Encompassing nearly 12,000 square miles of rugged volcanic peaks, the San Juan Mountains of southwestern Colorado are a classic example of mid-Tertiary mountain building in North America. The San Juan Mountains are part of an extensive volcanic field comprised of numerous domes and caldera-like structures, all produced during the latter half of the Tertiary Period.
Three pulses of volcanism have been recognized in the San Juan region. The first produced immense volumes of intermediate andesites, rhyodacites, and mafic-rich quartz latites. Lasting for some 5 million years, the first pulse poured forth an incredible amount of lavas, breccias, and ash before volcanic activity subsided. The second pulse produced eight major outpourings of ash and lava beginning about 29 million years ago. These eruptions were extremely violent as evidenced by the great volume of pyroclastic debris associated with this event. The third pulse of volcanism in the San Juan region produced vast amounts of basalt with minor rhyolites and associated pyroclastics. This final pulse began about 25 million years ago and is responsible for the thick blanket of basalt covering most of the eastern San Juans.
The Ute Creek watershed is dominated by Tertiary volcanic rocks bounded by ancient Precambrian metamorphics and mafic-rich granites. The Ute Creek area lies on the southwestern edge of the great San Juan Tertiary volcanic field described above. Along the headwaters of Ute Creek, basalts and basaltic andesites cover most of the area drained by East and Middle Ute Creeks while West Ute Creek cuts through a large formation of silicic ash-flow tuff. The silicic tuff butts up against Precambrian basement rock along the head of the creek, near the Continental Divide. The Tertiary volcanics are the product of a now-buried caldera complex called the Ute Creek Caldera. This caldera lies hidden beneath the Ute Creek watershed.
Mineralization in this part of the San Juans appears to be confined to the Beartown District, located 3 miles northwest of Ute Creek. The Beartown deposits were small but extraordinarily rich. Emplaced within the ancient Precambrian basement rock of the Uncompahgre Formation, the Beartown ore deposits were chock full of sylvanite and petzite.
The Ute Creek area has no obvious deposits of gold or silver although occasional samples of rich float have been found in the region. A U.S.G.S. mapping and sampling project during the 1940's suggested the Ute Creek area has good potential for hidden mineral deposits. It still remains to be seen if this is so.
The headwaters of Ute Creek comprise some of the most rugged and heavily wooded country in the lower 48. The Ute Creek area is extremely remote and borders the Continental Divide for nearly 15 miles in the heart of the San Juan Mountains. The area is stunningly beautiful and blessed with an abundance of freshwater springs that ring the valleys of East, Middle, and West Ute Creeks.
Mineralization in the San Juans is almost invariably associated with caldera structures. In the Ute Creek area, the caldera lies buried beneath the overlying volcanic rocks. Nevertheless, the potential for mineralization is present based on the U.S. Geological Survey of the 1940's. In addition, the close proximity of the rich Beartown deposits suggests the possibility of similar deposits somewhere near the headwaters of West Ute Creek. Occasional discoveries of rich float in Ute Creek hint at the presence of a hidden mineral deposit nearby. Caches and stashes of rich ore in the Ute Creek area seem to substantiate this.
Prospectors may want to concentrate on the western portion of the area of interest in their search for the Lost Ventana Mine. This includes Middle and West Ute Creeks. The eastern portion is predominantly overlain by basaltic rocks which are generally barren of mineral deposits. The head of West Ute Creek lies only a few miles southeast of the Beartown District. Maybe an offshoot of the rich Beartown vein crops out somewhere in the West Ute Creek area. Prospectors may want to restrict their search to areas in which the Rio Grande Pyramid and Window are visible. These famous landmarks are said to be visible from the portal of the mine. Prospectors must realize that the Ventana Mine is probably partially or even completely concealed. A metal-detector may be useful in the search for gold-bearing float.