The great Tertiary volcanic event in southwestern Colorado is only the latest of several events extending back to Precambrian time. But beginning about 40 million years ago, the southwestern portion of the state was rocked by wave after wave of volcanic eruptions. A vast amount of ash and lava was produced during a succession of volcanic pulses that lasted nearly 30 million years.
Three major volcanic pulses are recognized in the San Juan volcanic field. The earliest pulse produced vast amounts of intermediate volcanic rock, including andesites, rhyodacites, and mafic-rich quartz latites. After a brief interlude, the second pulse of volcanism began with a bang. During this episode of volcanism, silica-rich felsic rocks like quartz-rich latites and rhyolitic ash-flow tuffs were produced. Finally, a bi-model suite of basalts with minor rhyolite lavas and ash-flows was produced during the last volcanic pulse. This vast outpouring of basalt eventually covered most of the eastern San Juans before volcanism subsided.
The San Juan Mountains are home to 11 major caldera structures and more than 60 caldera-like, circular features. In the San Juans, most mineral deposits are associated with calderas. The collapse structures associated with many of these calderas have served as a locus for mineralization.
The Continental Divide north of Pagosa Springs comprises some of the most rugged country in North America. This vast area consists of a great exposure of Tertiary volcanics which overlies
much older sedimentary and igneous rock. Just west of the Tertiary volcanic field, an immense exposure of Mesozoic sedimentary rocks stretches southwestward. A huge wedge of Precambrian granite forms the core of this older basement rock and is also exposed west of the Tertiary volcanic field. The contact between these older rocks and the younger Tertiary volcanics roughly parallels the Continental Divide in this part of the San Juans. Faulting is prevalent along the western side of the Divide, especially near the headwaters of the Piedra River, Porphyry Gulch, Williams Creek, and Cimarrona Creek. Here, they occur as a concentric set of arcuate faults that trend roughly northwest-southeast. These ring-like, circular structures superficially resemble the collapse calderas at Silverton and Creede.
The Continental Divide in this part of the San Juans is overlain by quartz latite lavas and ash-flows, andesite lavas and ash-flows, and silicic ash-flow tuffs. The area is rich in pyroclastic rocks. Quartz latites occur on the east side of the Divide, near the headwaters of Fisher and Goose Creeks. Andesites and silicic ash-flow tuffs comprise most of the Divide area west of Piedra Pass.
The stretch of rugged mountains along the spine of the Continental Divide between Beartown and Summitville is nearly devoid of mineral deposits. The nearest zone of mineralization is at Beartown, located nearly 30 miles northwest of the area of interest. Here, the ore bodies consisted of the telluride minerals sylvanite and petzite.
The awesome peaks and ridges that form the backbone of the Continental Divide north of Pagosa Springs comprise some of the most rugged country in North America. "Stewart Country" includes an enormous tract of rugged peaks extending from Squaw Pass southeastward to Piedra Pass and then westward along the Divide to the Sawtooth Mountains. A number of factors combine to make this a difficult and unpromising area to prospect in. The area is incredibly vast and remote. In addition, elevations range from 12,000 to 13,000 feet and the air is thin. The local geology is not particularly conducive to the emplacement of significant mineral deposits and, in any case, placer deposits rarely form so high up. The Lost Stewart Placer is located in a seemingly barren part of the San Juans which is borne out by the lack of mining districts in the area.
The area of interest does contain at least two "circular features" which resemble craters or caldera-like structures. In the San Juan Mountains, mineral deposits are frequently associated with calderas. In addition, gold has been found along the Continental Divide, although it is indeed a rare occurrence. Small nuggets have been recovered along the Divide near the heads of Trout Creek and Ute Creek. It appears that most of this gold is glacially-derived.
Prospectors may want to begin their search in the small cirques and hanging valleys near the headwaters of the West Fork of the San Juan River, Cimarron Creek, Middle and East Forks of the Piedra River, Porphyry Gulch, Williams Creek, and Cimarrona Creek. It is an incredibly vast area. It is also home to the arcuate fault system described in the geology section above. Its resemblance to the well-known caldera structures and fault systems at Silverton and Creede may be more than superficial. It is quite likely that the small stream has been concealed, either by nature or by the hand of man. A metal-detector would be most useful in the search for the lost placer.