The beautiful Sierra Madre Range extends from Divide Peak (in Wyoming) southeastward to the Rabbit Ears Pass area, a distance of some 70 miles. The Sierra Madres make up the northern portion of the much larger and more extensive Park Range which also incorporates the Gore Range to the south. The Sierra Madre, Park, and Gore Ranges are all cut from the same cloth. They all share a similar composition and history, being composed of ancient Precambrian basement rock and having been uplifted and formed at the same time in Earth's history.
The core of this uplifted chain of mountains is mostly composed of ancient Precambrian metasediments and metavolcanics intruded by slightly younger orogenic granites. These ancient crystalline rocks are exposed as a result of uplift and erosion. The younger sedimentary rocks which once covered the Precambrian core are now exposed only along the flanks of the range and in some isolated spots along the crest.
Just west of the Sierra Madres, near Hahns Peak, a scattering of much younger Tertiary-age intrusives occur. These small Tertiary stocks crop out in a semicircle around Steamboat Lake. The Hahns Peak Mining District was founded on gold deposits hosted in these younger intrusives.
The Continental Divide between the headwaters of Bear Creek (which flows eastward to the North Platte River) and the headwaters of Gold Creek (which flows westward to the Elk River) forms the heart of the area of interest for the lost Sierra Madre mine.
This part of the mountain range consists almost entirely of ancient Precambrian metamorphic rock. This ancient basement rock crops out along much of the Divide in the Sierra Madre Range and forms the country rock into which Precambrian granite has intruded. The northern slopes of Mount Zirkel are made up of this ancient granite as are the mountains near Devils Peak, a few miles to the north, and also the area near Little Agnes Mountain, a couple of miles to the west.
The nearest mining districts to the area of interest along the Sierra Madre crest are the Slavonia District (located only 3 miles to the west) and the Pearl/Zirkel Districts (located about 11 miles to the north). Both are founded on gold and silver-bearing polymetallic sulfide deposits hosted within ancient Precambrian metamorphic country rock. The Pearl/Zirkel Districts were good silver producers for awhile but none of the Sierra Madre mining districts amounted to much, especially in comparison to the great bonanza camps on the Colorado Mineral Belt.
Prospectors have their work cut out for them in their search for the Lost Mine of the Sierra Madres. The mountains east of Hahns Peak along the headwaters of Elk Creek are extremely rugged and heavily forested. The crest of the range rises up to nearly 12,000 feet, presenting a formidable barrier to prospectors traveling from the Gold Creek drainage (on the west side of the Divide) to the Bear Creek drainage (on the east side).
Despite the ruggedness of the Sierra Madres, the area described above has been heavily prospected, especially during the late 1800's and early 1900's. If any large well-exposed deposits of gold were present they would certainly have been discovered at that time. Conversely, any small poorly-exposed deposits may just as easily have been overlooked.