COLORADO

The San Juan 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. Three pulses of volcanism are recognized in the San Juan Volcanic Field. The first occurred during the mid-Tertiary Period, about 35 to 40 million years ago. This episode of volcanism produced massive amounts of andesitic lava which coalesced to form the famous San Juan Formation. Then, about 28 to 30 million years ago, southwestern Colorado was subjected to a renewed sequence of eruptions. But this time, the lavas were more silicic and the eruptions more violent. Great quantities of silicic ash-flow tuff and breccia were produced during this volcanic event. Significantly, this episode of volcanism is 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 volcanic rocks produced during the previous episodes. Shortly thereafter, volcanic activity in the San Juans subsided.

Gold, silver, lead, zinc, and copper mineralization occurs in the San Juans in the form of hydrothermal veins and fracture fillings, replacement bodies, and pipe or "chimney" deposits. The majority of these ore deposits are associated with Tertiary volcanics. In the Silverton District, the ore deposits are emplaced within a deeply faulted structural feature known as the Eureka Graben. Formed as a result of uplift and doming some 27 million years ago, the Eureka Graben consists of a vast system of faults and fractures filled with rich mineral deposits.

The Coal Creek watershed drains the rugged northern slopes of Engineer Mountain and the southern flanks of Jura Knob. Engineer Mountain consists of a thick sequence of cyclic Pennsylvanian marine sediments on its lower slopes and younger Pennsylvanian-Permian "red beds" on its upper slopes. This sedimentary terrane is cut by many small dikes, sills, and stocks. Indeed, a number of small Tertiary intrusions and sills are located on Engineer Mountain itself and just northeast of the mountain, near Lime Creek.

The Lime Creek drainage basin appears to have some potential for future mineral strikes. Rich gold-bearing float has been found on Lime Creek and along its tributary, Coal Creek, for many years. The Lime Creek/Coal Creek area lies in the center of a richly mineralized section of the San Juans. Only 13 miles to the west lies the fabulous Rico District while the Silverton/Eureka District is located only 10 miles northeast of the area of interest. Silverton is one of the richest mining districts in the entire state. Mineralization at Silverton consists of polymetallic veins of gold and silver-bearing sulfides emplaced within a Tertiary quartz monzonite and andesite and along the faults and fractures that crisscross the area. The richest deposits are located along the southern rim of the Silverton Caldera, not far from the headwaters of Lime Creek.

 

Although there is a lack of evidence for any caldera structures in the Lime Creek/Coal Creek area, the presence of several small Tertiary intrusions is encouraging. The Lime Creek drainage is home to at least two well-documented lost mines: the Lost Sheepherder's Lode and the Baker brother's lost vein on Coal Creek. The area certainly seems promising for the lost mine enthusiast. Seekers of the lost vein may want to concentrate on the junction of Coal Creek (as it wends its way down from Engineer Mountain) and its tributary stream (which drains the southern slopes of Jura Knob). A metal-detector may prove useful in the search, especially if the vein is hidden beneath the soil or leaf mold.