COLORADO

The Lost Simpson Mine

GEOLOGY OF THE AREA


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The Sangre de Cristo Mountains of south-central Colorado are an uplifted, block-faulted chain of peaks consisting of ancient Precambrian basement rock and younger Pennsylvanian-Permian sediments. The Precambrian rocks include truly ancient metamorphic country rock intruded by slightly younger granites. Throughout most of its length, the crest of the range is capped by a thick sequence of tilted Pennsylvanian-Permian red beds. These beautiful red-colored sediments take on a distinctive ruddy hue during certain times of the day, hence the Spanish name Sangre de Cristo ("Blood of Christ"). The eastern slopes of the Sangres are also made up of these tilted Pennsylvanian-Permian sediments. On the western side of the Sangres, Precambrian crystalline rock is exposed along the base of the range. Higher up the slopes, younger Paleozoic sediments lie unconformably upon the ancient Precambrian basement rock. In some sections of the Sangre de Cristos, Precambrian granite crops out along the crest of the range. Examples include the Mount Blanca complex, the Culebra Peak area, and the Crestone Range.

In the La Veta Pass area, the Sangre de Cristo mountain chain is composed of ancient Precambrian metamorphic basement rock along the western edge, and younger Pennsylvanian-Permian sediments along the crest and on the eastern flanks of the range. The La Veta area is also home to a number of large mid-Tertiary granitic intrusions which form the core of several prominent landscape features. Mount Maestas, Rough Mountain, Silver Mountain, Iron Mountain, Sheep Mountain, Little Sheep Mountain, and the eroded eminence known as the Black Hills are all composed of these mid-Tertiary granites. Unlike most of the Sangre de Cristo mountain chain, the La Veta area is especially well-endowed with Tertiary intrusions. Most of these lie on the eastern side of the range.

The Sangre de Cristo Mountains were pushed up along a set of high-angle thrust faults that pretty much run the length of the chain. Faulting was initiated during Laramide times and continued sporadically throughout the Tertiary Period.

Rising up 10,522 feet, Silver Mountain lies just east of Mount Maestas and the La Veta Pass area. Like the Spanish Peaks, Silver Mountain lies at the hub of a series of radiating dikes that splay outward from the central peak. It is for this reason that the peak was once known as Dike Mountain. Silver Mountain is composed of the same granitic rock that makes up the Spanish Peaks, Mount Maestas, Sheep Mountain, and Little Sheep Mountain.

The only gold-producing districts in the immediate vicinity of La Veta Pass are the Grayback Mountain District, which lies just west of the pass, and the Silver Mountain District, which lies just east of the pass. In both districts, the ore deposits are associated with mid-Tertiary granitic intrusions. The Grayback District contained both lode deposits and placer deposits. The lode deposits consisted of gold-bearing pyrite and chalcopyrite veins which crisscrossed the southern slopes of Grayback Mountain. The placer deposits were located below the vein outcrops in the creeks and gulches that drain the mountainside. The best placers were in Spanish Gulch, Grayback Gulch, Giant Gulch, and Placer Creek.

The Silver Mountain District was never an organized mining district on a par with the Grayback District. The mines on Silver Mountain contained only very small veins of gold and silver.

PROSPECTING POTENTIAL

The La Veta Pass area is an important gateway to the San Luis Valley from the eastern plains of Colorado. It is one of the easiest pathways through the Sangre de Cristo Mountains. La Veta is remarkable for another reason. Unlike most of the Sangre de Cristo chain, the La Veta area is blessed with numerous Tertiary granitic intrusions. If we operate on the premise that gold-bearing ore deposits are frequently associated with Tertiary intrusive rocks (as in other parts of Colorado), then the La Veta area certainly offers some potential for future strikes. Indeed, Tertiary-age intrusions on both sides of La Veta Pass have small gold-bearing veins associated with them. In the case of the Grayback Mountain deposits, the veins contained gold-bearing pyrite in a matrix of quartz. Weathering of those veins produced significant placer deposits in the canyons and gullies below the ore bodies. The Silver Mountain deposits, on the other hand, were uniformly small and contained both gold and silver.

Prospectors may want to focus on the mid-Tertiary intrusives in the La Veta Pass area, especially Silver Mountain, Sheep Mountain, and Mount Maestas. In particular, the contact between the granite intrusions and the older country rock should be carefully checked for signs of mineralization. Silver Mountain merits special attention as most accounts of the story place the Lost Simpson Mine there.

The mine portal is almost certainly concealed. Prospectors may want to concentrate on the heavily forested sections of the mountain or on areas covered by landslide deposits and boulder fields. This presents the prospector with a daunting challenge. Mount Maestas, for example, is almost completely covered with talus and boulder fields. A metal-detector would be most useful in the search for gold-bearing float.

While in the area, prospectors may want to investigate the upper slopes of Grayback Mountain for the presence of a small, hidden telluride deposit. Some of the nuggets collected from Placer Creek and Grayback Gulch contained calaverite, but no telluride veins have ever been found on Grayback Mountain.


 

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