SYLVANITE: Au! IT'S GOLD

 Even on my best days I do not pretend to be any sort of a “real” mineralogist.  Yes, I have learned to enjoy minerals after giving up my career as a paleontologist/stratigrapher and administrator, and certainly try to learn as much as possible;however, there are many things about the formation of minerals that are beyond the scope of my limited abilities.  One of these areas of confusion remains the formation of the gold deposits in the Cripple Creek Mining District southwest of Colorado Springs.  I probably should leave any musings about Cripple Creek minerals to my friend Bob Carnein up at Lake George.  However, I recently picked up a specimen of sylvanite collected by one S. Willman in 1981 from Cripple Creek (anything more definitive than Cripple Creek is unknown).  Sylvanite [(AuAg)₂Te₄] is a mineral composed of silver, gold, and tellurium and that makes it even more interesting.

The element tellurium is a silver-white metalloid (possesses properties of both metals and non-metals) with the symbol Te and the atomic number of 52 (number of protons in the nucleus of the atom). It is a critical raw material used in the solar industry, copper and steel alloys, and semiconductors but its extraction also poses environmental risks for our planet (Missenab and others, 2020).

Tellurium exhibits oxidation states of 6+, 5+, 4+, 3+, 2+. 1+, 1-, 2-; however, only 6+, 4+ and 2- are stable. Tellurium can act as a cation with a 4+ oxidation state (a tellurite; IV) as in the uncommon mineral tellurite, TeO₂, or with a 6+ oxidation state (a tellurate; VI) as in jensenite, Cu₃TeO₆-2H₂O.   The telluride anion with a charge of 2- can combine with gold and silver cations in the minerals calaverite [AuTe₂], krennerite [Au₂AgTe₈], and sylvanite [(AuAg)₂Te₄]; all form major gold ores at Cripple Creek, Colorado.  In fact, Eckel and others (1997) noted that other than Cripple Creek, where krennerite and calaverite are the major sources of gold, sylvanite is probably the most valuable of the gold minerals in the other telluride gold deposits of Colorado. 

Although mineralogists have recognized eight gold-silver telluride minerals (calaverite, krennerite, sylvanite, petzite, muthmannite, empressite, hessite, and stuetzite), the gold-rich telluride species—calaverite, krennerite and sylvanite—are the most common and economically important minerals of the group (Zhao and Pring, 2019). Work by Cabri (1965) identified the following amounts of silver in each: krennerite 3.4 to 6.2 wt % Ag; sylvanite 6.7 to 13.2 wt % Ag; and calaverite 0 to 2.8 wt % Ag. Krennerite and calaverite both have over 40% gold while sylvanite somewhere in the mid-30s. However, native gold is the most common gold-bearing mineral, and the most valuable.  Since native gold is rare at Cripple Creek early miners roasted the ore to volatize the tellurium, a process that left rather pure gold as blisters or “sponge.”  See photos below. The current Cresson Pit uses a heap leach process.

Sylvanite has about a 1:1 to 3:1 gold to silver ratio.  In sylvanite [(AuAg)₂Te₄] the oxidation state of gold is 3+ while silver is 1+ [X 2 = 8+] while Te is 2- X [4 = 8-] and things balance out.  Since it is a metal, sylvanite is opaque and has a shiny, metallic luster with a silver-white streak. It comes in a variety of metal-like colors—white, silver-white, gray, sort of yellow, and is quite soft at ~1.5-2.0 (Mohs).  Crystals vary from platy to prismatic to granular.  My specimen has very tiny crystals and numerous non-descript grains.

Streaks and scattered grains/crystals of silver-gray to silver-yellow sylvanite.  The grains/crystals are submillimeter and are not very distinguishable.  Width FOV top: ~5 mm, bottom: ~4mm.

MinDat notes that sylvanite mostly is associated with low-temperature hydrothermal veins; however, it is also a late forming mineral in medium to high-temperature hydrothermal veins and is usually among the last minerals to form. But, the magmatic-hydrothermal processes causing the tellurium and gold enrichment in epithermal deposits (deposition by warm water shallow crustal environment associated with volcanic activity) is not well understood (Keith and others, 2020). Cripple Creek, with its deposits of gold tellurides, is a notable crustal laboratory for the study of epithermal deposits.   

Since it is near Halloween it is worth mentioning that the type locality of sylvanite is Baia de Aries, Romania. Now think or werewolves, vampires, and Vlad Dracula all living in in this Romanian region of Transylvania.  The name sylvanite comes from Transylvania. Just a little spooky trivia.

REFERENCES CITED

Eckel, E. B. and others, 1997 (original 1961), Minerals of Colorado: Putnam Press and Friends of Mineralogy-Colorado Chapter, Golden.

Cabri, L.J., 1965, Phase relations in the Au-Ag-Te system and their mineralogical significance: Economic Geology, Vol. 60.

Keith, Manuel, D. J., Smith, K. Doyle, D. A. Holwell, G. R.T. Jenkin, T. L. Barry, J. Becker and J. Rampee, 2020, Pyrite chemistry: A new window into Au-Te ore-forming processes in alkaline epithermal districts, Cripple Creek, Colorado: Geochimica et Cosmochimica Acta, Vol. 274.

Missenab, O.P., R. Rama, S.J. Millsb, B. Etschmanna, F. Reithcd, J. Shustercd, D.J. Smithe, and J. Bruggera, 2020, Love is in the Earth: A review of tellurium (bio)geochemistry in surface environments: Elsevier Earth-Science Reviews, Vol. 204, 103150.

Zhao, J. and A. Pring, 2019, Mineral transformations in gold-(silver) tellurides in the presence of fluids: nature and experiment: Minerals, Vol. 9, No. 3 (in  The Special Issue Mineral Surface Reactions at the Nanoscale).

And thinking of Transylvania and Dracula reminds me of my favorite Wm. Shakespeare quote:

 Eye of newt, and toe of frog,

Wool of bat, and tongue of dog,

Adder's fork, and blind-worm's sting,

Lizard's leg, and owlet's wing,—

For a charm of powerful trouble,

Like a hell-broth boil and bubble.

Double, double toil and trouble;

Fire burn, and caldron bubble.