The San Juan Mountains of southwestern Colorado are part of an extensive Tertiary volcanic complex consisting of a thick sequence of lavas, breccias, and 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. It is in the western and southwestern portions of the San Juan complex that we see this older basement rock exposed at the surface. The Needle Mountains, San Miguel Mountains, Rico Mountains, and La Plata Mountains all consist mostly of this underlying basement rock.
The San Juan volcanics are the product of 3 episodes of volcanism beginning about 40 million years ago and lasting on and off for some 20 million years. The first pulse produced massive amounts of intermediate lavas, breccias, and related tuffs. The famous San Juan Formation is a product of this initial phase of volcanism. About 28 to 30 million years ago, southwestern Colorado was subjected to a renewed sequence of eruptions. This second phase of volcanic activity produced great quantities of silicic ash-flow tuffs and breccias. Significantly, this episode of volcanism was responsible for most of the mineralization in the San Juan Mountains. Fifteen separate calderas have been identified in the San Juans that date from this event. Significant mineralization is associated with many of these calderas. The third and final phase of volcanism began about 25 million years ago and was concentrated in the central and eastern portions of the San Juans. These third-phase volcanics are largely absent in the western part of the San Juan complex. This final phase of volcanism produced a bimodal sequence of dark-colored basalts and light-colored, high-silica rhyolites.
The Mount Wilson Complex forms the rugged spine of the San Miguel Mountains, near the western edge of the great San Juan Volcanic Field. The core of the Mount Wilson Complex consists of a large, mid-Tertiary dioritic intrusion. Nearly every peak in the area is composed of this diorite. The exceptions are Lizard Head and parts of Mount Wilson which consist of older sedimentary rock and younger volcanics.
Precious and base metal deposits occur in the San Juans in the form of veins and fracture-fillings, replacement bodies, and pipe or "chimney" structures. The majority of these ore deposits are associated with Tertiary volcanics. In the Mount Wilson District, the ore deposits consist of gold-bearing sulfide veins emplaced within a Tertiary diorite intrusion. The sulfide veins contain pyrite, chalcopyrite, galena, sphalerite, tetrahedrite, and arsenopyrite. The richest deposits were found on the western slope of Mount Wilson, near the head of Big Bear Creek. Here, a swarm of faults and fractures was discovered crisscrossing the diorite intrusion. Each fault and fracture was filled with gold-bearing sulfide ore.
The San Miguel Mountains of southwestern Colorado are extremely fertile ground for the modern-day prospector but they also present a unique set of challenges. First and foremost, the San Miguels contain some of the most rugged country in North America. The ruggedness of the area has always limited exploration and development to a certain extent. Secondly, the area of interest is quite extensive. It comprises all of the Mount Wilson Complex which forms the heart of the San Miguel Mountains. Three towering peaks mark the center of this complex: Mount Wilson, Wilson Peak, and El Diente Peak. Fifteen square miles of heavily forested mountain slopes and rugged alpine peaks make up the Mount Wilson Complex. But the area does have a proven history of gold production. During the 1870's, the lode deposits on Mount Wilson yielded nearly 25,000 ounces of gold.
The Tertiary diorite intrusion should probably be the focus of any prospector's search for the Lost Trail Mine. In the Mount Wilson District, all gold-bearing deposits are associated with this Tertiary diorite. It was said that the bones of the two prospectors were found in a small park or meadow high up on Mount Wilson - maybe the upper slopes of the peak should be looked at closely. Veins that occur as fracture-fillings tend to be thin and narrow and are easily overlooked. This is especially true in rugged and broken country. A metal-detector may be useful in the search for gold-bearing "float" hidden in the leaf litter or buried in alluvium.