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. The rugged Needle Mountains are a good example of these older, exhumed wedges of basement rock.
The great piles of Tertiary volcanics that blanket most of the San Juan massif are the product of three episodes or "pulses" of volcanism. The first pulse occurred 35 to 40 million years ago and produced vast quantities of andesitic lavas and breccias. The second pulse occurred 28 to 30 million years ago, but this time the chemistry of the volcanics was different. Now, for some 3 million years, immense outpourings of silica-rich rhyolitic ash-flow tuffs and lavas covered the land. Then, between 23 and 25 millions years ago, a new series of lavas began to appear. Basalts and high-silica alkali rhyolites were now the dominant rock type being extruded. This bimodal sequence of basalt and rhyolite was radically different from the previous lavas. Shortly thereafter, volcanic activity in the San Juans subsided.
The precipitous Needle Mountains, located north-northeast of Durango, consist of an uplifted block of ancient Precambrian igneous and metamorphic rocks exposed by a combination of uplift and deep erosion. This block of uplifted Precambrian crust consists of a belt of ancient metamorphic country rock intruded by a number of granitic plutons. The oldest intrusive rock in the Needle Mountains is the
1.78 billion year old Twilight Granite. Emplacement of the Twilight Granite was followed by a period of active tectonism in which a north-northeast trending foliation was imparted upon both the Twilight Granite and the metamorphic country rocks (which include Archean schists and gneisses and the Irving Greenstone). The 1.72 billion year old Tenmile and Bakers Bridge granites were emplaced shortly thereafter. Igneous activity was resumed some 360 million years later with the formation of a number of large granitic plutons. The Eolus Granite, a biotite- and hornblende-bearing monzogranite, was emplaced during this period of time. The West Needle Mountains are mostly composed of this granite.
Gold, silver, lead, zinc, and copper mineralization occurs in the San Juans in the form of veins and fracture fillings, replacement bodies, and pipe or "chimney" deposits. The majority of these ore deposits are associated with Tertiary volcanics, but a few of them are much older in origin. The small, gold-bearing veins in the Needle Mountains District are a case in point. These polymetallic ore deposits consist of gold and silver-bearing pyrite, sphalerite, chalcopyrite, galena, and tetrahedrite emplaced within faults and fractures in the ancient granites and metavolcanics.
The rugged Needle Mountains of southwestern Colorado certainly have potential for future mineral strikes, but prospectors have their work cut out for them. The Needle Mountains are remote, heavily forested, and nearly trackless. In addition, the area appears to be only weakly mineralized. The mountains are home to only one small and relatively insignificant mining district. But on the other hand, the Needle Mountains District did produce thousands of ounces of gold for a short period during the late 1870's. And the rugged, heavily wooded terrain only enhances the possibility that hidden veins lie somewhere in the area.
A number of other factors suggests the presence of additional gold-bearing veins in the Needle Mountains. First of all, the Needle Mountains are virtually surrounded by rich mining districts. Beartown lies only 4 miles northeast of the Needle Mountains while the great Silverton District lies 8 miles straight north. The Rico District is located 15 miles west of the Needles while the La Plata District lies 20 miles to the southwest. Secondly, the Needle Mountains are home to at least two well-documented lost mines: Levi Carson's sylvanite vein and the Lost Estes Mine. Both are located in the West Needle Mountains, near Twilight Peak. Rich gold-bearing float in Twilight Creek seems to indicate the presence of a hidden vein somewhere in the area.
When American prospectors first penetrated the Needle Mountains, they discovered signs of ancient Spanish mining activity. The old Spanish prospect pits and mine shafts were a clear indication that the area was mineralized. The geology seems to bear this out. The presence of several Precambrian (Proterozoic) igneous intrusions emplaced within an older, metamorphic belt (which includes the Irving Greenstone) should excite any gold prospector. Greenstone belts in particular seem to have an affinity for gold. Virtually every Greenstone belt in the world has gold associated with it.
Gold occurs in many of the Proterozoic (1.7 billion years old) sulfide deposits of Colorado. Research has revealed the presence of similar gold and silver-bearing sulfide deposits in other Precambrian belts throughout the world. These deposits are especially abundant in regionally metamorphosed basement rock of Proterozoic age.
Prospectors may want to focus their search for the Lost Carson Mine along the rugged northern slopes of the West Needle Mountains. Most accounts of the story place the mine somewhere in this area. Prospectors may also want to look closely at any bands of iron-rich rock that they happen to find in the Irving Greenstone Formation. Gold is sometimes associated with the ironstones in these ancient rocks. A metal-detector would certainly be of use in locating any gold-bearing float hidden in the soil or leaf mold.