Information on the Gem-Emerson gold bearing vein system is derived from a review of published and unpublished literature on 12 mines and 3 tunnels along with limited field work.

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           This study starts at the Franklin mine at the southeast portion of the vein system and progresses west-northwest to the Knickerbocker tunnel, a distance of approximately 3 miles.

 

           The southeast portion of the vein system is located approximately 1 and ½ miles north of the town of Idaho Springs, in Clear Creek County, Colorado and extends northwest approximately 3 miles. The area is accessed by the Oh My God road which runs between Idaho Springs and Central City.

 

           The country rock is primarily metamorphics that have been injected with numerous Tertiary dykes that followed along fault and fracture zones created during the Laramide Orogeny; this was followed by a period of massive hydrothermal activity in which mineral bearing waters were injected from below into zones of weakness. As the temperature and pressure declined the minerals precipitated out of the water, filling fissures and impregnating the country rock. There is abundant mineralization in the area with numerous veins of various size and richness crisscrossing one another, in many cases forming large and rich ore bodies.

 

           The Gem-Emerson vein system is of extraordinary width and persistency and is unquestionably the mother vein of this section with many cross veins where rich ore shoots formed.

 

          The Franklin mine, located on the southeastern portion of the system, was developed to a depth of 670 feet by an inclined shaft with 7 principle levels. Production of 1,382 tons of smelting ore shipped between 1956 and 1959 yielded 273 oz gold, 8,354 oz silver, 5,400 lbs. copper and 90,500 lbs. zinc.

 

          The Freighters Friend mine was opened before 1900 and it was worked last in 1958. The mine exploits a part of the Gem vein system, which in this mine consists of three principle veins—the Freighters Friend (Gem footwall vein), the Gem hanging wall vein and the Washington Vein. Most of the mining has been done on the Freighters Friend vein, but the Washington and Gem veins have been developed and stoped at places. The U.S. Bureau Mines data indicate that at least 6,458 tons of crude ore and 1,295 tons of concentrates have been shipped since 1904, yielding 3,446 oz gold, 28,931 oz silver, 18,607 lbs copper, 221,277 lbs lead and 53,055 lbs zinc.

 

         The Gem  mine is the major mine in the southeast portion of this vein system, here the vein varies in width from 30 feet at the surface to 58 feet at the Argo Tunnel level, 1900 feet below (measured on the dip). There are two streaks of ore within the vein of about equal width, varying from 2 to 8 feet, one on the footwall and the other on the hanging wall. The zone between the two principle streaks is altered country rock with varying amounts of mineralization, sometimes enough to be considered mill ore for the entire width of the vein.  The average of 170 assays of samples cut in 1921 was .28 oz of gold, 6.73 oz silver and 10% to 16% lead. Nearly all of the production has come from the footwall ore streak. The hanging wall ore streak has been extensively tested in the mine and in a 2000 foot drift to the northwest where the ore is reported to run an average of .30 ounces of gold per ton for the entire distance.

 

        The Doves Nest mine was opened in the late 1880s when the main Doves Nest shaft was sunk to a depth of 504 feet. Within the Doves Nest mine there are two veins separated by a porphyry dyke 4 to 20 feet thick. The vein on the north side is described by E.E. Chase in a court deposition dated February 27th, 1892 as “ a well defined vein eight to ten feet in width, with a mineralized ore body on both the foot and hanging walls and more or less mineralized throughout its entire width”. This vein is nearly stoped out to the 5th level. In a report by H.S. Henderson, Manager, The North American Mining Co. stated “ This mine is without doubt the most important former producer bought in by our company. Soon after the Doves Nest started producing in quantity, litigation arose with the Comstock mine (adjoining it on the west), also a producer of note. Although some $200,000 had been extracted from the upper workings, the litigation continued to such an extent that the pumps were pulled and the enterprise abandoned for many years. Some 14,200 tons were shipped with an average value of $14.60 in gold and silver alone. I am informed that pending litigation, experts cut 425 samples, which averaged $17.60 per ton”.

 

       The Comstock mine, to the west-northwest of the Gem, was opened by a 385 foot shaft in the 1880s. Tenor of the ore shipped from 1902 to 1917 averaged .35 oz gold, 11.4 oz silver and 9.7% lead. The mine was re-opened  by Albert Hoyl in 1981 and 1982. The shaft was reconditioned and deepened to a depth of 620 feet.  He encountered a vein 29 feet wide with two streaks of ore, the footwall streak being 5 feet wide and the hanging wall streak 6 feet wide. No information is available as to the tenor of the ore found.

 

       The Big 5 tunnel cuts this vein system approximately 2800 feet northwest of the Comstock mine at a depth of approximately 2000 feet (measured on the dip). Here the vein consists of a zone of sheared, altered and silicified rock about 30 feet thick with a footwall “pay streak” that ranges from 3 to 9 feet thick. Extensive drifts have been driven both east and west of the tunnel. One shipment of 5734 tons of ore averaged .68 oz gold, 2.23 oz silver and .56% copper per ton.

 

       The next major mine, to the northwest, on the system is the Crown Point and Virginia which has a shaft 720 feet deep. The exact width of the vein at this location is unknown, however there are reported to be several parallel veins. The footwall ore streak is from 5 to 8 feet wide and has been nearly stopped out of high grade ore to the sixth level. The tenor of the ore is variable but appears to average approximately .35 oz gold, 5 oz silver with some copper, lead and zinc.

 

       The Lucania tunnel cut the vein system approximately 1600 feet northwest of the Crown Point and Virginia mine at a depth of approximately 1925 below the surface. The vein at this point had 42 feet of low grade mineralization.

 

       The Knickerbocker tunnel has been driven along the vein from the west 2100 feet to connect with the J.L. Emerson shaft at 600 feet. The vein ranges from 6 to 25 feet in width. Sutherland and Hays reported regarding the vein in the Knickerbocker tunnel; “ There are two large ore shoots, one at 950 feet and the other at 220 feet from the portal of the tunnel. These ore bodies are of immense proportion but of low grade”.

 

       Based on a review of the production history of the mines and tunnels from the Gem mine to the Crown Point and Virginia mine it is estimated that the ore body will be nearly continuous between these mines and will average 6 feet in width and extent to a depth of at least 2000 feet. Therefore, each 1000 feet of strike along the vein will have approximately 1,000,000 tons of ore averaging .35 oz gold, 5 oz silver with some copper, lead and zinc.

          

Sources of information:

 

Bastin and Hill (1917) USGS PP 94 Economic Geology of Gilpin County

and Adjacent Parts of Clear Creek and            Boulder Counties, Colorado

 

Lovering and Goddard (1950) USGS PP 223 Geology and Ore Deposits of

The Front Range,            Colorado

 

Sims, Drake and Tooker (1963) USGS PP 359 Economic Geology of the

           Central City District Gilpin County, Colorado

 

Sims and others (1963) USGS PP 371 Geology of Uranium and Associated

Ore Deposits Central Part of the Front Range Mineral Belt, Colorado

 

Sims and Gable (1964) USGS PP 474-C Geology of Precambian Rocks

Central City District, Colorado

 

Moench and Drake (1966) USGS Bulletin 1208 Economic Geology of the

Idaho Springs District Clear Creek and  Gilpin Counties, Colorado

 

Moench and Drake (1966) USGS Open File Report Mines and Prospects,

Idaho Springs District, Clear Creek  and Gilpin Counties, Colorado

 

H. E. Henderson (Met. E) Report of Properties of the North American

Mining Company Situated in Clear Creek, Gilpin,  Boulder and San

Juan Counties, Colorado

 

H. J. Miller (M.E.) (1912) Report on the Gem Group of Properties for the

Idaho Mining, Reduction and Transportation Tunnel Co.

 

Court Proceedings (1891) Edwards et. al. vs Balmoral Mining Company