjoining Ohwiler Range) form the southwest corner of the San Juan massif. This part of the San Juan uplift consists of Pennsylvanian-Permian red beds in the central and eastern part of the complex and Cretaceous sediments in the western portion. These sedimentary rocks form the country rock into which the younger Laramide igneous rocks have intruded. The Laramide intrusions consist of various concordant and discordant igneous bodies ranging in composition from syenite to granodiorite.
An extremely diverse array of mineral deposits occurs in the La Plata Mountains and nearby Ohwiler Range. Four major types of precious metal deposits are recognized:
A unique stockworks of gold and platinum-group metals associated with a Laramide syenite intrusion on Copper Hill, (between Bedrock Creek and Boren Creek); the stockworks contain chalcopyrite, magnetite, native gold, native platinum, and native palladium;
Gold-bearing pyrite veins and replacement deposits near the old site of La Plata City; the veins and replacement bodies occur within early-Mesozoic limestones and hornfels;
Ruby silver veins which occur in a belt extending from Fassbinder Gulch to the central portion of Cumberland Basin; these remarkable deposits contain proustite, pyrargyrite, stephanite, argentite, and native gold;
Gold and silver telluride veins on Diorite Peak, Lewis Mountain, Madden Peak, and Parrott Mountain; the gold and silver tellurides occur within Permian red beds and Jurassic sandstones along faults and fractures; these deposits contain sylvanite, calaverite, krennerite, hessite, petzite, and coloradoite.
Ore-bearing faults and fractures containing gold and silver tellurides make up the vast majority of precious metal deposits in the La Plata Mountains. Tellurides have been by far the most important ores in the La Plata mining country.
The La Plata Mountains, and Parrott Peak in particular, have fairly good potential for future mineral strikes. First of all, the history of the area suggests that hidden lodes still exist. Parrott Mountain is home to three well-documented lost mines: the Clubfoot Mine, the Hollingsworth lode, and Lone Wolf's vein. The Root Gulch area is home to two of them! Secondly, Parrott Mountain has a way of concealing its riches until, for some inscrutable reason, it decides to give them up. Although prospected since the mid-1700's, Parrott Mountain has yielded its precious metal deposits only grudgingly. Indeed, the last major strike took place as recently as the 1930's. Thirdly, the geology of the La Platas is most favorable for the emplacement of mineral deposits. The area is richly mineralized and contains many types of mineral assemblages and modes of emplacement. The presence of small Laramide intrusions throughout the range with proven mineral deposits associated with most of them makes the district attractive to prospectors.
Prospectors may want to concentrate on the rugged slopes of Parrott Mountain between Root Gulch and Snowslide Draw. Hollingsworth was quite certain that the elusive sylvanite vein cropped out in that area.