"Rock of the Month #176, posted for February 2016" ---

A maroon-coloured slag

from southwest Colorado, U.S.A. Williams and Chronic (2014, pp.269-273) provide a concise illustrated description of the geology of the Silverton to Durango district. A map of the rail route is available (D&SNGRR & Fort Lewis College, 2012). Railway construction centred on Durango in 1880, and lines reached out to Silverton (July 1882), then Ouray (1887) and Lake City (1889: see Kile and Staebler, 2011, pp.6-7). Durango became the regional smelting centre.

The Durango area has seen a wide range of mining and smelting activities for diverse products: coal, uranium, base and precious metals. At the time of our last visit in September 2015, the Animas River ran clear, but amongst the cobbles by the shore was an orange gel, a ferruginous precipitate from a mine tailings breach earlier that summer. Much information is available on the environmental aspects of the long history of metal mining and refining (e.g., Church et al., 2007; Degraff, 2007). The smelting and metal-refining activity was essentially two-stage, as summarised concisely here:

"Smelting in Durango began ... in the 1880s, with the construction of a mill that processed lead, gold, silver and copper from the San Juan Mountains. The smelter and mountains of slag occupied a strip of land bounded north and south by today's County Road 210 and Lightner Creek and east and west by the Animas River and a steep ridge that tops out 1,200 feet above the city. The peak came to be called Smelter Mountain. The smelter, which closed in 1930, was replaced in 1942 with a United States Vanadium Corp. mill that refined uranium ore for all but two years until 1963. The uranium was destined for the Manhattan Project, which produced the atomic bomb. The end of World War II halted production from 1945 to 1947, but the Cold War brought renewed activity" (Rodebaugh, 2012).

The samples shown here are clearly old material of the historical period of base- and precious-metal smelting. The slag would have been readily available for use as railroad ballast, since the Smelter Mountain site is just southwest of the rail terminus in downtown Durango. The samples were collected beside the Durango-Silverton railway line, along a siding above the right (west) bank of the Animas River. The Needleton Siding is about 31 of the 45 miles north from Durango towards Silverton, near Mile Marker 481 on the line, and near some historic mine sites in the hills.

Figure 4. Here is a polished thin section, 46x26 mm in area, resting on an American quarter (coin diameter 23 mm). The slag is clearly flow-banded in transmitted light, but the feature evident in this image is the presence of abundant sulphide melt, largely concentrated into the 12 largest spherical blebs, clearly visible here. The blebs average about 1 mm in diameter, and are aligned parallel to the direction of flow of the slag when still molten.

By the late 19th century, Colorado smelters were tackling a variety of challenging ore types, variably rich in pyrite and arsenic, from sources in Colorado, Utah and Nevada (Pearce, 1890). The smelters produced matte rich in Cu, Ag and Au, which might ideally have contained about 40% Cu plus 400 oz/T Ag and 6 oz/T Au The slag generated by the smelters was typically dominated by silica and Fe oxides, plus oxides of Mn, Ca, Zn and lesser Al, Mg, Pb and Cu (Pearce, 1890). More recent studies of surface chemistry have shown that in near-neutral tailings with carbonate-buffered pH, up to 50% of Pb occurs adsorbed on Fe oxide and oxyhydroxide species. In contrast, in sulphide-rich low-pH samples Pb jarosites are the main Pb host (Morin et al., 1999). The blebs of sulphide matte in the slag are an obvious hint that this aggregate is from the early smelter period in the late 19th and early 20th centuries.

Figures 5-8. Four photomicrographs at nominal magnification 50X, long-axis field of view 1.7 mm. The rather dull slag reveals quite exquisite beauty under the microscope! The banded slag is composed of pale yellow-green, brown, and near-opaque glasses. In the green and brown glasses, incipient nucleation has produced fern-like growths of silicate crystallites. These are yellow-green pleochroic, probably iron-rich pyroxenes. Clear, sometimes curved crystallites of (?) calcic feldspar nucleate in the near-opaque glass. The spherical blebs are probably a copper-rich Cu-Fe-S intergrowth of "intermediate solid solution", with minor alteration to secondary minerals such as goethite and/or copper salts. The silicate crystal masses aggregate in glass, and also on the margins of pre-existing sulphide blebs. Figures 5-6 show a sulphide bleb 0.9 mm wide in glass and pyroxene matrix. in both transmitted and reflected plane-polarized light. Figure 7 shows two more sulphide spheroids, 1.2 and 0.6 mm in diameter. Figure 8 is a spectacular close-up of some of the skeletal pyroxene microcrystallites.

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References

Church,SE, von Guerard,P and Finger,SE (editors) (2007) Integrated Investigations of Environmental Effects of Historical Mining in the Animas River Watershed, San Juan County, Colorado. USGS Prof.Pap. 1651, 1096pp. plus CD-ROM, in 2 volumes.

Degraff,JV (editor) (2007) Understanding and Responding to Hazardous Substances at Mine Sites in the Western United States. GSA Reviews in Engineering Geology 17, 180pp.

D&SNGRR & Fort Lewis College (2012) Geology of the Durango-Silverton Train Route. Fort Lewis College, CO, 2-sided colour map, 1:125,000 scale.

Kile,D and Staebler,GA (editors) (2011) The San Juan Triangle of Colorado: Mountains of Minerals. Lithographie No.15, 105pp.

Morin,G, Ostergren,JD, Juillot,F, Ildefonse,P, Calas,G and Brown,GE (1999) XAFS determination of the chemical form of lead in smelter-contaminated soils and mine tailings: importance of adsorption processes. Amer.Mineral. 84, 420-434.

Pearce,R (1890) Progress of metallurgical science in the West. Trans.AIME 18, 55-72.

Rodebaugh,D (2012) Paying for toxic smelter? Families search for clues of uranium contamination in Durango's past. Durango Herald, 21 April.

Williams,F and Chronic,H (2014) Roadside Geology of Colorado. Mountain Press Publishing Company, Missoula, 3rd edition, xvi+399p.

Figures 9-10. A particularly large mass of slag on Needleton Siding of the railway line north of Durango, roughly 30x30x12 cm in size. At right is a collection of slag pieces, including the two samples, plus two pieces of coal (the fuel for the steam engines is mined locally, in southwest Colorado).

Graham Wilson, 25 October 2015, 19 November 2015, 17 December 2015, 15-16 January 2016

Compare the sulphide matte in this sample with textures in a copper slag from southern Ontario" .

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