The vast San Juan volcanic field contains a multitude of circular or crescent- shaped structures, many of which have been positively identified as calderas. Nearly 60 circular or curvilinear features are recognized throughout the volcanic field. Many of these are quite large. A few, like the La Garita Caldera, are truly colossal in size.
The tremendous La Garita Caldera is home to a number of smaller structural features including the famous Creede Caldera and the rugged, uplifted block of volcanic rock known as the La Garita Mountains. It is also fragmented and extraordinarily uniform in composition. The volcanic rocks produced during the La Garita event are without exception dacitic in composition.
The La Garita Mountains consist mostly of highly-weathered dacitic lavas known as the Fish Canyon Tuff. These dacites are a product of the La Garita event mentioned above. Although the La Garitas consist almost entirely of porphyritic dacite, the northern portion of the range is home to a few small outcrops of younger third-phase volcanics. These bimodal rocks consist mostly of dark-colored basalts and high-silica rhyolites.
Older Tertiary intrusive rocks crop out in the southern portion of the La Garita Mountains. Two large masses of Tertiary intrusive rock lie near Cyclone Mountain and just north of the peak, along the upper stretches of La Garita Creek. Two smaller bodies lie on the other side of the La Garita divide, near the headwaters of Deep Creek and Wannamaker Creek. The abandoned site of the remote ghost town known as Sky City lies close by.
Mineralization in the mountains west of the upper San Luis Valley consists of polymetallic gold- and silver-bearing sulfide veins, telluride deposits, and breccia pipes. The Bonanza District, located in the Cochetopa Hills, 30 miles northeast of the La Garitas, contains both telluride and sulfide deposits. Both occur within faults and fractures in the Tertiary volcanics. The Crystal Hill District, located just west of the town of La Garita, is home to a collapse breccia structure containing native gold and native silver. The precious metal deposits of Crystal Hill were initially quite rich. The Embargo Creek District, located 20 miles west of La Garita, contains gold and silver-bearing sulfide veins emplaced within faults and fractures in the older pre-ash flow lavas. These polymetallic veins were located along Embargo Creek in Saguache and Rio Grande Counties.
Most accounts of the Lost Mine of Saguache Creek place the deposit on the Saguache side of the La Garita Mountains, across the divide from Carnero Creek. Depending on which fork of Carnero Creek the survey party ascended, the area of interest extends northeastward from the flanks of Bowers Peak (elevation 12,449 feet) all the way to Carnero Pass. It is a vast area describing an arc roughly 15 miles long. The Saguache side of the range is drained by several major tributaries that have their headwaters in this area. These include Johns Creek, California Gulch, Moon Creek, West Park Creek, and Houselog Creek. If the survey party ascended the South Fork of Carnero Creek, then they probably crossed the divide at Moon Pass. This would place them at the headwaters of Moon Creek, just south of Hat Mountain. Descending Moon Creek, the party would eventually reach Saguache Creek via California Gulch. Prospectors may want to focus on this section of the La Garitas first, then extend their search along the Saguache side of the mountain range either to the northeast or southwest.
It would be tempting to head southwest toward the old site of Sky City. Sky City is located on the Saguache side of the La Garita Mountains, near Wannamaker Creek. It lies directly across the divide from the Embargo Creek headwaters. The Sky City area is home to two small Tertiary intrusive bodies. Similar bodies lie on the Embargo Creek side of the range near Cyclone Mountain and along the upper reaches of La Garita Creek. Sky City is home to the now abandoned Sky City Mine. Although initially quite promising, the ore bodies near Sky City turned out to be only isolated pockets that did not persist at depth. Maybe the "rich gold streaks" reported by the survey party back in the early 1870's were of a similar nature. In any case, the presence of small Tertiary plugs in the Sky City area makes it attractive to the prospector.