The San Juan Mountains of southwestern Colorado consist of a domed and uplifted pile of Tertiary volcanic rocks resting upon older Mesozoic, Paleozoic, and Precambrian basement rock. Tertiary volcanic rocks cover most of the San Juan region but in some areas the older basement rocks have been exposed as a result of uplift and erosion. Most of these exposures of ancient country rock occur in the western and southwestern portions of the San Juans. The Needle Mountains and the cliffs and mountain crags surrounding Ouray are good examples of this exposed basement rock.
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. Each phase of volcanism is marked by a change in the type of volcanic rock produced. The main difference between the 3 phases is chemistry. First-phase volcanics are intermediate in composition while second-phase volcanics are distinctly felsic and silica-rich. Third-phase volcanics are different still. This last pulse of volcanism in the San Juan region produced a bimodal sequence of volcanic rocks consisting of dark-colored basalts and light-colored, high-silica rhyolites.
The Bear Creek drainage heads near Engineer Mountain and flows generally westward to the Uncompahgre River, just above the town of Ouray. The Bear Creek basin consists mostly of younger Tertiary igneous rocks, but at its mouth a belt of Precambrian metamorphic rocks crops out. These ancient beds of metamorphosed sediments are
tilted 90° vertically. The sight of these rock formations turned on their side provides a dramatic demonstration of crustal deformation as mountains are pushed up. The upper reaches of Bear Creek cut through younger Tertiary igneous rocks consisting mostly of early, first-phase volcanic rocks of the San Juan Tuff. Slightly older Tertiary intrusive rocks crop out along the divide separating Bear Creek from Henson Creek.
A large east-west trending fault cuts through the mountains just south of Ouray. The fault extends roughly 6 miles from the slopes of Mount Ridgeway eastward to the area near Engineer Mountain.
Gold, silver, lead, zinc, and copper mineralization occurs 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 second-phase Tertiary volcanic rocks, the rest with older Laramide intrusives and older-still Precambrian metamorphic and igneous rocks. In the Ouray Mining District, the ore bodies consist mostly of veins and fracture fillings. Three types of deposits are recognized:1) polymetallic gold-silver telluride veins containing calaverite, petzite, hessite, auriferous pyrite, chalcopyrite, tetrahedrite, galena, and native gold 2) silver-lead veins containing pearcite, pyrargyrite, silver-bearing galena, tetrahedrite, pyrite, and sphalerite 3) gold-bearing pyrite veins containing native gold, chalcopyrite, and auriferous pyrite
All three types of deposits are associated with nearby Tertiary intrusions. The Ouray District is well-endowed with mineral deposits. Over $125,000,000 in gold, silver, lead, copper, and zinc has been recovered from the district.
The Bear Creek area southeast of Ouray is the best and at the same time worst prospecting country in the lower 48. The Bear Creek watershed presents the prospector with dazzling possibilities for rich mineral strikes, but there are no mountains in North America more rugged than those surrounding Bear Creek. The Bear Creek basin is home to one of the richest deposits of calaverite in mining history. The richness of the ore is well-documented. The eye-witness accounts of a number of knowledgeable mining men who actually saw samples of Lindstrom's ore attest to its incredible richness. The assay records of Lindstrom's sale of the ore in Ouray confirm the bonanza quality of the calaverite vein. Seventy pounds of ore brought Lindstrom $7300!
But the Bear Creek basin is not easily penetrated. The country is extremely rugged and nearly inaccessible. As one gazes up Bear Creek from Highway 550 he is met with the sight of ancient Precambrian metasediments standing up on end. It is a bit disconcerting to see these primeval rock formations jutting up from the mountainsides. The country is stark and forbidding.
Rising up at the head of Bear Creek, the high peak known as Engineer Mountain is home to a scattering of rich mineral claims. The Polar Star Mine, located near the top of the peak, was the richest and largest mine in the area. A number of other rich deposits were discovered on Engineer Mountain, including the Syracuse Pride vein, the Annie Woods vein, and the Siegel group of claims.
Prospectors should probably begin their search for the Lost "Crazy" Swede vein in the middle and upper basins of Bear Creek. In this rugged country that's a tremendous amount of ground to cover. Hundreds of small crannies, notches, and rock clefts dot the mountainsides. Prospectors should keep a sharp eye out for fragments of rich "float" in the creek. Careful tracing of the float may help pinpoint the location of the vein. Prospectors may want to focus on the upper basin of Bear Creek. The presence of Tertiary intrusive rocks in the area makes this part of the basin attractive. Recall that the ore deposits in the nearby Ouray District are all associated with Tertiary intrusives. In addition, the area near Engineer Mountain has already been shown to harbor mineral deposits.