Sunday, February 21, 2010

Gold Deposits of Colorado

This photo taken by the author in the San Juan Mountains
shows an area that is essentially one giant gossan
(rusty rock produced by oxidation & weathering of
sulfide minerals often associated with gold). Even though
several mine dumps are seen, one can
 be guaranteed that several gold deposits were missed).

(Revised 2/16/2022) Colorado is one of the more productive gold states, ranking third, behind Nevada and California production. Historically, Colorado produced more than 50 million ounces of gold since the yellow metal was discovered in the 19th century. Essentially all gold deposits in the state are located in the mountainous regions of the western half of the state. Considerable gold was identified in a northeast-trending belt known as the Colorado Mineral Belt that runs from Boulder Colorado (in the northeast) to the San Juan Mountains to the southwest. The Cripple Creek mining district, the most important gold district in the state, lies in a separate zone southeast of the main mineral belt (Ralph, 2009). An area with similar mineralization is located north in the Rattlesnake Hills of Wyoming.

The Colorado Mineral Belt contains abundant ore deposits stretching northeast from the La Plata Mountains in southwestern Colorado to the Front Range near Boulder. Most historic metal mining camps of Colorado lie within this area, the most important exception being Cripple Creek. Over 25 million ounces was extracted from the area.

"The refining pot is for silver and the furnace for gold, And a man is valued by what others say of him" - Proverbs 27-21

The Colorado Mineral Belt also includes molybdenum mines at Climax near Leadville and Henderson. Major historic sites for gold included Central City which yielded 4.2 million ounces; Leadville on the Arkansas River where gold was found in the 1860s, and later lead, silver and zinc lodes found in 1874. In the San Juan Mountains the major discoveries were at Ouray, Silverton, Mount Sneffels and Telluride. More than 10 million ounces of gold was mined from this region. Gold placers were found at Breckenridge and Fairplay.

Mines in the San Juan Mountains, Colorado
Parts of the mineral belt follow Precambrian shear zones. Those shear zones were active at several times during the Precambrian and Phanerozoic. Laramide intrusives are associated with this belt and once were thought to be responsible for most ore deposits; however, many important ore deposits are currently interpreted to be genetically related to more recent magmatism, some as young as 25 million years.

Gold was first reported in Colorado in 1858. Further finds in 1859 prompted the Pikes Peak gold rush which eventually yielded 1.25 million ounces of gold.  Gold was discovered in 1858 in the South Platte River. Gold was discovered in placers at Empire, Georgetown, Idaho Springs, Breckinridge, Blackhawk. Other prospectors moved up the Arkansas River and discovered gold at California Gulch near what is now known as the Leadville district.

The area is underlain by igneous and metamorphic rocks, including contorted schists, interlayered gneiss, migmatite, and sheets, dikes and masses of granite and granodiorite. The veins generally fill north-northeast to east trending fractures and faults in Precambrian igneous and metamorphic rocks. The Precambrian complex is intruded by a variety of Tertiary plutons and dikes. Mineralization ranges in age from late Cretaceous through late Tertiary and is found in veins that range from a few inches to nearly 40 feet thick. Common ore minerals include gold, silver, pyrite, chalcopyrite, galena, sphalerite and tennantite and are generally grouped into:
1) pyrite-quartz veins; 
2) copper veins with quartz, pyrite, chalcopyrite, tennantite, minor galena and sphalerite; 
3) lead-zinc quartz veins containing quartz, pyrite, galena, sphalerite and subordinate chalcopyrite and tennantite, and 
4) lead-zinc carbonate veins containing galena, sphalerite, and small amounts of chalcopyrite, tennantite and pyrite. 

Specimen with molybdenite in fracture fillings surrounding
quartz. The molybdenum is the distinct, silver,
 metallic material.
All contain gold, but the most important are lead-zinc quartz veins. A few gold telluride veins are found in the southeast part of the Central City district. The gold is most commonly associated with pyrite and chalcopyrite, while silver is more closely associated with the tennantite.

After Carlin and Homestake, Cripple Creek is the third most productive gold district in the US. The ores from Cripple Creek have an origin different from other gold deposits in the Front Range. The gold at Cripple Creek was formed within or at the margin of a Tertiary volcanic breccia. These rocks occupy a steep-walled volcanic caldera about 4 miles long and 2 miles wide that was a result of gaseous eruptions. Explosive fracturing and subsidence produced shear zones in the breccia and adjacent Precambrian rocks. Along these shear zones, various dikes and irregular masses of volcanic rocks were emplaced, including latite, syenite, phonolite and lamprophyre. These dikes were more porous to fluids than the surrounding Precambrian granitic rocks, and in many places became the conduits for the ore bearing brines.

Breccia. A broken rock rehealed (cemented) by
later mineralized solutions
These ore deposits are epithermal and found along persistent veins. Many of these veins lie close to the margin of the breccia mass while others persist into the breccia or extend into the adjacent Precambrian rocks. The veins are narrow, high-grade zones mineralized with gold, silver and copper tellurides, pyrite, sphalerite, galena and tetrahedrite.

The gangue minerals (associated minerals with no value) are chiefly quartz and fluorspar, with dolomite, ankerite and celestite. 

The most spectacular discovery at Cripple Creek was the Cresson Vug. This cavity, roughly 25 feet by 12 feet by 40 feet high, was found in 1914, and was lined with gold tellurides, quartz, celestite and clay. Nearly 20,000 ounces of gold had been mined from this vug.

While most gold mined in Colorado was from lodes, there was significant placers. Placer mines included the Gregory placer which yielded 50,000 ounces. Others were mined at Blackhawk, Central City and Gamble Gulch. About 90,000 ounces of gold were mined from Cache Creek area near Granite. Other placers were worked along the Arkansas, Lost Canyon Gulch, Chalk Creek, Cottonwood Creek, Pine Creek, Bertschey’s Gulch, Gold Run Gulch, Gilson Gulch, Oregon Gulch and Ritchey’s Patch.

Nearly 140,000 ounces of placer gold was mined in Clear Creek and its tributaries, including Chicago Creek near Idaho Springs. Additional important placers were mined in Nevada and Illinois gulches and Missouri Flats. About 350,000 ounces of placer gold was mined from Fairplay, Tarryall and the Alma districts and 360,000 ounces of gold was placered from Iowa and California Gulches, tributaries of the Arkansas River, in the Leadville district. 

Shorty panning for gold - one of
many sketches by the author.
More than 750,000 ounces of placer gold were recovered from the Breckenridge district. Rich gold placers were discovered along Georgia Gulch on the north side of Farncomb Hill, and soon afterward placers were discovered along many streams and gulches in the district. Farncomb Hill is famous for its rich deposits of native gold. Bench gravels in the area were mined by hydraulic methods, and deep placers were worked by bucket line dredge. Lesser amounts of placer gold were also taken from McNulty Gulch in the Tenmile district.

An extensive area of placer deposits exists for a considerable length along the Arkansas River. For most of this area, the gold in the Arkansas is very fine in size. Much of the headwater area of the Arkansas River as it comes down out of the Leadville area.

The Cripple Creek and Victor open pit mine in the Cripple Creek District is the only active large-scale gold mining operation in Colorado. The ores yield average grades of 0.02 to 0.03 ounce per ton. A similar gold district was discovered in Wyoming in 1981 by W. Dan Hausel, but the district has yet to be developed even though drilling has identified commercial amounts of gold at shallow depth. Several small underground operations also are working in the Front Range gold belt. 

In 1890, gold was discovered at the Independence Lode, producing one of the largest gold strikes in history. It is reported that $500,000,000 worth of gold ore was dug from Cripple Creek. 

Through 2005, the Cripple Creek district produced about 23.5 million ounces of gold. Open pit operations began in1994 east of Cripple Creek at Victor.

The Cripple Creek Mining District is located within a Tertiary age alkaline volcanic/diatreme complex dated at 33.4 Ma (Jensen, 2003). Surrounding country rocks are Precambrian granites and metamorphic units. Gold mineralization within the Cripple Creek district occurs in two principal styles. Mineralization occurs as (1) broad zones of low-grade, gold-pyrite mineralization which is microfracture-controlled and disseminated and (2) as fracture zones containing high-grade, gold-silver tellurides. The low-grade mineralization commonly contains microcrystalline native gold attached to pyrite and was deposited in permeable host rocks and/or microfractures adjacent to the narrow, high-grade structures. In many cases, the high-grade gold mineralization occurs adjacent to the contacts between Cripple Creek Breccia and the sills, dikes, domes, plugs, and/or Precambrian contacts. 

Both types of mineralization are associated with potassium metasomatism. High-grade mineralization occurs as either native gold with iron oxides after gold-silver tellurides and pyrite, or as the gold-silver tellurides (calaverite, krennerite, and sylvanite) in association with pyrite. Minor gangue mineralization consists various occurrences of quartz, fluorite, and carbonate. The majority of the historical production came from these high-grade gold-silver telluride vein portions of the deposit. 

There are two late-stage, prominent hydrothermal breccia pipes in the northwest corner of the district. Gold mineralization within these late features is almost entirely in the clasts indicting that much of the activity was post mineral (Seibel, 1991). 

A flawless 14.2 carat octahedral diamond from
Kelsey Lake Colorado (photo courtesy
of Howard Coopersmith).
The State of Colorado is essentially bankrupted. This lack of cash flow could be corrected by cutting taxes (something most politicians have no concept of) but also by providing serious incentives for exploration and mining and by cutting ridiculous regulations associated with exploration. All of the western states including Colorado would do well to study the Canadian models for exploration, land acquisition, mining and regulation. Colorado still has some incredibly rich gold deposits, many to be found, and along the way, the state also has many diamond deposits that could also be exploited.

Look close at the depression found in northern Colorado - it looks like a pond, and it does produce
sporadic ponds, but the rock under this pond near the Wyoming State Line, is formed of relatively
soft diamond-bearing kimberlite! Find a pond, and you may hit the jackpot.

Diamond-bearing kimberlite at Kelsey Lake, Colorado. This brecciated blue ground yielded many
gem-quality diamonds.