The San Juan Mountains of southwestern Colorado are part of a vast Tertiary volcanic field consisting of a thick sequence 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 Needle Mountains, Grenadier Range, and Rico Dome are all composed of this older, exposed basement rock. The San Juan volcanics are the product of three episodes of volcanism beginning about 40 million years ago and lasting some 20 million years. The first pulse of volcanism produced massive amounts of andesitic lavas and breccias. This andesite-producing stage was followed by a second pulse of volcanism that was more silica-rich and more violent than the first pulse. This second episode of volcanism produced immense outpourings of rhyolitic lava and ash-flow tuff. This rhyolite-producing stage was followed by a third and final stage of volcanism that was again different from the first two. In this case, a bimodal sequence of basalts and high-silica alkali rhyolites was extruded. Volcanic activity ceased after the production of these lavas.
The Beartown District lies near the southwestern edge of the great Tertiary volcanic field described above. Indian Ridge and Ute Ridge rise up just east of Bear Creek and the Beartown District. Both ridges are composed of Tertiary volcanic rocks. The Beartown District itself is developed on ancient Precambrian slates, schists, and quartzites of the famous Uncompahgre Formation. These metasediments crop out along the Continental Divide in this part of the San Juans.
Gold, silver, lead, zinc, and copper mineralization occurs in the San Juan Mountains 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, the rest with Precambrian granites and metamorphics. In the Beartown District, the ore deposits consist of gold and silver-bearing telluride veins emplaced within the Precambrian country rock. The most important ore minerals were sylvanite and petzite.
The headwaters of Bear Creek near the old mining camp of Beartown have pretty good potential for future mineral strikes for a number of reasons. This part of the San Juans is home to a number of lost mines including the Lost Ventana Mine in nearby Ute Creek basin and the subject of this story, the hidden mine on Bear Creek. Well-documented accounts like these usually have some basis in fact. The area is certainly mineralized. The Beartown District was a proven producer of gold and silver for a number of years. Although the Beartown veins were small and paltry, they were nevertheless extraordinarily rich. Ore from the Sylvanite Mine near Kite Lake assayed out at $4000 a ton!