COLORADO

The Lost Sheepherder Lode

GEOLOGY OF THE AREA


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The San Juan (and West Elk) Mountains of southwestern Colorado are part of an extensive 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. Gold, silver, lead, zinc, and copper mineralization occurs in the San Juans in the form of veins, replacement bodies, and pipe (or chimney) deposits. The majority of these ore deposits are associated with Tertiary volcanics.

During middle Tertiary time (35-40 million years ago), southwestern Colorado was the site of massive volcanic eruptions of andesitic lava. These immense outpourings of lava produced the San Juan Formation (in the San Juan volcanic field) and the West Elk Breccia (in the West Elk volcanic field). Then, about 28 to 30 million years ago, southwestern Colorado was subjected to a renewed sequence of eruptions. This episode of volcanic activity produced great quantities of silicic ash-flow tuffs and breccias. Significantly, this episode of volcanism was also responsible for most of the mineralization in the San Juans. Fifteen calderas have been recognized in the San Juans that date

 

from this event. Significant mineralization is associated with many of these calderas. About 26 to 27 million years ago, uplift and doming of the San Juan region occurred, followed by renewed volcanism. Between 23 and 25 million years ago, a new series of lavas began to appear. Basalts and high-silica alkali rhyolites were now the dominant rock types being extruded. This bimodal sequence of basalt and rhyolite was radically different from the previous andesitic lavas. Shortly thereafter, volcanic activity in the San Juans subsided.

The headwaters of Lime Creek are located only 5 miles southwest of the rich Silverton (or Eureka) Mining District. Home of the famous Sunnyside vein, the Silverton District is one of Colorado's richest. The vein deposits are associated with the Silverton caldera and are emplaced within radial and concentric fissures, faults, and fractures. In the Sunnyside vein, the ore deposits consist of gold-tellurides and gold-bearing sulfides.

The headwaters of Lime Creek are situated in a terrane of Paleozoic sedimentary rocks intruded by a Tertiary-age feldspar porphyry stock. Along the southern edge of the Tertiary intrusion, the Paleozoic sediments form a doubly-plunging syncline. These sedimentary rocks consist of limestones, sandstones and shales that have somehow survived the uplift and erosion that has stripped away most of the older sediments in this part of the San Juans.

PROSPECTING POTENTIAL

Mining men and prospectors have long suspected that a rich vein of gold ore lies somewhere along the headwaters of Lime Creek. Occasional discoveries of gold-bearing float in Lime Creek seem to confirm the existence of a hidden vein in the area. The Lime Creek basin is home to at least two lost mines: the gold-bearing vein discovered by Sull and Charley Baker on Coal Creek (a tributary of Lime Creek) and Sasario Silva's vein. Certainly the presence of gold-bearing float in the stream coupled with the accounts of Sasario Silva and the Baker brothers makes the Lime Creek area an exciting prospect for the modern-day gold-seeker.

Another factor that favors the Lime Creek area as a locus of hidden veins is its proximity to the Silverton Caldera and the Eureka Mining District. Known as the "Treasure Chest of the Silvery San Juans", Silverton is one of Colorado's richest mining areas. Mineralization in the Eureka (Silverton) District occurs primarily in the southern portion of the caldera complex, fairly close to the Lime Creek headwaters.

The geology of the Lime Creek area is certainly favorable for the emplacement of ore-bearing mineral deposits. Although the Lime Creek area is devoid of any craters or caldera structures, the presence of a Tertiary-age feldspar porphyry intrusion near Highway 550 gives the region fairly good potential for future mineral strikes.

Prospectors may want to concentrate on the upper headwaters of Lime Creek, extending from the small Tertiary intrusion near Highway 550 northward toward the sources of West Lime and North Lime Creeks. The Tertiary intrusion is an attractive place to start, especially along the contact between the stock and the surrounding country rock. The limestones in particular provide excellent host rocks for small veins or replacement bodies.

Prospectors should not overlook the meadows and pastures that constitute much of the upper Lime Creek country. The vein is thin and may have been exposed by Silva's sheep as they grazed the meadows. Indeed, it may even now be exposed at the surface in some high meadow along Lime Creek, too small to be noticed unless you happen to be standing right over it.


 

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