The San Juan Mountains of southwestern Colorado are comprised of a vast, composite Tertiary volcanic field consisting of a thick pile of lavas, breccias, and ash-flow tuffs. These Tertiary volcanics are underlain by Precambrian, Paleozoic, and Mesozoic rocks which have been exposed in some areas as a result of uplift and erosion. The San Juan volcanics are the product of 3 episodes of volcanism beginning about 40 million years ago and lasting off and on for some 20 million years, The first phase of volcanism produced massive amounts of andesitic lava and fine breccia. This initial stage of volcanism lasted some 5 million years before volcanic activity subsided. Then about 28 to 30 million years ago, southwestern Colorado was subjected to a renewed sequence of eruptions. This episode of volcanic activity produced great quantities of silicic ash-flow tuffs and breccias. Significantly, this second phase of volcanism was responsible for most of the mineralization in the San Juans. About 26 to 27 million years ago, uplift and doming of the San Juan region occurred, followed by renewed volcanism. This third and final phase of volcanism produced a new series of lavas consisting of dark-colored basalts and high-silica rhyolites. This bimodal sequence of basalt and rhyolite was radically different from the previous lavas. Shortly thereafter, volcanic activity in the San Juans ceased.
The Ouray, Sneffels, and Camp Bird districts lie along the northwestern edge of the San Juan volcanic field. Older, first-phase volcanics make up most of the terrain in this part of the San Juans. Only where the rivers and streams have cut deep canyons do we see older Mesozoic, Paleozoic, and Precambrian rocks. The steep cliffs surrounding the town of Ouray are made up of these older sedimentary rocks. Younger, second-phase volcanics are mostly absent in this part of the San Juans. Only the peaks towering above Sneffels Creek are composed of these younger volcanics. Intrusive igneous rocks also occur in this part of the San Juans. Mount Sneffels, Mount Wetterhorn, Mount Ridgway, and Whitehouse Mountain are all home to Tertiary intrusive rocks.
As mentioned above, the canyon walls surrounding Ouray are made up of older Mesozoic and Paleozoic sedimentary rocks. South of Ouray, these sedimentary rocks are in direct contact with ancient Precambrian basement rock. West of Ouray, along the slopes of Whitehouse Mountain, the sedimentary rocks are in contact with Tertiary intrusives. Oak Creek drains this part of Whitehouse Mountain. From the floor of the Uncompahgre valley to the head of Oak Creek, one first passes through a thick sequence of Paleozoic sedimentary rocks followed by an equally thick layer of Mesozoic sandstones and shales. Further up the creek, Tertiary intrusive rocks are encountered.
A large, east-west trending fault cuts through the mountains just south of Ouray. Roughly 6 miles long, it extends from Mount Ridgway eastward to the Engineer Mountain area.
Gold, silver, lead, zinc, and copper mineralization occur in the San Juans in the form of veins and fracture fillings, replacement bodies, and pipe or "chimney" deposits. The majority of these ore deposits are associated with Tertiary volcanics. In the Ouray District, the ore bodies are Laramide in age. Ore bodies consist of both veins and replacement bodies containing native gold, pyrite, chalcopyrite, galena, and sphalerite. In addition, some gold and silver telluride veins occur just north of Ouray. These telluride-bearing veins are emplaced within a brecciated "paleokarst" deposit consisting of mixed Mississippian and upper Mesozoic strata. These ancient landslide deposits contain native gold, hessite, calaverite, petzite, gold-bearing pyrite, tetrahedrite, chalcopyrite, and sphalerite. This unique landslide formation is also home to rich silver-lead veins containing pearcite, pyrargyrite, silver-bearing galena, tetrahedrite, and pyrite. Both types of deposits are associated with Tertiary intrusive rocks.
The high mountain peaks surrounding Ouray, Colorado are a prospector's dream. First of all, the area is highly mineralized. The mountains are quite literally laced with mineral deposits. In the Ouray Mining District alone, polymetallic gold-silver telluride veins, gold-bearing sulfide veins, and rich silver-lead deposits all occur. Then, there's the richly-mineralized Sneffels District, located only a few miles west of Ouray. This mining district includes a number of rich claims in Yankee Boy Basin. And finally, there's the fabulous Camp Bird District near Imogene Creek. Highly mineralized and extraordinarily rich, the Camp Bird area has produced a fortune in gold and silver since 1895.
The geology of Mount Ridgway and Whitehouse Mountain appears to be favorable for the emplacement of small ore bodies. The slopes of Whitehouse Mountain in particular, consist of thick layers of Paleozoic and Mesozoic sedimentary rocks capped by Tertiary
intrusive rock. The presence of Tertiary intrusive rock at the head of Oak Creek is certainly encouraging to the prospector. Although most of the ore deposits in the Ouray District are Laramide in age, the overwhelming majority of San Juan ore bodies are mid-Tertiary in age. The Tertiary intrusion near the head of Oak Creek may also have small mineral deposits associated with it.
Prospectors should probably focus on the entire Oak Creek watershed in their search for the lost mine. Particular attention should be paid to the upper headwaters of the creek where Tertiary intrusive rock crops out. The contact between the intrusion and the surrounding country rock should be scrutinized. The vein is probably concealed by an overburden of soil, sod, or vegetation. A metal-detector would be most useful in the search for the lost vein and also any "float" that may have been carried away by erosion.