New Mexico Mineral Symposium — Abstracts

The precious metal, silver, may occur in a variety of mineralogical forms within base metal, gold, or silver ores, including:

native silver
silver sulfide (argentite/acanthite)
silver halides (chlorargyrite/bromargyrite)
alloyed with gold (electrum) gold-silver tellurides
silver sulfosalts (proustite, polybasite, etc.)
substituting in other sulfosalts (tetrahedrite, etc.) substituting in galena
substituting in other sulfide minerals
in supergene minerals (Mn-oxides, jarosite, other sulfates, and carbonates)

Of the primary (hypogene) silver-bearing minerals, aside from those sulfosalts that contain essential silver (proustite-pyrargyrite, pearceite-polybasite, stephanite, matildite, miargyrite, etc.), a major host for silver is the tennantite-tetrahedrite-freibergite series. This solid-solution series of isometric "gray copper ores" can be characterized by the general formula (Cu,Fe,Ag)₁₂ (As,Sb)₄S₁₃, or more accurately as (Cu,Ag)₁₀ (Fe,Zn,Cd,Hg,Cu)₂(As,Sb,Bi)₄S₁₃. A complete series extends from tennantite, ideally Cu₁₀(Fe,Zn)₂As₄S₁₃, to tetrahedrite, Cu₁₀(Fe,Zn)₂Sb4S₁₃. From this binary series, solid solution extends part way to freibergite, (Ag,Cu)₁₀(Fe,Zn)₂(Sb,As)₄S₁₃, and to the recently (1988) described argentotennantite, (Ag,Cu)₁₀(Zn,Fe)₂(As,Sb)₄S₁₃. Silver-bearing members are more common in the tetrahedrite (Sb-rich) members of the group. True freibergites are rare; silver contents in tetrahedrites commonly range from <0.1 to approximately 15-20 wt % Ag (equivalent to approximately 25-35 mole % freibergite).

Silver enters galena through a coupled substitution with either bismuth or antimony: Ag + Bi = 2 Pb or Ag + Sb = 2 Pb. Thus, in the absence of these semi-metals, synthesis experiments have produced not more than about 0.4 wt % Ag in galena. Galenas containing Bi or Sb, however, can contain as much as 10 wt % Ag. Low-silver galena (less than a few percent Ag) may be homogenous, but high-silver galenas (>5% Ag) usually contain exsolved silver-bearing sulfosalts, such as tetrahedrite or matildite. Estimates of the average or typical concentration of silver in galena worldwide are on the order of 0.1 wt % Ag (= 1000 ppm, = 29.16 oz/ton of galena), but this may range from <0.005% to 1% or more in different types of ore deposits.

Silver contents in other sulfide and sulfosalt minerals worldwide, reported in the literature plus some analyzed by the author via KEVEX semiquantitative energy-dispersive x-ray spectrometry, fall into the following (all highly variable) ranges:

chalcopyrite <1 - 20,000 ppm (median = 55-300, mean = 90-550)
sphalerite <1 - 8,000 ppm (median = 55-110, mean = 110-535)
enargite <100 - 6000 ppm
bornite 80 - 4000 ppm
pyrrhotite 10 - 1600 ppm (median = <10-15, mean = 15-25)
arsenopyrite 100 ppm
pyrite <10 - 300 ppm (median = <10-30, mean = 15-70)
marcasite 2 ppm

The higher values reported for some of these minerals probably represent bulk admixture of native silver or other silver-rich minerals.

Few accurate analytical data exist for tetrahedrite-tennantites from New Mexico. Galenas from the Hansonburg and other districts of lead-zinc-barite-fluorite replacement deposits are quite low in silver (<0.01 to 0.02 wt % Ag), a feature they have in common with the Mississippi Valley-type ore deposits they resemble. Some galena from other deposits in the state, probably higher temperature and more closely igneous-related ore deposits, shows higher silver values (one sample from the Groundhog mine, Grant County, contains 0.25 wt % Ag); Northrop (1959) quoted eight analyses of galena, ranging from 0.02 to 0.17 wt % Ag. A specimen of coarsely bladed enargite (Denver Mus. Nat. Hist. #2334) from New Mexico, locality unknown, contains 0.03 wt % Ag. Analyses by Burnham (1959) showed a range from 1 to 20,000 ppm Ag in chalcopyrite from New Mexico mines (median = 65 ppm; n = 36), and 2 to 1000 ppm Ag in sphalerite (median = 50 ppm; n = 42). Two analyses, by this author, of New Mexico sphalerite show 0.00 (Groundhog mine) and 0.02 (Mason Tunnel, Hanover district, Grant County) wt % Ag. More analyses are in progress and new results will be reported at the symposium.

Suggested reading
Fleischer, Michael, 1955, Minor elements in some sulfide
minerals: Economic Geology, 50th Anniversary volume, pp. 970¬1024.

Foord, E. E., and Shawe, D. R., 1989, The Pb-Bi-Ag-Cu-Hg chemistry of galena and some associated sulfosalts--a review and some new data from Colorado, California, and Pennsylvania: Canadian Mineralogist, v. 27, no. 3 (in press).

Foord, E. E., Shawe, D. R., and Conklin, N. M., 1988, Coexisting galena, PbSss and sulfosalts: evidence for multiple episodes of mineralization in the Round Mountain and Manhattan gold districts, Nevada: Canadian Mineralogist, v. 26, pp. 355-376.

Heyl, A. V., Hall, W. E., Weissenborn, A. E., Stager, H. K.,
Puffett, W. P., and Reed, B. L., 1973, Silver; in United States Mineral Resources: U.S. Geological Survey Professional Paper 820, pp. 581-603.

Nishiyama, Takashi, and Kusakabe, Yoshihiko, 1986, Silver content in some common ore minerals: Mining Geology (Japan), v. 36, pp. 425-437.

Pattrick, R. A. D., and Hall, A. J., 1983, Silver substitution into synthetic zinc, cadmium, and iron tetrahedrites: Mineralogical Magazine, v. 47, pp. 441-451.

References:

1. Burnham, C. W., 1959, Metallogenic provinces of the southwestern United States and northern Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 65, 76 pp.
2. Modreski, P. J., 1988, The silver content of galena and sulfosalt ' minerals from hydrothermal ore deposits in Peru, Colorado, and New Mexico; in Mineralogy of precious metal deposits, a symposium on the mineralogy of gold and silver deposits in Colorado and other areas: Friends of Mineralogy,Golden, Colorado, August 12-15, 1988, pp. 70-79.
3. Northrop, S. A., 1959, Minerals of New Mexico, revised edition: Albuquerque, University of New Mexico Press, 665 pp.