"Rock of the Month #161, posted for November 2014" ---

Gold-silver telluride minerals, and gold ores from Colorado, II

The tellurides shown in this item occur as relatively coarse, lustrous grains that may be visible to the native eye. Tellurium occurs as a native element and a wide range of tellurides, reduced compounds with such metals as Cu, Bi, Ag, Pb, Hg, Fe, Ag, Pt and Pd. The optical properties of 26 tellurides were compiled by Uytenbogaardt and Burke (1971, pp.233-253). The tellurides of the platinum group elements, as with most discrete minerals of the PGE, are generally microscopically small, <0.05 mm in diameter. Gold deposits associated with alkalic igneous bodies commonly contain tellurides, including the gold-silver tellurides petzite, sylvanite and krennerite, and the Au telluride calaverite.

This month's article is an extension of the recent introduction to tellurides from Colorado. To read more on a different, Pb-Ag-Bi telluride association, see Rock of the Month 67 on rucklidgeite.

Cripple Creek (Carnein and Bartos, 2005) is one of many gold camps worldwide with appreciable gold production from telluride ores. Many telluride minerals have been described from mesothermal to epithermal gold vein deposits around Kalgoorlie in Western Australia (Stillwell, 1953; Shackleton et al., 2003); Kirkland Lake, Ontario (Todd, 1928); and at the Emperor mine in Fiji (Pals and Spry, 2003). Many American occurrences are very well-documented, from such early works as Genth (1868) and Silliman (1874) to more recent observations (e.g., Geller, 1993). The geological and mineralogical literature for such a relatively obscure element as tellurium is surprisingly extensive. As of 01 November 2014, the MINLIB bibliography had 1,354 records of Te and tellurides, from 1849 to the present, including 118 on calaverite, 85 on sylvanite and 20 on the Cu telluride, rickardite.

Tellurides tend to occur as small (mm-scale and smaller) crystals, but the larger end of the size distribution is, not surprisingly, found in high-end private and museum collections. Examples from Cripple Creek include calaverite and sylvanite (Barlow et al., 1996; Carnein and Bartos, 2005, pp.163-167,180; Geffner, 2013) and even pseudomorphs of the nickel telluride melonite replacing calaverite (Carnein and Bartos, 2005, p.174; Wilson, 2009, p.77).

Figure 2. Photomicrographs of sample 1665. Left: an elongate mass of creamy calaverite plates small, equant, euhedral pyrite grains within poorly-reflective gangue minerals. Right: in this view the opaque ore minerals appear in outline, adjacent to coarse, strained quartz and K-feldspar, in (top right) the very fine-grained feldspathic matrix. Nominal magnification 100X, long-axis field of view 0.9 mm, plane-polarized reflected light and (right) crossed-polarized transmitted light.

Figure 3. Photomicrographs of samples 1665 and (right) 1445. Left: carbonate and quartz inclusions in a small K-feldspar phenocryst, in the fine-grained trachytic-textured groundmass of the rock. Right: a thin section of a chip (sample 1445) from the nearby Portland mine. This sample is labelled as "sylvanite in phonolite breccia". It was purchased from David New in 1992. The rock is a medium to pale grey breccia with pale phonolite clasts in a dark cement containing shiny ore minerals. Small quartz-lined vugs host discrete silvery-grey platy crystals of the monoclinic telluride sylvanite, (Au,Ag)₂Te₄. This image shows one of the scattered, large (up to 0.8 mm) crystals of shiny sylvanite, partially overgrowing small, equant, subhedral to euhedral crystals of fractured pyrite in a matrix of strained quartz. Nominal magnification 50X, long-axis field of view 1.7 mm, in crossed-polarized transmitted and (right) plane-polarized reflected light.