"Rock of the Month #158, posted for August 2014" ---

Gold-silver telluride minerals, and gold ores from Colorado

Arguably, tellurium-rich gold ores are developed in Colorado as well as anywhere on Earth, both in economic terms (tonnage of contained gold) and in light of the occurrence of diverse well-crystallized mineral species. Thus the MINLIB bibliography (21 July 2014) contains 111 records of publications concerning Au, Te and the state of Colorado (1874-2013). Given the wealth of information on the telluride ores, just a brief sampling of the literature is presented here. The mining districts and their ores have been described by some of the giants in American geology, including Silliman, Lindgren and Emmons. Prior to the mining period, Silliman (1874) described the geology and telluride mineralogy of the Gold Hill area, comparing the veins to the more complex ores from Nagyag in Transylvania. Brief notes follow on three mineral districts and localities in Colorado.

1) The Boulder County region includes Au- Ag- Te ore occurrences in the Jamestown, Gold Hill and Magnolia mining districts, part of a broad, north-trending area of precious-metals telluride mineralization, about 13 km² in area, at the northeast end of the Front Range mineral belt. A pioneering study of the complex suite of early Tertiary ores, including telluride veins, identified some 67 vein mineral species, often in fine intergrowths. The chief ore minerals are the tellurides sylvanite, petzite and hessite, plus native gold. In some mines calaverite and krennerite are also important, and ten other tellurides plus native Te are also found. Subordinate gangue phases include roscoelite green mica, ankerite, calcite, fluorite and barite (Kelly and Goddard, 1969). Their paragenetic scheme includes early sulphide, then native tellurium, then tellurides of progressively lower Te content, and then native gold.

The Boulder Telluride Belt, including the Gold Hill mining district, is rich in gold ores with a complex mineralogy, including native gold, electrum, native tellurium and tellurides. Supergene decomposition of Au tellurides may yield metallic, native gold. Altaite, calaverite and other tellurides occur in the district (Geller, 1993). The detailed paragenesis is a sequence of mineral deposition commencing with Au tellurides and native Te (± tellurantimony and tetradymite), followed by melonite, then altaite, then Ag tellurides ± nagyagite, and native Au and finally petzite (Geller and Atkinson, 1991; Geller, 1993).

2) Historically, the Cripple Creek district in Teller county is one of the most storied gold and silver mining camps in North America, along with such other areas as the Mother Lode of California, the Comstock Lode in Nevada and the Klondike district in the Yukon. The Cripple Creek gold district was an 1891 discovery, and production rose in spectacular fashion to a zenith in 1900, falling by the time Lindgren and Ransome (1906) published a detailed description of the mines in their peak years.

The Cresson mine was perhaps the most famous of all the local mines, in part because of the Cresson Vug, a spectacular cavity plated with telluride crystals, almost all of which were apparently sent to the smelter, to the great sorrow of collectors and mineralogists alike (see, e.g., Smith et al., 1985; Carnein and Bartos, 2005; Anon, 2006). The cavity was lined with sylvanite and calaverite crystals, as well as native gold.

A federal survey found sylvanite and calaverite in the area in 1873. The filing of the El Paso lode claim by Robert Womack on 20 October 1890 started the real "gold rush" (Grimstad and Drake, 1983). Concise historical descriptions of the Cripple Creek camp include those of Emmons (1917) and Rickard (1932).

As of 1988, the Cripple Creek district had produced some 653 tonnes of gold and at least 60 tonnes of silver, of which the Cresson mine produced about 50 T Au in 1903-1935 (Saunders, 1988). The gold mineralization is associated with, and hosted by, Tertiary igneous rocks emplaced circa 34 Ma, with the gold mineralization dated at about 30-28 Ma (Saunders, 1988).

The local intrusions have pronounced alkaline, syenitic affinity: syenite, trachyte, phonolite and alkalic basalt. The Cresson deposit is hosted in a diatreme-like breccia pipe, the "blow-out". Gold occurs in cavities ranging from a few mm to >10 m in size, located between breccia fragments. Calaverite is the main Au mineral in Cresson ore, but other tellurides are also present, e.g., petzite, krennerite, altaite, melonite and coloradoite (Saunders, 1988; Kelley et al., 1998). Gold at Cripple Creek has been recovered from veins, small diatremes, hydrothermal and tectonic breccias, and disseminated mineralization in wallrock, in an Oligocene nested diatreme- alkaline intrusive complex, located on structural controls. The complex consists of 3 coalesced diatremes, two of which yielded most of the gold. Veins and hydrothermal breccias display low total sulphide content, abundant fluorite, carbonate, oxides and sulphates, evidence of boiling hydrothermal fluids at low temperature and low salinity, and early adularia K-feldspar. Multiple boiling and sealing events resulted in extensive brecciation and alteration (Thompson, 1991).

Figure 2. Three small samples from the Cresson mine in the Cripple Creek mining district. Left: fine-grained, very pale, porphyritic and somewhat vuggy phonolite, enclosing disseminated grains and a few larger crystals of lustrous calaverite, AuTe₂. The sawn piece (left, sample 1824) was collected by Kenny Mumford in 1956 (purchased from David Shannon Minerals in 1997). The two smaller, angular pieces (sample 1665) of silicified phonolite were collected in 1992 by Brad Bowman when the old mine dumps were being reworked for the gold content of the tailings (from David Shannon Minerals, 1995). They each display small lustrous crystals of telluride. The ore includes some fine-grained pyrite and fluorite. Right: A close-up (nominal magnification 25X, long-axis field of view 5 mm) of a large, elongate calaverite crystal (top left of middle rock chip in the companion photograph) in a quartz-rich matrix. This monoclinic gold telluride forms elongate crystals striated parallel to length.

3) The Sunnyside mine worked veins which have a complex paragenesis, as noted concerning an earlier featured specimen of manganese silicates and base-metal sulphides, with manganese-rich pyroxenoid minerals. The mine geology and mineralogy is well-documented (22 MINLIB records, 1959-2011). Incidentally, there are also lesser mines named Sunnyside in the Hedley area of southern British Columbia and the Tobacco Root Mountains of Montana, a Sunnyside porphyry in Arizona, and a lake of the same name in Quebec. The Sunnyside ores contain Cu, Pb, Zn and Cd (the latter pair largely in sphalerite, no doubt), as well as Au and Ag (Thompson, 1986). The Sunnyside mine produced >800,000 ounces Au and 14 million ounces Ag, the veins averaging 0.16 oz/T Au. Sunnyside mineralogy includes pyrite, sphalerite, galena, tetrahedrite, rhodochrosite, manganocalcite, and other phases. The Sunnyside veins are younger than the others in the area, and contain a late stage of Au-Te mineralization. The major mines in the western San Juans show a close spatial and temporal relationship to small intrusions of rhyolitic quartz porphyry. Sunnyside was the biggest producer (Bartos, 1993)

Figure 3. Two photomicrographs of sulphide-related gold in the Sunnyside sample, in plane-polarized reflected light. Left: Buttery-yellow native gold and small granules of greenish-grey (?) tetrahedrite in a matrix of pitted, cleaved galena, fringed by chalcopyrite against the host vein quartz. Nominal magnification 50X, long-axis field of view 1.7 mm. Right: rounded blebs of native gold as inclusions in pale pyrite, with galena on margin against vein quartz. Nominal magnification 100X, long-axis field of view 0.9 mm.

Tom Casadevall documented a 6-stage paragenesis of the ores in his doctoral thesis. These include: 1) pyrite-quartz ore, 2) banded quartz-sulphide ore, 3) massive galena- sphalerite- chalcopyrite- bornite- hematite ore, 4) Au- Te- quartz ore, 5) Mn ores and 6) quartz-fluorite-carbonate-sulphate ores (Casadevall and Ohmoto, 1977).

Figure 4. Left: A photomicrograph of sulphides in the Sunnyside sample, in plane-polarized reflected light. Pyrite, chalcopyrite, pale grey galena plus sphalerite (medium grey) and darker quartz gangue. Nominal magnification 100X, long-axis field of view 0.9 mm. Right: in the same sample (2520), gold is bright yellow against distinctly greenish-yellow chalcopyrite, with sphalerite (medium grey) and galena (pale grey) in a dark quartz gangue. Nominal magnification 200X, long-axis field of view 0.45 mm.

References

Anon (2006) Cresson Vug photo found! Mineral.Record 37, 194-195.

Bartos,PJ (1993) Comparison of gold-rich and gold-poor quartz-base metal veins. SEG Newsletter 15, 1,6-11, October.

Carnein,CR and Bartos,PJ (2005) The Cripple Creek mining district. Mineral.Record 36, 143-185.

Casadevall,T and Ohmoto,H (1977) Sunnyside Mine, Eureka mining district, San Juan County, Colorado: geochemistry of gold and base metal ore deposition in a volcanic environment. Econ.Geol. 72, 1285-1320.

Emmons,WH (1917) The Enrichment of Ore Deposits. USGS Bull. 625, 530pp.

Geller,BA (1993) Mineral Studies in the Boulder Telluride Belt. PhD Thesis, 2 volumes, 731pp., University of Colorado, Boulder.

Geller,BA and Atkinson,WW (1991) Paragenetic sequence of telluride and related mineral stages in the Boulder County telluride belt, Colorado. GSA Abs.w.Progs. 23 no.5, 486pp., 417, San Diego.

Grimstad,B and Drake,RL (1983) The Last Gold Rush: a Pictorial History of the Cripple Creek and Victor Gold Mining District. Pollux Press, Victor, Colorado, 159pp.

Kelley,KD, Romberger,SB, Beaty,DW, Pontius,JA, Snee,LW, Stein,HJ and Thompson,TB (1998) Geochemical and geochronological constraints on the genesis of Au-Te deposits at Cripple Creek, Colorado. Econ.Geol. 93, 981-1012.

Kelly,WC and Goddard,EN (1969) Telluride Ores of Boulder County, Colorado. GSA Memoir 109, 237pp.

Lindgren,W and Ransome,FL (1906) Geology and Gold Deposits of the Cripple Creek District, Colorado. USGS Prof.Pap. 54, 516pp., 29 plates and maps.

Rickard,TA (1932) A History of American Mining. McGraw-Hill Book Company, Inc., New York, 1st edition, 419pp.

Saunders,JA (1988) Textural and geochemical characteristics of gold mineralization from Cresson mine, Cripple Creek district, Colorado, U.S.A. Trans.Inst.Min.Metall. B97, 36-39.

Silliman,B (1874) Tellurium ores of Colorado. Amer.J.Sci. 108, 25-29.

Smith,AE, Raines,E and Feitz,L (1985) The Cresson Vug, Cripple Creek. Mineral.Record 16, 231-238.

Thompson,TB (1986) Field Trip Guide Book to Colorado Ore Deposits. 14th General Meeting of IMA, 25pp.

Thompson,TB (1991) Cripple Creek revisited. GSA Abs.w.Progs. 23 no.4, 99.

Graham Wilson, 08,11-12 June 2014, upgraded 22,25 July 2014, 22 October 2014, 01 November 2014

See the companion article on tellurides from Colorado.

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